Comment on Chu et al. Acetaminophen’s Role in Autism and ADHD: A Mitochondrial Perspective. Int. J. Mol. Sci. 2025, 26, 8585

The United States Food and Drug Administration (FDA) has now linked acetaminophen use in pregnancy to risk of offspring autism and has recommended leucovorin as a possible therapeutic for ASD [1]. This is a controversial move given recent observational evidence suggesting an inconsistent effect of acetaminophen use during pregnancy and risk of offspring neurodevelopmental disability, including autism spectrum disorder (ASD) [2,3].

Chu et al. [2] have recently published a review suggesting a theoretical framework of maternal acetaminophen use-induced mitochondrial damage in utero. In the review, the authors described the results of a series of observational studies investigating in utero exposure to acetaminophen and risk of offspring neurodevelopmental disorders. These studies have often been plagued with methodological concerns about self-reported use, imprecise dosage quantification, and the overuse of questionnaire-based methodologies, yielding inconsistent results and, therefore, no widespread change to the medical guidelines for the use of acetaminophen. Murine studies revealed that embryonic exposure to acetaminophen increased anxiety-like vocalizing behavior and reduced ambulation, while post-natal exposure reduced maze completion in adults, delayed habituation, and reduced pain thresholds. Moreover, while the authors do elaborate on the role of oxidative stress and mitochondrial abnormality in ADHD and ASD, the evidence that acetaminophen specifically induces these abnormalities or any core phenotypic expression of ASD/ADHD is rather underwhelming, especially given that the majority of ingested acetaminophen is metabolized and harmlessly excreted.

We have two main critiques of the authors’ review: Firstly, the review acknowledges that acetaminophen first entered the market in 1956, more than a decade after infantile ASD was first described. This temporal fact considerably weakens the argument of causation. Indeed, acetaminophen may be a surrogate indicator to the real cause of ASD, as is the main point of our commentary. Secondly, many of the described neurobehavioral and biochemical toxicological findings, while evident, were non-descript or occurred from drug administration above the recommended human dose equivalent and did not constitute part of the core diagnostic criterion of common neurodevelopmental pathology.

Given the policy recommendations that have been announced by the FDA, our organization thought it would be important, albeit belated, to convey our ongoing research about a potential significant environmental cause of ASD and how this factor may compel maternal acetaminophen use during pregnancy. The purpose of our commentary is three-fold.

• Introduce our proposed environmental trigger for neurodevelopmental pathology using data from our prior epidemiological investigations;
• Explain the mechanism characterizing the potential confounding role of our variable of interest;
• And, unlike Chu et al. [2], highlight how our variable displays temporal precedence with the first description of infantile ASD in 1943.

1. What Is the Environmental Trigger of Neurodevelopmental Pathology?

IHER has recently published a commentary with original data analysis [4], supporting our contention that ASD is a disorder of opioid dysregulation that is specifically induced from environmental emissions of nitrous oxide (N₂O). N₂O is well-known as a potent greenhouse gas, with a warming potential 300 times greater than that of carbon dioxide. The compound is widely used in medicine and dentistry as an analgesic, as it is thought to carry little to no harm in the acute medical setting. The compound is not metabolized by the human body, which is why medical scavenging units are critical during administration of the gas. Elevations in striatal dopamine release characterize the acute exposure effects of N₂O, supporting its clinical use as a therapeutic for patients experiencing treatment-resistant depression and validating its public perception as a “laughing gas”.

However, N₂O also exists in the environment as an air pollutant, derived predominately from synthetic nitrogen application in agricultural soil management. Exposure to this pollutant likely confounds the role of pesticides, and particularly glyphosate, in ASD risk. Our prior research shows a significantly positive, county-level association between glyphosate and farm use of synthetic nitrogen fertilizers [5]. Glyphosate impairs nitrogen fixation in soil through chelation of critical minerals, like manganese, necessitating the use of synthetic nitrogen in agriculture.

Unlike glyphosate and other pesticides, however, trace levels of exposure to N₂O induce cognitive and attention deficits in healthy human subjects [6] and alter key striatal dopaminergic neurotransmission in a preclinical model of chronic, intermittent exposure [7] through putative release of dynorphin opioid peptides and activation of the kappa opioid receptor (KOR) [8]. These clinical and neurobiological effects are consistent with an ADHD phenotype and explain the therapeutic role of psychostimulants in restoring dopaminergic signaling within the limbic system. Consistently, our prior investigations link farm use of synthetic nitrogen, as the greatest source of environmental N₂O pollution, to increased healthcare utilization for ADHD diagnosis (representing a severely impaired phenotype) in a U.S.-based cohort [9].

Conversely, farm use of synthetic nitrogen reduces hospitalization risk for autistic disorder diagnosis in the same healthcare seeking U.S. population [4]. These two-way (state and year) fixed effects regression analyses accounted for healthcare utilization for autistic disorder in HCUP states over nearly a decade, during the aftermath of improved diagnostic recognition (DSM-IV in 1994) [10] and expanded data collection efforts and awareness with the establishment of the CDCs Autism and Developmental Disabilities Monitoring (ADDM) network in 2000 [11]. These population-based data suggest, but do not prove, that, unlike ADHD, gestational exposure to environmental sources of N₂O rewires ASD brain during critical periods of brain development, revealing an opioid dependence. Genetic studies show a comprehensive autistic syndrome in µ-opioid receptor null mice [12]. We, therefore, suspect that a heightened KOR system, an antagonistic opioid receptor subtype implicated in negative stress adaptation, socio-communicative deficits (a key diagnostic feature in ASD), drug-seeking and a strong negative regulator of brain catecholaminergic activity, then manifests in ASD brain to adapt to a lifetime exposure of chronic N₂O emissions [4]. In addition to the manifold biological links, various climate factors have also been implicated in ASD risk, including precipitation, ozone, particulate matter, temperature, etc., which can all be explained by environmental N₂O pollution [4].

