Cannabinoids Drugs and Oral Health—From Recreational Side-Effects to Medicinal Purposes: A Systematic Review

Background: marijuana, the common name for cannabis sativa preparations, is one of the most consumed drug all over the world, both at therapeutical and recreational levels. With the legalization of medical uses of cannabis in many countries, and even its recreational use in most of these, the prevalence of marijuana use has markedly risen over the last decade. At the same time, there is also a higher prevalence in the health concerns related to cannabis use and abuse. Thus, it is mandatory for oral healthcare operators to know and deal with the consequences and effects of cannabis use on oral cavity health. This review will briefly summarize the components of cannabis and the endocannabinoid system, as well as the cellular and molecular mechanisms of biological cannabis action in human cells and biologic activities on tissues. We will also look into oropharyngeal tissue expression of cannabinoid receptors, together with a putative association of cannabis to several oral diseases. Therefore, this review will elaborate the basic biology and physiology of cannabinoids in human oral tissues with the aim of providing a better comprehension of the effects of its use and abuse on oral health, in order to include cannabinoid usage into dental patient health records as well as good medicinal practice. Methods: the paper selection was performed by PubMed/Medline and EMBASE electronic databases, and reported according to the PRISMA guidelines. The scientific products were included for qualitative analysis. Results: the paper search screened a total of 276 papers. After the initial screening and the eligibility assessment, a total of 32 articles were considered for the qualitative analysis. Conclusions: today, cannabis consumption has been correlated to a higher risk of gingival and periodontal disease, oral infection and cancer of the oral cavity, while the physico-chemical activity has not been completely clarified. Further investigations are necessary to evaluate a therapeutic efficacy of this class of drugs for the promising treatment of several different diseases of the salivary glands and oral diseases.


Introduction
Cannabis, also known as marijuana, has always been one of the illicit drugs most commonly used at recreational levels worldwide [1]. On the other hand, medical use of this plant dates back more than 2000 years ago and has been described in almost all of the ancient cultures [2]. Recreational and ritual use of cannabis and its derived compounds (called cannabinoids) has an important historical meaning, mostly due to the various psychological and physiological effects on the human body, particularly the intense euphoria experience. At the same time, cannabinoids have always been provided to patients, for pain treatment and management, as well as treatment for other types of diseases. Phytocannabinoids have been proposed as dietary supplements to improve the gastrointestinal tract function [3][4][5][6]. However, acute and long-term cannabinoid intoxication has several adverse effects which span from unconscious health problems such as tachycardia, immune depression and increased cancer risk [7] to motor impairment and catalepsy [8], interference with cognitive function, panic attacks and a higher risk of developing psychosis [9]. In regard to therapeutic administration, the cannabinoids reported a clinical capability towards anxiety and depressive symptoms regulation [10].
In recent years, many states legalized and promoted the use of cannabinoids for therapeutical purposes, and in some states recreational cannabis became legal and its prevalence markedly rose [11]. Given the present and future increase in health issues related to cannabinoid consumption, it is mandatory for oral healthcare providers and dentists to know and understand the oral effects of cannabis.
This review briefly summarized the components of cannabis and the endocannabinoid system, as well as its cellular and molecular mechanisms of biological cannabis action in human cells and biologic activities on tissues. We will also look into oropharyngeal tissue expression of cannabinoid receptors together with a putative association of cannabis to several oral diseases. Therefore, this review elaborates the basic biology and physiology of cannabinoids in human oral tissues, with the aim of providing a better comprehension of the effects of its use and abuse on oral health, in order to include cannabinoid usage into dental patient health records as well as good medicinal practice.

