Molecular Hydrogen Therapy—A Review on Clinical Studies and Outcomes

With its antioxidant properties, hydrogen gas (H2) has been evaluated in vitro, in animal studies and in human studies for a broad range of therapeutic indications. A simple search of “hydrogen gas” in various medical databases resulted in more than 2000 publications related to hydrogen gas as a potential new drug substance. A parallel search in clinical trial registers also generated many hits, reflecting the diversity in ongoing clinical trials involving hydrogen therapy. This review aims to assess and discuss the current findings about hydrogen therapy in the 81 identified clinical trials and 64 scientific publications on human studies. Positive indications have been found in major disease areas including cardiovascular diseases, cancer, respiratory diseases, central nervous system disorders, infections and many more. The available administration methods, which can pose challenges due to hydrogens’ explosive hazards and low solubility, as well as possible future innovative technologies to mitigate these challenges, have been reviewed. Finally, an elaboration to discuss the findings is included with the aim of addressing the following questions: will hydrogen gas be a new drug substance in future clinical practice? If so, what might be the administration form and the clinical indications?


Introduction to Hydrogen Therapy
The most frequently used medical gases include medical air, oxygen, nitrogen, nitrous oxide and carbon dioxide, and hydrogen gas (H 2 ) is a promising newcomer with unique antioxidant properties.Demonstrated to selectively counteract deleterious reactive oxygen species (ROS), such as the hydroxyl radical, H 2 can maintain tissue homeostasis and can be more clinically useful than strong antioxidants that indiscriminately neutralize both beneficial and harmful ROS species [1].Initially introduced in diving gas around 1970, H 2 was thought to be non-toxic and biologically inert [2].The interest in using H 2 for disease treatment has increased since the antioxidant properties were unveiled in 2007 [3].Since then, over 2000 scientific publications have elucidated its therapeutic promise from in vitro, in vivo animal and human studies.
The expanding body of literature substantiates therapeutic effects of H 2 and clinical trials have been conducted within major disease areas like cardiovascular, respiratory and cancer, with a focus on diseases associated with the accumulation of ROS.Yet, translation into standard clinical practice presents challenges.One challenge is administration of a decent H 2 dose due to its low water solubility and its explosive properties when mixed with oxygen gas (O 2 ).Drinking H 2 -saturated water has been reported to be a common and feasible method but H 2 has a low water solubility of 1.57 mg/L (1.57ppm), corresponding to approximately 19 mL/L at standard ambient temperature and pressure (SATP) conditions.Therefore, to obtain several milligrams of H 2 per day, the ingestion of several liters of saturated hydrogen water per day is required [4].The injection of H 2 -rich saline is also limited by its low aqueous solubility and can only be performed with proper equipment,

•
Will H 2 emerge as a novel drug in clinical practice?

•
Which administration form optimizes the efficacy of H 2 ?

•
What are the potential clinical applications for H 2 ?
These three questions form the scientific basis for the current review publication.

Methods
Published scientific literature related to H 2 therapy was identified by specific searches using the terms "hydrogen therapy", "medical use of hydrogen" and "hydrogen gas" in combination with "human study" and "clinical trial", in several readily searchable publication databases including PubMed.Scientific papers written in a different language than English and those studying human effects of alkaline ionized water were excluded.An overview of the clinical trials was gathered from www.clinicaltrils.gov(accessed on 30 September 2023), the international clinical trials register by U.S. Department of Health and Human Services (National Institutes of Health, National Library of Medicine), and www.umin.ac.jp/ctr/ (accessed on 30 September 2023), the Japanese preregistration system for approved clinical trials (UMIN, University Hospital Medical Information Network).The relevant clinical trials were found by searching for "hydrogen", "hydrogen gas" and "H2" (trials about "hydrogen peroxide" and "hydrogen breath test" were excluded).Publications related to the clinical trials were also included in the present review.This research resulted in 47 and 34 clinical trials, from cliniclatrials.govand UMIN, respectively, and 64 publications on human studies (in addition to several relevant animal and in vitro studies).A review article published in 2015 made a summary of the clinical trials up until that year, and the present summary therefore mainly focuses on the clinical data produced in the period after [10].
This review publication is sectioned as follows.Section 3 of this review presents and discusses the quantitative data of clinical trials and publications related to H 2 therapy.Section 4 discusses safety of H 2 administration, which is the first important topic upon clinical translation of a novel drug.Section 5 includes a tabular overview of the clinical trials registered at clinicaltrials.govand UMIN and discusses the findings there.Section 6 qualitatively investigates the data from published reports after human studies, including clinical trials, by describing the study methodology and the main findings.These results formed the basis for the discussion in Section 7.

Quantitative Overview of Clinical Trials and Scientific Publications
A quantitative overview of the number of clinical trials and scientific publications about H 2 therapy in humans are shown as a function of the publication year, and the first publication date for the clinical trials, in Figure 1.The number of clinical trials has increased from one registered in 2011, five in 2015, with a boost to ten in 2016 and 12 in 2019, to 6-10 each year in the period 2020-2023.The number of scientific publications does not follow the same trend and has been more stable over the period 2012-2022 (data only until August 2023), with an average of approximately five publications per year, and a maximum of 11 publications in 2019.The increased publication activity during 2019-2020 can possibly be explained by the high number of clinical trials initiated in 2016 and the sudden interest in hydrogen therapy in COVID-19 studies.Therefore, the high number of clinical trials registered from 2019 to 2023 might result in a subsequent upswing in the number of scientific publications in the near future.The publications and clinical trials sorted after the indication area and administration route are shown in Figure 2. The major disease areas globally are represented in this graph, including central nervous system (CNS), cancer, cardiovascular and respiratory.Lifestyle-related conditions also have a high number of both trials and publications, and this area is becoming increasingly important with a global population on average increasing in age and weight.Inhalation is the dominating administration method, followed by drinking H2-enriched water, infusion of H2-enriched saline.H2-bath/eye-drops and H2-dialysis have also been used in a few trials.The publications and clinical trials sorted after the indication area and administration route are shown in Figure 2. The major disease areas globally are represented in this graph, including central nervous system (CNS), cancer, cardiovascular and respiratory.Lifestylerelated conditions also have a high number of both trials and publications, and this area is becoming increasingly important with a global population on average increasing in age and weight.Inhalation is the dominating administration method, followed by drinking H 2 -enriched water, infusion of H 2 -enriched saline.H 2 -bath/eye-drops and H 2 -dialysis have also been used in a few trials.