2. The Mechanism of Potential Confounding

As it relates to the role of maternal acetaminophen use in pregnancy and risk of offspring ASD, it may be prudent to consider how chronic pain tolerance develops in human subjects exposed to N₂O. The magnitude of analgesic effect from 35% N₂O had decreased by almost half and more than a third for detection and pain thresholds, respectively, after six exposures, compared to the first N₂O exposure [13]. Use of acetaminophen can be part of a multi-modal pain management technique for patients who have developed a tolerance to opioids [14].

Therefore, any implication of maternal acetaminophen use in offspring ASD risk should explicitly consider how chronic environmental N₂O exposures may be driving opioid tolerance/fever/headache among pregnant women (conditions for which acetaminophen use is clinically indicated) and thereby increase use of other pain management therapies, like acetaminophen (Figure 1). Moreover, given the long-standing, cumulative evidence from independent studies about how trace N₂O exposure explains the core ASD symptomatology (Figure 1), we believe that environmental emissions of N₂O are a confounding factor in the relationship between maternal acetaminophen use during pregnancy and risk of offspring neurodevelopment pathology, including ASD. Even more noteworthy, leucovorin, now being suggested as a possible approved therapeutic for ASD, has also been clinically shown to reverse the hematologic depression induced from N₂O, presumably through the putative restoration of folate metabolism, as we have highlighted previously [4].

3. Temporal Precedence

Farm use of synthetic nitrogen fertilizers is widely recognized as the most significant contributor to environmental N₂O emissions since 1940 [36]. The introduction of synthetic nitrogen in agriculture occurred early in the preceding decade [37], while Leo Kanner’s first reported case series of ASD in children was published in 1943 [38]. The chronological order of these events is consistent with our hypothesis that rising N₂O emissions from an increasingly industrialized agricultural system cause neurodevelopmental abnormality (Figure 2), demonstrating temporal precedence. Interestingly, world use of synthetic nitrogen peaked in 1988, as shown in Figure 2 [37]. This time period coincides with the year that HHS Secretary RFK Jr. has cited as when an unknown environmental toxin triggered the worrisome rise in prevalence of neurodevelopmental disorders.

4. Future Directions

To further our prior epidemiological investigations, IHER has begun a program of individual assessment of chronic environmental exposure to N₂O. Preliminary assessments from direct combustion sources indicate a level of exposure that matches or exceeds levels that have been shown to clinically alter human cognitive functioning and neurotransmission in animal models of N₂O neurotoxicity (≥50 ppm). Future comprehensive assessments should be conducted in at-risk populations (i.e., those diagnosed with ADHD) and from a wide variety of environmental sources (including farmland, vehicular pollution, bodies of water, low-level elevation hotspots where environmental N₂O emissions could be expected to pool, and even bacterial denitrification processes in human models of disease pathogenesis [39], etc.) to clearly delineate potential individual exposure to environmental emissions of N₂O.

However, exposure to N₂O is not recommended for pregnant women due to the known risk for reproductive harm. While the American Dental Association (ADA) recommends the use and proper maintenance of scavenging systems and setting exposure limits for occupational exposure, the ADA also specifically advises that pregnant patients should avoid it whenever possible [40]. According to NIOSH, the exposure limit is 25 ppm for exposure to N₂O in the dental settings [41]. Worryingly, IHER’s preliminary emissions testing indicates levels of direct environmental N₂O emissions from combustion sources to be at least more than double this estimate. Given such concern from regulatory and scientific bodies, we concur that exposing pregnant women to N₂O to prospectively observe ASD outcomes or acetaminophen use is an unethical research question to pursue. Care must be taken to ensure the least amount of harm when investigating the link between environmental emissions of N₂O and neurodevelopmental outcomes.

Perhaps most importantly, our novel findings support decades of independent research linking trace N₂O exposure to all key phenotypic manifestations of ASD (Figure 1). We are not aware of any other factor that has been able to explain the core phenotype, as well as notable comorbidities, of an ASD diagnosis, and exhibit temporal precedence with the first description of infantile ASD in 1943. Therefore, we advocate additional study on the magnitude of N₂O emissions, development of individual exposure models, and especially well-designed studies of the effects of environmental N₂O pollution on neurodevelopmental risk. Our research also critically links known agents of global climate warming to direct human health costs and presents a stark, perhaps inconvenient, reality check to the scientific community.

Disclosure

The Institute of Health and Environmental Research (IHER) is a 501c(3) research-based non-profit organization. Both authors are IHER directors and participated in the research and the writing of the commentary. The institute has received no funding for this work and has no conflicts of interest.

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a.

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1.

The IHER is organized exclusively for charitable, religious, educational, and scientific purposes, including, for such purposes, the making of distributions to organizations that qualify as exempt organizations under Section 501(c)3 of the Internal Revenue Code, or the corresponding section of any future federal tax code.

b.

To work towards the betterment of environmental health and well-being by protecting natural ecosystems and fostering sustainable and environmentally sensitive agricultural systems.

c.

To work towards the betterment of public health by prevention and control of disease and illness, thereby preventing the distress due to these now and in the future;

d.

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e.

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(i)

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(ii)

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(iii)

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