Phyto-Cannabinoids
The Cannabis sativa plant contains more than 500 components. Amongst them, more than 100 compounds which possess an aromatic hydrocarbon have been identified and called cannabinoids [12]. All these cannabinoids have bind-described bind/activate cannabinoid receptors [13,14]. The plant-derived cannabinoids are also called phytocannabinoids, in order to distinguish them from synthetic cannabinoids and endogenous counterparts (endocannabinoids). Among phyto-cannabinoids there are three major compounds derived from cannabigerol-type (CBG) molecules, delta-9-tetrahydrocannabinol (THC, the main psychoactive compounds from cannabis), cannabinol (CBN), and cannabidiol (CBD) [15]. They were isolated and structurally identified by nuclear magnetic resonance as well as by mass spectrometry [16]. The majority of phyto-cannabinoids are characterized by different affinities to cannabinoid receptors, despite possessing the basic structural types described above. THC, a highly hydrophobic and lipophilic compound, is the most abundant in cannabis [17]. This compound binds to both cannabinoid receptors with similar affinities for CB 1 and CB 2 (both Ki values are around 40 nM), but has been shown to possess less intrinsic affinity to CB 2 than CB 1 [14]. THC administration to animal models as well as to human subjects highlighted the enormous and potent psychoactive properties of this compound, with a plethora of effects on locomotion, anxiety, pain, cognition and reality perception [1,18]. On the other hand, CBD has always been considered to be an isomer of THC devoid of psychoactive activity. When compared to THC, CBD has significantly lower affinity for CB 1 and CB 2 receptors, with Ki values at M levels (in nM for THC) [14], but several other brain targets and molecular effectors have been proposed for this compound other than cannabinoid receptors, including numerous classical ion channels, receptors, transporters, and enzymes (reviewed in [19]). However, some CBD effects at these targets in in vitro assays only manifest at high concentrations, which may be difficult to achieve in vivo, particularly given CBD's relatively poor bioavailability [20]. Several reports also suggest that CBD might also affect the bioavailability, receptor binding and molecular actions of THC [21].
CBN is a product of THC metabolism and has only mild psychoactive activity if compared to its parental molecule [22] with higher affinity to CB 2 than CB 1 receptors. To date, there are three main forms of cannabis consumption: marijuana, hashish, and hash oil [23]. Hemp, a preparation of cannabis dried leaves and flowers, contains 0.5%-5% THC. On the other hand, cannabis flower heads compressed to form small light brown or black blocks, so called hashish, contains 2%-20% THC. The recently formulated hash oil, which is an oily liquid derived from hashish, can include up to 15%-50% THC and represents the highest percentage obtained in natural products so far [23].

Synthetic Cannabinoids
Historically, the use of the marijuana-derived ∆ 9 -THC as well as synthetic analogues was actually the golden tool for the discovery and characterization of CB 1 [24]. Among the synthetic cannabinoid agonists, we will briefly mention some of them, since they are widely used in experimental models ( Figure 1). HU-210, characterized by a like-3 ring structure as in THC, is the most potent synthetic compound belonging to the HU series and was first synthesized and characterized in Israel. Bi-and tricyclic analogs of ∆ 9 -THC, such as CP-55,940, characterize the second group of CB 1 agonists used in pharmacological studies. As a third group of ligands, amino-alkylindols, such as WIN-55,212, exhibit potent CB 1 agonistic activity [12].
All the above reported compounds also show some ability to bind and activate CB 2 receptors. Amongst the selective CB 1 agonists, ACEA (arachidonoyl-2 -chloroethanolamide) is the first one ever characterized and has a very potent and extremely selective CB 1 agonist without activity at CB 2 [25]. Synthetic ligands showing antagonistic properties at the cannabinoid receptors have been developed in the past. The compounds specific to CB 1 and most widely used in both pre-clinical and clinical studies are SR141716 [26], AM251 [27] and AM281 [28]. Instead, CB 2 receptor antagonists such as SR144528 and AM630 have different actions on effector cells and tissues by targeting the receptors [29]. Finally, two classes of compounds are normally used to interfere with the endocannabinoid system, although not acting directly on cannabinoid receptors. These compounds are represented by inhibitors of endocannabinoid re-uptake, such as AM 404 [30], VDM-11, UCM-707 and OMDM-2 [31], and by inhibitors of anandamide hydrolysis, such as URB532, URB597 [32]. More recently, synthetized compounds are the two inhibitors of 2-AG degradation, such as JZL184 and JZL195 [33,34]. These classes of compounds seem to have been shown to selectively increase the concentration of endocannabinoids, possibly avoiding some of the side effects due to generalized cannabinoid receptor activation by direct agonists.