Human Safety of H2 Administration
Ensuring safety is the foremost consideration when the aim is clinical translation of a new drug substance.As opposed to other medical gases like carbon dioxide, nitric oxide, and hydrogen sulfide, H2 does not bind to hemoglobin in the bloodstream, thus H2 will not induce heme-related toxic effects.Extensive animal studies have shown that H2 administration is safe, yet comprehensive documentation of human safety across diverse administration methods remains to be demonstrated.A mixture of H2:O2 or H2:O2:He gas has been employed by deep-sea divers to prevent decompression sickness for a long time, therefore also confirming human safety of H2 inhalation.Two studies from 1988 and 1994 concluded that breathing mixtures comprising 49-56% H2 during dives to 450-500 m below sea level can alleviate some symptoms of the high-pressure nervous syndrome and confirmed the safety in use [11,12].Notably, recent clinical studies aim to document human safety of H2 therapy, for instance the clinical studies NCT04046211 and UMIN000013221 (Tables 1 and 2).One study involving ten cerebral ischemia patients demonstrated that inhaling 3% H2 for 30 min did not change any physiological parameters, reaffirming safety.Additionally, an increase in H2 blood levels, equivalent to findings in animal studies, was observed.However, some inconsistencies in the H2 levels between the individuals were also observed [13].The high number of the clinical trials conducted (Tables 1 and 2) underpins the safety and the nearly absent toxicity of H2 administration by drinking H2-saturated water, inhaling H2 gas of different levels, injection of H2-rich saline and other methods such as topical application and dialysis.Very few adverse reactions from human H2 consumption have emerged across the reported clinical studies, and

Human Safety of H 2 Administration
Ensuring safety is the foremost consideration when the aim is clinical translation of a new drug substance.As opposed to other medical gases like carbon dioxide, nitric oxide, and hydrogen sulfide, H 2 does not bind to hemoglobin in the bloodstream, thus H 2 will not induce heme-related toxic effects.Extensive animal studies have shown that H 2 administration is safe, yet comprehensive documentation of human safety across diverse administration methods remains to be demonstrated.A mixture of H 2 :O 2 or H 2 :O 2 :He gas has been employed by deep-sea divers to prevent decompression sickness for a long time, therefore also confirming human safety of H 2 inhalation.Two studies from 1988 and 1994 concluded that breathing mixtures comprising 49-56% H 2 during dives to 450-500 m below sea level can alleviate some symptoms of the high-pressure nervous syndrome and confirmed the safety in use [11,12].Notably, recent clinical studies aim to document human safety of H 2 therapy, for instance the clinical studies NCT04046211 and UMIN000013221 (Tables 1 and 2).One study involving ten cerebral ischemia patients demonstrated that inhaling 3% H 2 for 30 min did not change any physiological parameters, reaffirming safety.Additionally, an increase in H 2 blood levels, equivalent to findings in animal studies, was observed.However, some inconsistencies in the H 2 levels between the individuals were also observed [13].The high number of the clinical trials conducted (Tables 1 and 2) underpins the safety and the nearly absent toxicity of H 2 administration by drinking H 2saturated water, inhaling H 2 gas of different levels, injection of H 2 -rich saline and other methods such as topical application and dialysis.Very few adverse reactions from human H 2 consumption have emerged across the reported clinical studies, and all trials have concluded that H 2 administration is safe for humans.Still, concerns about H 2 flammability warrant continued consideration.

Overview of Registered Clinical Trials with H 2 Administration
Once the safety of H 2 administration is confirmed, the subsequent objective is to document the therapeutic efficacy of a novel drug substance, a prerequisite for its integration into everyday clinical practice.Unlike conventional medications typically evaluated for specific diseases, H 2 , as an antioxidant, can offer versatile application across various medical conditions.An overview of the 47 clinical trials registered at clinicaltrials.govand 34 clinical trials registered in the UMIN database investigating hydrogen therapy is tabulated in Tables 1 and 2, respectively (as of August 2023).The tables are sorted after disease area and administration method.Many of the listed trials are within large disease groups like cardiovascular, CNS, respiratory and cancer.
These encompass the major healthcare challenges of today demanding innovative pharmaceutical interventions.Successful outcomes of these trials bear the potential to significantly contribute to addressing the foremost sources of disease and mortality within the present global landscape [14].
A few different technological solutions have been employed to facilitate H 2 administration in the clinical trials tabulated above.This includes inhalation machines that aim to make the mixture of H 2 and O 2 safer and more manageable.A hydrogen-oxygen generator device developed by Shanghai Asclepius Meditech Inc. (Shanghai, China) has been used for the inhalation of a gas mixture of 66% H 2 and 33% O 2 (3 L/min, model AMS-H-03).This technology was used in several trials for COVID-19, cancer, respiratory and cardiovascular diseases, where many of which Shanghai Asclepius Meditec Inc. was the responsible party or sponsor.Similarly, Qingdao Haizhisheng Corp. (Qingdao, China) has introduced a hydrogen generator (model HZS-2700A) delivering a flow rate of 2 L/min, which was employed in a type 2 diabetes trial.Additionally, investigations have explored the inhalation of lower H 2 concentrations, typically around 2%.A Japanese company, Doctors Man Co., Ltd.(Yokohama, Japan), provides products for H 2 administration such as H 2 water generators and gas inhalers.
Several suppliers of products related to H 2 -rich water consumption have also been identified in the compiled clinical trials.Notably, electrolyzed water or magnesium (Mg) tablets that produce H 2 from reaction with water, said to give concentrations of 2 ppm (2 mg/L) H 2 and above, are widely used with intake of typically 0.5-1 L/day.DrinkHRW (New Westminster, Canada) is one supplier of Mg tablets and Nihon Trim Co., Ltd.(Osaka, Japan) is a supplier of electrolyzed water.Oral administration is one of the easiest to implement for self-administration at home.Moreover, suppliers of ready-to-drink H 2 -rich water used in clinical trials include Rejuvenation, HRW Natural Health Products Inc. (New Westminster, BC, Canada), Aquastamina-R, Nutristamina (Ostrava, Czech Republic and Melodian Co., Ltd.(Osaka, Japan).Some claim to have achieved H 2 concentrations of over 7 ppm (7 mg/L) in 500 mL of water and 15 ppm (15 mg/L) in 250 mL of water [15].Another company, HoHo Biotech (Taipei, Taiwan), has developed capsules of porous coral material that they claim can absorb and carry hydrogen.Administered orally, the nanoscale carrier can release the enclosed H 2 within the body [16].This technology has been applied in trials involving patients with rheumatologic conditions [17].
The commercial interest in developing technologies for H 2 administration and the geographical diversity of clinical trials show an excitement for H 2 therapy spreading across continents.Predominantly, these trials are conducted in Europe, USA and Asia.A major part of the reported clinical trials has been conducted in Japan (Table 2); however, China also emerges as a center for many of the trials, particularly those related to COVID-19.Notably, the exercise-related studies are conducted in Serbia and Czech Republic.

Efficacy Results of H 2 Administration in Human Studies
An overview of the conducted and to-be conducted clinical trials in a certain therapeutic field, such as the one provided above, can say something about the interest in the field, however it does not consider the outcomes.The efficacy of H 2 administration has been reported in at least 64 referee reviewed scientific publications that are qualitatively discussed below; many but not all are results of the clinical trials listed above.In the following presentation, the articles are categorized into sections according to the primary disease areas investigated.These categories encompass cardiovascular, cancer, respiratory, CNS, infections, lifestyle-related and other diseases, and quantitatively discuss the findings from human studies.

Cardiovascular Diseases
Cardiovascular diseases are the leading cause of deaths worldwide [14], and by such are in need of new therapies.Several clinical studies have concluded that H 2 inhalation or drinking can improve the outcomes of cardiovascular diseases, most in combination with standard treatments.An early phase/initial clinical trial assessed the effect of inhaling 2% H 2 gas, in combination with target temperature management, in a total of ten patients with post-cardiac arrest syndrome.H 2 inhalation was found to maintain a favorable neurological outcome in patients, even though improvements were not statistically significant.However, the 90-day survival rate was significantly better in the H 2 -group, compared to control [18].In another study involving five post-cardiac arrest syndrome patients given the same treatment as above, arterial H 2 concentration was measurable and indications of reduced amounts of oxidative stress and cytokine levels were found [19].A slightly larger study including 20 patients found health-promoting effects of hydrogen therapy for adverse left ventricular remodeling after percutaneous coronary intervention for patients after myocardial infarction.At the 6-month follow-up after initiating treatment with 1.3% H 2 inhalation, the improvement in the left ventricular stroke volume index and ejection fraction was numerically greater than in the control group, even though the latter was not significant.Indications were given for initiating a large-scale efficacy trial [20].H 2 has also been shown as a useful modulator of blood vessel function.Flow-mediated dilation was significantly improved for patients who drank H 2 -rich water (7 ppm = 7 mg/L), compared to placebo [21].These studies show that the inhalation of a low concentration H 2 -containing gas and drinking of H 2 -rich water can be useful in treating cardiac and vascular conditions, respectively.

Cancer
Cancer is the second leading cause of death in the world today [14].H 2 has been administered to cancer patients with the purpose of controlling tumor progression, for combination treatment and to alleviate side-effects and adverse events of standard cancer treatment in a variety of cancer diseases.Additionally, H 2 administration has been shown to promote antitumor immune responses.Figure 3 summarizes the benefits of hydrogen therapy in cancer patients.
Two separate case studies with cancer patients with multiple metastases have shown tremendous effects of H 2 inhalation therapy.One of the patients suffered from recurrent gallbladder carcinoma and was given daily H 2 inhalation therapy.In the first month, the tumors continued to progress following a gradual decrease in tumor size and tumor marker levels that eventually returned to normal.After around two and a half months, the patient could resume normal life and survival was reported still after 10 months.A pseudo-progressive remission after H 2 therapy was observed, which may resemble the pattern that occurs following PD-1 antibody treatment.This suggests that H 2 can affect the immune system [22,23].The other case report was a non-small cell lung cancer patient that underwent H 2 gas inhalation as monotherapy after oral and surgical treatments had stabilized the first lesions.The brain metastases reduced in size after four months of H 2 treatment and completely disappeared after one year.The liver and lung metastases were also stabilized after one year, and survival was lengthened [24].These case studies are tremendous observations and show that H 2 might elicit significant control of tumors after standard cancer treatments have failed.However, it does not empirically prove that H 2 has medical effects in cancer patients and for this we will need statistical grounds from systematic clinical trials.
One clinical study including 58 patients with advanced non-small cell lung cancer reported in 2020 that H 2 therapy was able to relieve pulmonary symptoms compared to a control group that received no treatment.The hydrogen-group was administered H 2 by inhalation for 4-5 h per day for 5 months.The same hydrogen-treatment was also given to non-small cell lung cancer patients in combination with either chemotherapy, targeted therapy or immunotherapy.After 16 months, the progression-free survival was higher for the hydrogen-only treatment group, and significantly higher for all the combinationtreatment groups, as compared to the control group [25].Another trial reported benefits of H 2 treatment in 42 lung cancer patients treated with nivolumab.Significantly longer overall survival was found for the combination treatment with H 2 gas compared to those treated with nivolumab only.It was suggested that the two therapeutic agents might exhibit synergistic effects as mitochondrial activators [26].It is reasonable to treat lung cancer with H 2 inhalation to target the disease site, even though systemic effects have been observed.Other cancer types that have shown positive outcomes from H 2 therapy by inhalation or drinking include liver, nasopharyngeal and colorectal cancer [27][28][29] as well as head and neck cancer (see Table 1).
Interesting immune-modulating effects of H 2 administration have been observed in cancer patients.A high proportion of immune cells of the type CD8+ T cells expressing the programmed cell death protein-1 (PD-1) is often seen in cancer patients and can be associated with poor cancer prognosis.PD-1 is an immune checkpoint receptor that guards against autoimmunity and is often involved in immunotherapy treatment (PD-1 inhibitors) as it makes the immune system "oversee" cancer cells.The administration of H 2 has been shown to reduce the proportion of PD-1+ CD8+ T cells in the blood of cancer patients.This has been observed in both late-stage colorectal carcinoma and lung cancer patients in separate clinical trials and H 2 has shown to improve cancer prognosis in both patient groups [26,29].Furthermore, H 2 can improve the efficacy of nivolumab treatment in cancer patients with high levels of PD-1+ CD8+ T cells, which previously had a poor response to nivolumab [26].A significant decrease in the proportion of PD-1+ CD8+ T cells after H 2 inhalation treatment was also seen in a case study of a gallbladder carcinoma patient [22].The loss of immunological activity in the CD8+ T cells may be due to mitochondrial dysfunction, in which H 2 is found to be an important player.The colorectal carcinoma patients treated with a combined therapy of H 2 and nivolumab showed a significantly longer overall survival than the patients who were treated with nivolumab alone [30].Another study found that patients with non-small cell lung cancer that inhaled H 2 :O 2 (2:1) without other treatments resulted in reestablishment of normal levels of six cell subsets involved in our immune system, including cytotoxic T cells, T helper cells and natural killer cells [31].From these appealing results, the use of H 2 could be interesting in combination with immunotherapy for modulating immune reactions towards cancer cells.
In addition to the direct effects of H 2 therapy in controlling tumor progression, several cell and animal studies [32][33][34][35] have shown that H 2 can be effective in alleviating sideeffects without reducing anti-tumor activity of standard cancer treatment.The array of side-effects may be experienced as devastating for the quality of life of cancer patients and survivors alike.Therefore, clinical studies have investigated the effect of H 2 on chemo-and radiotherapy induced injuries.A study with 134 colorectal cancer patients found that hydrogen-rich water can alleviate mFOLFOX6 chemotherapy-induced liver injuries [36].Non-small cell lung cancer patients undergoing chemotherapy, targeted therapy or immunotherapy treatment experienced a decrease in adverse events after H 2 administration, and for some it even disappeared [25].
Radiotherapy is an important treatment for several cancer diseases.Side-effects of radiotherapy are often associated with increased generation of ROS that can potentially be reduced with H 2 treatment.One study tested whether six weeks of drinking H 2rich water improved the quality of life for 49 patients with liver tumors who received radiotherapy.It was shown that H 2 -rich water consumption reduced the biological response to oxidative stress induced by radiation without comprising the anti-tumor effects of radiotherapy.Quality of life-scores were significantly improved for patients receiving H 2 -therapy in combination with radiotherapy compared to patients receiving placebo water [27].An observational study found that H 2 inhalation treatment can significantly alleviate radiotherapy-induced bone marrow damage, such as the reducing effects of white blood cells and platelets, without compromising the anti-tumor effects [37].Adverse reactions like difficulty of swallowing, brain injury and hearing loss are often experienced after radiotherapy for nasopharyngeal cancer.Three nasopharyngeal cancer patients were reported to have moderate-extremely severe hearing loss and needed hearing aids after radiotherapy.The patients received H 2 administered by inhalation for four hours every day (2:1 = H 2 :O 2 ).After 1-2 months, the patient's binaural hearing had improved considerably, and one of the patients no longer needed a hearing aid [28].These findings are promising, however, more research is needed to reach definitive conclusions.

Respiratory Diseases
When listing the leading causes of death worldwide, respiratory conditions come third [14], and they are still in need of new pharmaceutical solutions.A gas mixture of helium and oxygen has been used for decades to treat obstructive pulmonary disease for its lower density, higher viscosity and reduced airway resistance compared to the conventional nitrogen-oxygen mixtures [38].Using oxygen-hydrogen mixtures can have similar effects, in addition to the therapeutic antioxidant effects of H 2 .
diotherapy for nasopharyngeal cancer.Three nasopharyngeal cancer patients were re ported to have moderate-extremely severe hearing loss and needed hearing aids after ra diotherapy.The patients received H2 administered by inhalation for four hours every day (2:1 = H2:O2).After 1-2 months, the patient's binaural hearing had improved considerably and one of the patients no longer needed a hearing aid [28].These findings are promising however, more research is needed to reach definitive conclusions.In 2020, a study that examined the acute effects of inhalation of H 2 -containing gas found that inflammatory status in asthma and COPD patients was attenuated by the treatment.The trial included 20 asthma and COPD patients that inhaled a 2.4% H 2 -containing steam mixed gas for a single inhalation period of 45 min.This treatment significantly decreased inflammation markers like monocyte chemotactic protein 1, IL-4 and IL-6 levels in both COPD and asthma patients [39].In 2021, a larger clinical trial was conducted with 108 patients with acute exacerbation of COPD receiving either H 2 :O 2 or O 2 therapy.They found superior improvement in symptoms in the H 2 :O 2 group with significant results in certain test scores [40], showing that COPD is an interesting indication for H 2 therapy.The authors have not been successful in identifying more extensive studies of hydrogen therapy for asthma, despite the positive findings.Another clinical study from 2018 evaluated the efficacy and safety of breathing H 2 in acute severe tracheal stenosis patients.Thirty-five patients were administered H 2 :O 2 gas mixture (2:1) for 15 min and 120 min (6 L/min) in two consecutive breathing steps.All the measured endpoint parameters except vital signs were improved after inhaling H 2 , including inspiratory effort as assessed by diaphragm electromyography (EMGdi), transdiaphragmatic pressure (Pdi), Borg score, and impulse oscillometry (IOS) [41].These clinical studies show that H 2 inhalation is a promising treatment option for respiratory diseases, which is a suitable administration method both for targeting the diseased tissue and for convenience as breathing devices are often used by respiratory disease patients.Additionally, the inhalation of H 2 has also been tested in COVID-19 infected patients for treating respiratory symptoms, with very promising results (see Section 6.5).

Central Nervous System Diseases
Conditions affecting the central nervous system (CNS) represent one of the major public health challenges today.Dementia and other diseases causing cognitive decline are correlated with an aging population which is an increasing problem and new medications to treat these diseases are badly needed.Molecular hydrogen is the smallest molecule, and its very small size and nonpolar nature makes it highly diffusible.It can even pass through the blood-brain barrier which is a major obstacle for medical treatment of the brain [8].
A few clinical studies have investigated the effects of hydrogen therapy on acute damage to the brain caused by cardiac occurrences, all of them showing benefits of H 2 administration.In one study, thirty-seven patients with poor-grade subarachnoid hemorrhage were given a H 2 -rich solution by infusion for 14 days together with intracisternal magnesium sulfate infusion, or only intracisternal magnesium sulfate infusion.Incidences of cerebral vasospasm and delayed cerebral ischemia were significantly lower in the treatment groups and H 2 -therapy had additional effects by decreasing serum malondialdehyde, a marker of oxidative stress, reduced biomarkers for neuronal damage and physical therapy improvement by the Barthel Index [42].In another study, 25 patients with cerebral infarction were given H 2 inhalation (3% H 2 gas) treatment for one hour twice a day.H 2 inhalation, compared to control, gave significant effects on lower relative MRI signal, indicating less severe infarction site, neurological improvement seen by NIHSS scores and improved Barthel Index scores [43].Improved MRI indices for brainstem infarction patients were also observed after intravenous administration of H 2 -rich saline, in combination with edaravone.The results were better for the combined treatment than for edaravone treatment alone [44].For giving direct access to the brain via the blood-brain barrier, H 2 -rich saline is more often administered intravenously in clinical trials involving brain disorders as compared to drinking and inhalation that is more widely used for other conditions.This can also be due to convenience because critically ill patients are hospitalized and are typically prepared for IV administration of various drugs.However, other administration routes might also be useful for treating critically ill patients.A large-scale trial with 73 patients at 15 hospitals in Japan studied the effect of H 2 inhalation on neurological outcomes for patients with brain ischemia during post-cardiac arrest.The patients were randomly assigned to receive O 2 with or without 2% H 2 for 18 h.An increase was seen in the primary neurological outcome, however, this was not statistically significant.The reported statistical significance in the secondary outcomes, on the other hand, including an increase in 90-day survival, suggests that H 2 inhalation may have therapeutic benefits without neurological deficits [45].
Clinical trials with hydrogen therapy have also been conducted with patients with cognitive disorders including Alzheimer's and Parkinson's disease, as reduced neurodegeneration have been seen from several animal studies with H 2 [46][47][48].In one human study, 73 patients with mild cognitive impairment drank 300 mL of H 2 -enriched water or placebo water every day for a year and their Alzheimer's Disease Assessment Scalecognitive subscale (ADAS-cog) scores were then determined.The study found that carriers of the apolipoprotein E4 (APOE4) genotype significantly improved their total ADAS-cog scores after drinking H 2 water when compared to the control group.This indicates that genetic variations can affect how subjects respond to hydrogen treatment.However, there was no significant difference between the H 2 and the control groups in ADAS-cog scores after 1 year [49].Another study involving eight patients diagnosed with Alzheimer's disease suggests that 3% H 2 inhalation can relieve symptoms and has disease-modifying effects.Improvements in ADAS-cog scores and integrity of neurons, seen by diffusion tensor imaging (DTI), were significant during the 6-month follow-up but non-significant after one year, compared to untreated patients [50].
A trial tested H 2 inhalation in 20 Parkinson's disease patients.The inhalation of either low dose H 2 of ~1.3% in air or placebo for 10 min twice a day for 4 weeks did not result in any beneficial effects.However, another interesting finding of increased 8-hydroxy-2deoxyguanosine (8-OHdG, an indicator of cellular oxidative stress) and other reported stress responses, suggest that beneficial effects of hydrogen therapy are partly or largely mediated by hormetic mechanisms [51].A larger-scale similar study following 178 patients over 72 weeks revealed no disease improvement by oral H 2 consumption (1 L/day) in patients with Parkinson's disease [52].Even though beneficial effects of hydrogen therapy were not found in this large-scale study of Parkinson's disease, combinatory treatment might show more benefits.A pilot study including 18 patients with Parkinson's disease found significant improvements in the total Unified Parkinson's Disease Rating Scale (UPDRS) scores compared with the baseline after combined treatment of daily drinking H 2 -saturated water and photobiomodulation (PBM) over a two-week period.The report attributes the positive effects to "PBM targeting the brainstem may facilitate neuronal activity, and the concomitant H 2 may clear additional ROS produced by PBM" [53].A randomized, double-blind pilot study performed in 2013 including 18 Parkinson's disease patients treated with levodopa showed that drinking 1 L of H 2 water every day for 48 weeks significantly improved the UPDRS scores [54].The improvements can, however, be attributed to the alleviation of side-effects of the other drug taken in combination, as observed for cancer treatments.

Infections
A flourishment of H 2 therapy occurred as the pandemic of the Coronavirus disease 2019 (COVID-19) spread globally.H 2 inhalation has been used to treat respiratory-related COVID-19 symptoms in several clinical studies, primarily for the same reasons as its use in other respiratory conditions, namely a reduction in gas density and airflow resistance.However, the studies do not rule out possible beneficial antioxidant effects of H 2 .
A pioneering clinical study conducted with 100 patients at seven different Chinese hospitals found exclusively positive results of continuous inhalation of H 2 :O 2 2:1 gas mixture (3 L/min) in all the end-point parameters, compared to standard oxygen treatment, as shown in Figure 4.The primary endpoint was disease severity, and the secondary endpoints were dyspnea, cough, chest distress, chest pain and oxygen saturation, and all were significantly improved after just two days of treatment compared to control [55].These excellent outcomes resulted in the inclusion of this specific treatment (H 2 :O 2 inhalation 2:1) as a recommendation by the China National Health Commission in the "Chinese Clinical Guidance for COVID-19 Pneumonia Diagnosis and Treatment (7th edition)" [56], next to standard oxygen therapy.This practice does not seem to have attracted attention globally in the combat against COVID-19, and additional clinical trials have been performed (see Table 1).A study published in 2022 investigated the use of H 2 therapy in rehabilitation of 50 acute post-COVID-19 patients.They found significant improvements in 6-min walking test distance, forced vital capacity and expiratory volume when administered H 2 by inhalation.The protocol described at-home inhalation of 100% H 2 at low flow rate (250 mL/min) for 60 min twice a day for two weeks [57].
A retrospective study, also published in 2022, of medical records of twelve COVID-19 patients that were subject to H 2 :O 2 therapy by inhalation found suppression of inflammatory responses, compared to standard treatment.For the H 2 group, the results showed a significant decrease in neutrophil percentage and the concentration of C-reactive protein [58].In summary, H 2 inhalation can be beneficial for COVID-19 patients for at least three different reasons; (i) a reduction of ROS levels, (ii) reduced airflow resistance for eased respiratory conditions and (iii) a reduction in inflammatory responses.All the positive results of both improved physical and respiratory function in COVID-19 patients have demonstrated the clinical usability of this novel therapeutic strategy.Hydrogen therapy in the treatment of other infections has shown health-promoti effects in various animal models including sepsis and periodontitis, a serious gum inf tion, by reducing inflammation and sepsis-induced injuries [59][60][61].No in-human stud have investigated the effect of H2 on sepsis, but one clinical study found elevated oxidati stress levels in patients with chronic hepatitis B. Sixty patients were included, some we administered 1.2-1.8L/day of H2-rich water, and some were given routine treatment fo consecutive weeks.A significant reduction in ROS was seen after H2 therapy, compar to the control group.Improved liver function and hepatitis B DNA levels were comparab after both H2 and control treatment, and the report suggests that longer-term stud might be needed to confirm the physiological effects of H2 on the reduction of oxidati stress [62].To summarize, the treatment of infection-related symptoms using H2 h shown positive outcomes, but there has been no documentation that the underlying inf Hydrogen therapy in the treatment of other infections has shown health-promoting effects in various animal models including sepsis and periodontitis, a serious gum infection, by reducing inflammation and sepsis-induced injuries [59][60][61].No in-human studies have investigated the effect of H 2 on sepsis, but one clinical study found elevated oxidative stress levels in patients with chronic hepatitis B. Sixty patients were included, some were administered 1.2-1.8L/day of H 2 -rich water, and some were given routine treatment for 6 consecutive weeks.A significant reduction in ROS was seen after H 2 therapy, compared to the control group.Improved liver function and hepatitis B DNA levels were comparable after both H 2 and control treatment, and the report suggests that longer-term studies might be needed to confirm the physiological effects of H 2 on the reduction of oxidative stress [62].To summarize, the treatment of infection-related symptoms using H 2 has shown positive outcomes, but there has been no documentation that the underlying infection itself is affected by H 2 therapy.

Lifestyle-Related Conditions and Exercise
Hydrogen therapy has also been used in the treatment of lifestyle-related and metabolic conditions, which is an increasing problem with an increasingly heavier population.Many studies also point to the beneficial effects of H 2 therapy in relation to exercise and sportsrelated injuries, including administration forms like H 2 inhalation, drinking of H 2 -rich water and topical H 2 application.A pilot study found that oral intake of H 2 -rich water prevented an elevation of blood lactate and decreased muscle fatigue during heavy exercise in ten male elite soccer players and suggests that it can be a suitable means of hydration for athletes [63].Another study involving eight male cyclists found similar results, oral intake of H 2 -saturated water improved performance in intermittent cycling exercises when the duration was longer than 30 min (anaerobic) [64].A clinical trial involving ten men and ten women found an increase in peak running velocity up to 4.2% in a running time-to-exhaustion test after seven days of inhaling 4% H 2 for 20 min each day [65].One study showed that drinking H 2 -rich water before and after strenuous exercise reduced the exercise-induced increase in ROS levels in eight male volunteers.This might help prevent accumulative muscular fatigue even though exercise performances were not significantly different over the two 3-day consecutive exercise tests, compared to placebo water [66].A larger-scale study investigated the effects of drinking H 2 -enriched water right before cycle ergometer exercise sessions in healthy non-trained (n = 99) and trained (n = 60) participants.They found that H 2 -water, compared to placebo water, enhanced endurance and relieved psychometric fatigue as measured by maximal oxygen consumption and Borg's scale and visual analogue scales, respectively [67].Additionally, healing of acute sports-related soft tissue injuries can be positively impacted by both oral and topical H 2 treatment for 2 weeks, next to standard-care [68].Flipping the coin from high-level exercise, H 2 therapy has also been studied for managing lifestyle-related disease states.Typically drinking 1-2 L of H 2 -rich water daily is shown to reduce ROS levels and contribute to body fat reduction in overweight people [69], as well as managing lipid and glucose metabolism in patients with diabetes type 2 [70] and metabolic syndrome [71][72][73][74].

Other Diseases and Conditions
Outside the major disease areas discussed herein, several clinical studies investigating H 2 on other indications related to high oxidative stress levels have been reported.Rheumatoid arthritis is an autoimmune disease characterized by chronic inflammation and the destruction of bone and cartilage.Elevated hydroxyl radical levels are thought to be involved in pathogenesis, for which H 2 therapy can have a selective scavenging effect.Both drinking and injection of H 2 -enriched water and saline have been tested for the treatment of rheumatoid arthritis in separate clinical trials.In one study, twenty patients drank 530 mL of H 2 -rich water/day for 4 weeks in two separate periods.In another study, 24 patients were randomly assigned either H 2 -saline or placebo-saline which was administered intravenously by drop infusion of 500 mL daily for 5 days.The results of both studies showed that H 2 significantly reduced biomarkers for oxidative stress (for instance urinary 8-hydroxydeoxyguanine) and disease activity score in 28 joints (DAS28), by Creactive protein levels.This was accompanied by significant improvements in rheumatoid arthritis symptoms [75,76].Chronic graft-versus-host-disease can also be considered an autoimmune disease.A longer-term study over 12 months found therapeutic effects of daily drinking H 2 -rich water at a dose of 12 mL/kg.Out of the 24 patients enrolled, 18 had an objective response as measured in seven domains (skin, mouth, gastrointestinal, liver, eyes, lungs and joints and fascia) using the National Institutes of Health (NIH) Consensus form for measuring therapy response for chronic graft-versus-host-disease. The survival time and the survival rate at 4 years was significantly prolonged in the response group, compared to the nonresponse group.There is currently no standard therapy for chronic graft-versus-host-disease patients refractory or dependent to corticosteroid treatment, and therefore H 2 can serve as a good option [77].
Another worldwide health issue, without any approved medications, is non-alcoholic fatty liver disease (NAFLD), causing hepatic dysfunction.Metabolic impairment plays a major role in NAFLD pathogenesis, so pharmaceuticals that advance lipid and glucose metabolism are of interest for treating this disease.Hydrogen possibly has these properties, as seen from the results of the clinical trials with diabetes and metabolic syndrome patients.NAFLD can also be associated with inflammation, excess oxidative stress and aberrant cellular signaling.Two separate clinical trials studied the effect of oral intake of H 2 -rich water at 1 L/day.One of them included 12 subjects and found that after one month, the H 2 therapy significantly reduced liver fat accumulation, as compared to placebo, with no significant differences in body weight and composition [78].The second study involved 30 subjects and lasted for 2 months and found non-significant decreases in levels of NAFLD disease markers, as well as weight and body mass index.Interestingly, nonsignificant increase tendencies of oxidative stress markers (8-hydroxy-2 -deoxyguanosine and malondialdehyde) were also found, which was explained by the hormetic effects of H 2 occurring prior to the significant clinical improvements over the longer-term [79].Another study tested 13-week inhalation of H 2 :O 2 (2:1) gas mixture at rate of 3 L/min for 1 h/day in 43 NAFLD patients with moderate-severe cases.Improvements in serum lipid and liver enzymes and significantly improved liver fat content was found.Additional studies in mice models revealed that this effect of H 2 was possibly caused by promoting hepatic autophagy [80].Some specific conditions have been studied for the effect of H 2 -enrichment in other administration forms such as solutions for dialysis and via the eyes.One study demonstrated significant effects of using H 2 solutions during cataract surgery (phacoemulsification) to restore vision.The procedure employs ultrasound which produces free radicals that can potentially be neutralized with H 2 -enriched solutions.Thirty-two patients with cataracts in both eyes were treated with the conventional method on one eye and using H 2 -enriched solution on the other eye.Reduction rates of endothelial cell density, the primary endpoint, were significantly smaller in the H 2 group at all the measured time points [81].
Hemodialysis patients experiencing chronic inflammation have poor prognoses and therapeutic approaches are limited.Therefore, the anti-inflammatory properties of H 2 can be utilized in the dialysis solution for these patients.Two studies performed by the same research group, published in 2010 and 2018, investigated the effects of H 2 -infused dialysate, with H 2 from water electrolysis, in the treatment of hemodialysis patients.In the 2010 study, 21 patients were switched to H 2 -enriched dialysis solution for 6 months, resulting in significant decreases in systolic blood pressure compared to before the test period.A significant decrease in plasma inflammation markers were also identified [82].The 2018 study was longer-term and had a larger test group of 161 and 148 patients administered H 2 -enriched and conventional dialysis solutions, respectively, over a 3.3-year observation period.Reductions in post-dialysis hypertension were found and multivariate analysis revealed H 2 -enriched dialysis as an independent significant factor for the primary endpoint, which was a composite of all-cause mortality and development of non-lethal cardio-cerebrovascular events [83].In both studies, H 2 caused minimal changes in dialysis parameters but the positive results including improved blood pressure control and ameliorated inflammatory reactions could improve the prognosis of chronic hemodialysis patients.
Others have studied H 2 therapy for the treatment of various diseases involving inflammation and pain.Four patients with acute erythematous skin diseases with fever and/or pain experienced significant improvement in symptoms, that did not recur, after infusion with H 2 -enriched fluid (500 mL/day for 3 days) [84].Twenty-eight patients with interstitial cystitis/painful bladder syndrome were given H 2 -rich water or placebo water for 8 weeks.Even though H 2 -rich water was extremely effective in improving the bladder pain score in 11% of the patients, the improvements were not significantly different from that of placebo [85].H 2 -enriched water has also been tested on patients with mitochondrial and inflammatory myopathies.Observations included improved mitochondrial dysfunction in mitochondrial myopathy patients and inflammatory processes in patients with polymyositis/dermatomyositis.Significant improvements in certain disease markers were observed, even though the improvement in clinical symptoms was not significant.Two different therapy schemes were followed, where 14 patients drank 1 L/day for 12 weeks and 22 patients drank 0.5 L/day for 8 weeks.Less prominent effects were seen for the lower amount scheme, illustrating a dose-dependent response of H 2 therapy [86].

Summary and Discussion
Qualitative and quantitative analysis of the current data related to human use of H 2 therapy has been conducted herein, encompassing over 64 clinical studies and 81 registered clinical trials.All the studies have unequivocally confirmed the safety and legitimacy of H 2 in human consumption in all the administration methods tested, and therefore set the foundation for clinical trials for a variety of indications.The high numbers of ongoing clinical trials registered over the recent years underscores the global effort for rapid introduction of H 2 therapy in various disease areas spanning chronic to acute stages.Phase 2 and 3 clinical trials are ongoing, yet the area is still in its infancy.
The exploration of H 2 consumption's efficacy has encompassed several interesting indications spanning the major disease areas.Noteworthy positive outcomes have emerged among patients suffering from myocardial infarction, an array of cancers (lungs, liver, colorectal and gallbladder), asthma, COPD, brain ischemia, COVID-19, rheumatoid arthritis, non-alcoholic fatty liver disease and in hemodialysis patients.Also, lifestyle-related indications have shown promising results, including type 2 diabetes, obesity and exercise performance.Given the molecular effects of mitigating the most damaging reactive oxygen species, H 2 should be most effective in treating diseases correlated with high levels of oxidative stress.Moreover, most disease states involve dysregulation of ROS and therefore it can be a diverse medication.Promisingly, several clinical studies have identified the reduction of oxidative stress biomarkers, confirming the mechanism of action.Despite these robust indicators, the road to clinical translation might prove more intricate compared to conventional new drugs that are developed by pharmaceutical companies and tailored to specific indications.
Respiratory health emerges as a promising arena for hydrogen therapy, particularly in the context of respiratory diseases and COVID-19 infections.This is evidenced by the integration of H 2 inhalation in the China National Health Commission's recommendation for COVID-19 treatment.While the reduction in airway resistance due to the low density of H 2 is a possible factor, its antioxidant potential can have additional benefits.
Cancer remains an interesting field for H 2 therapy, offering not only direct effects on cancer progression but also demonstrating relief from side-effects of standard treatments.Furthermore, synergistic effects from combining H 2 with conventional treatments, such as chemotherapy and radiotherapy, hold promise for optimizing therapeutic outcomes and enhancing overall quality of life.The observed immuno-modulating effects of H 2 have the potential to complement immunotherapy, which further augments its therapeutic potential.Hydrogen therapy has been used to complement conventional therapies in clinical trials including cardiac arrest, brainstem infarction and Parkinson's disease patients, in addition to cancer.Due to low side-effects and potential beneficial effects, the health risks can be greater for not using H 2 in combination with standard treatments across a broad spectrum of conditions from cancer to sports-related injuries.
Ideally, H 2 therapy would be beneficial for diseases that currently have no treatment options.Cognitive disorders, which are increasing concurrently with the elderly population, exemplify such unmet needs.While small-scale human trials offered hope for therapeutic effects of H 2 in Parkinson's and Alzheimer's disease, large-scale studies have found no significant positive results.Use in other diseases without standard medical regimes, however, such as non-alcoholic fatty liver disease or hemodialysis, shows promise.In summation, the landscape of H 2 therapy's clinical utility is multifaceted and promises both challenges and opportunities, yet its integration with conventional treatments warrants further exploration.
Undeniably, the administration method plays a major role in disease treatment.Oral consumption of H 2 -rich water, suitable for most diseases, and inhalation of H 2 gas mixtures, used prominently for respiratory symptoms, are emerging as the predominant methods.Low-dose (4% or below) H 2 inhalation is often used to mitigate the explosion risk.Injection is most relevant for hospitalized patients and the use of H 2 -enriched dialysis solutions and water baths show potential in suitable disease contexts.Another factor in the therapeutic equation is the dose of H 2 administered.Inhalation enables sustained, safe high-dose administration, while drinking may necessitate consumption of either large amounts of saturated water or high concentration of supersaturated water in order to obtain an optimal H 2 dose.
Innovations within micro-and nanomaterial drugs for in vivo H 2 generation are promising for advancements of the field [87,88].These new drugs can allow for feasible oral administration of a tablet that will dissolve in the gastrointestinal (GI) system and, in reaction with the water available, produce H 2 that can freely diffuse throughout the body.Some candidate materials include magnesium (Mg), magnesium hydride (MgH 2 ) and silicon (Si) that can react with water to produce H 2 in the following reactions: Reaction (1) follows the same principle as the Mg effervescent tablets for H 2 -rich water generation.The theoretical capacity (at SATP: 1 bar and 25 • C) is 1.02 L/g and animal studies have shown that Mg-containing materials can produce H 2 for treatment of osteoarthritis and gastric cancer [89][90][91].MgH 2 can produce more H 2 , up to 1.89 L/g, and oral MgH 2 administration to mice has been shown to alter gene expression to enhance fatty acid metabolism [92].The use of MgH 2 in vase water is also shown to be applicable in postharvest flower preservation by extending the vase life of cut roses [93].Si has a theoretical H 2 generation capacity of 1.77 L/g (reaction 3) and Si nanoparticles have been shown as an effective candidate for H 2 generation in simulated GI environments [94].Animal studies have confirmed positive effects of orally administered H 2 -generating Si in several disease models including pneumonitis, ischemia-reperfusion injury and Parkinson's disease [95][96][97].Other materials have also been studied, including H 2 -producing coral calcium hydride that has been shown to reverse brain damage and alleviate oxidative stress and neuroinflammation induced by methamphetamine exposure in mice [98].The realization of these types of technologies could enable feasible at-home administration of high doses of H 2 .For instance, from a 65 mg Si tablet, more than 100 mL of H 2 can be generated inside the body, which is equivalent to drinking more than 5 L of H 2 -saturated water.
Hydrogen precursors, for instance Mg or Si particles, can be regarded as hydrogen prodrugs and will, as new chemical entities in drug products, require preclinical and clinical documentation including safety of the materials and the by-products.The in vivo kinetics of H 2 generation after ingestion will also have to be documented and potentially optimized.Nevertheless, hydrogen therapy holds the potential to emerge as the next breakthrough in the clinical application of nanomedicine.Despite this promise, clinical trials utilizing this cutting-edge technology are yet to be conducted.
The realization of H 2 therapy's full potential faces nuanced challenges.Unlike conventional drugs, the medical use of H 2 cannot directly be protected by intellectual property rights.This might dampen pharmaceutical industry support for clinical trials.However, patenting technology for H 2 production devices, such as inhalation devices, and new delivery systems is possible.Therefore, the development of these technologies might be necessary for realizing the clinical potential of H 2 therapy.

A
quantitative overview of the number of clinical trials and scientific publications about H2 therapy in humans are shown as a function of the publication year, and the first publication date for the clinical trials, in Figure 1.The number of clinical trials has increased from one registered in 2011, five in 2015, with a boost to ten in 2016 and 12 in 2019, to 6-10 each year in the period 2020-2023.The number of scientific publications does not follow the same trend and has been more stable over the period 2012-2022 (data only until August 2023), with an average of approximately five publications per year, and a maximum of 11 publications in 2019.The increased publication activity during 2019-2020 can possibly be explained by the high number of clinical trials initiated in 2016 and the sudden interest in hydrogen therapy in COVID-19 studies.Therefore, the high number of clinical trials registered from 2019 to 2023 might result in a subsequent upswing in the number of scientific publications in the near future.

Figure 1 .
Figure 1.The number of clinical trials and scientific publications about hydrogen therapy in humans, sorted after publication year from 2010 to 2023 (per August 2023).

Figure 1 .
Figure 1.The number of clinical trials and scientific publications about hydrogen therapy in humans, sorted after publication year from 2010 to 2023 (per August 2023).

Figure 2 .
Figure 2. Registered clinical trials (t) and scientific publications (p) about hydrogen therapy, sorted by disease indication and administration method.

Figure 2 .
Figure 2. Registered clinical trials (t) and scientific publications (p) about hydrogen therapy, sorted by disease indication and administration method.

Figure 3 .Figure 3 .
Figure 3. Schematic overview of the effects of hydrogen therapy in cancer treatment.The top images show MR imaging of brain metastasis (arrow) before (left) and after (right) H2 treatment, adapted from [24]; OncoTargets and Therapy 2019:12 11145-11151-originally published by, adapted and used with permission from Dove Medical Press Ltd., Macclesfield, UK.The middle figures show alterations in cytotoxic and helper T cell levels in cancer patients after H2 treatment, adapted from [31].The parallel red lines show the normal range of cell levels and the black short lines show the measured average values at each time point.The bottom left figure shows number of months (mon with progression-free survival (PFS) of lung cancer patients receiving no treatment (control), H Figure 3. Schematic overview of the effects of hydrogen therapy in cancer treatment.The top images show MR imaging of brain metastasis (arrow) before (left) and after (right) H 2 treatment, adapted from [24]; OncoTargets and Therapy 2019:12 11145-11151-originally published by, adapted and used with permission from Dove Medical Press Ltd., Macclesfield, UK.The middle figures show alterations in cytotoxic and helper T cell levels in cancer patients after H 2 treatment, adapted from [31].The parallel red lines show the normal range of cell levels and the black short lines show the measured average values at each time point.The bottom left figure shows number of months (mon) with progression-free survival (PFS) of lung cancer patients receiving no treatment (control), H 2 therapy (H 2 only), or combined treatments with H 2 and immunotherapy (immuno), targeted therapy (target) or chemotherapy (chemo).The black short lines show the average PFS values for each treatment.Adapted from [25].The bottom right figure shows the quality of life (QOL) scores of patients treated with radiotherapy for liver tumors, with or without H 2 water, adapted from [27].A higher score reflects more symptoms and lower QOL.* p < 0.05, ** p < 0.01, *** p < 0.001, for all figures.

Figure 4 .
Figure 4. Trial overview and results from a successful COVID-19 study, showing all end-point parameters were positive after H 2 -O 2 inhalation compared to control.* p < 0.05, ** p < 0.01, EOT = endof-treatment.Reproduced with permission from [55], Journal of Thoracic Disease; published by AME Publishing Company, Hong Kong, 2020.

Table 1 .
Clinical trials studying the safety and therapeutic effects of H 2 administration registered at clinicaltrials.gov,outlining disease studied, administration method, trial status, the date the trial was first submitted and location.NA = not applicable.