Cannabinoid Receptors
CB 1 , the first identified cannabinoid receptor identified, was cloned in rat, human and mouse tissues [24,35,36]. The characterization and the cloning of the other wellknown cannabinoid receptors, designated CB 2 , were subsequently also realized in the three species [37,38].
The analysis of the primary amino acid sequence of CB 1 and CB 2 receptors led to assigning them to the large family of G protein-coupled receptors (GPCRs). A combination of mutagenesis experiments and three dimensional models of these two receptors identified important structural determinants of the structure/function relationships and ligand binding/effector triggering (reviewed in [39]). CB 1 and CB 2 are encoded by different genes but possess 44% amino acid homology. In humans, CB 1 was preferentially localized in the brain and the spinal cord but nowadays is accepted to be ubiquitously expressed throughout the body [14]. In contrast, CB 2 is expressed at high levels in leukocytes, neutrophils, keratinocytes, the spleen, natural killer cells, and, at a lower extent, in the muscle, liver, intestines and testes [40], as well as in the adipose tissue [41]. However, the second isoform of CB 2 seems to be present in additional tissues, especially in the brain and kidney [40]. Although CB 1 and CB 2 are well known and characterized, numerous pharmacological studies suggest the existence of additional cannabinoid receptors. Recent data point to two other GPCRs, G protein-coupled receptor 55 (GPR55) and G protein-coupled receptor 119 (GPR119) as novel potential cannabinoid receptors (reviewed [42]), besides the transient receptor potential vanilloid type 1 (TRPV 1 ) ion channel, which is well-known to bind some endocannabinoid ligands. The human orphans GPR55 and GPR119, originally identified through a bioinformatic approach [43], were both cloned in mice, rats and humans [44]. The human GPR55 shares only 14% sequence identity with the CB 1 and CB 2 receptors and is mainly expressed in the brain (caudate and putamen, cerebellum) [44,45]. Thus, GPR55 might be involved in learning, memory, and motor function given its high expression in the brain, especially the basal ganglia and cerebellum [44,45]. The human GPR119 is encoded by a protein of 335 amino acids, and isoforms of this receptor are present in various mammalian species [44]. Expression profiles of GPR119 mRNA receptor seem to be restricted to the pancreas, fetal liver and gastrointestinal tract in humans [46,47].

Biological Effects of Cannabinoids via Their Receptors
Cannabinoids exert their physiological and pathophysiological effects mainly by binding to various cannabinoid receptors and triggering different signaling pathways ( Figure 2). Here, we will mainly focus on the best described amongst them, which is the CB 1 receptor [48]. The central mechanism of action of CB 1, when activated, is to inhibit adenylate cyclase, a second messenger system, in a dose-dependent manner via Gi/o proteins, which reduce intracellular levels of cyclic adenosine monophosphate (cAMP) [49,50]. This turn results in a downregulated activity of cAMP-dependent protein kinase (PKA), which in turn reflects on downstream signaling pathways, such as ion channels, and electrical properties of the cell, triggering several mitogen-activated protein kinases (MAPK) [51]. Amongst other signaling pathways which have been shown to play a key role in the cellular and behavioral effects of THC PI3K/Akt signaling, the mTOR pathway and neurosteroid synthesis are worth mentioning (reviewed in [48]). Furthermore, a series of recent studies point out that apart from their canonical plasma-membrane localization and signaling, CB 1 receptors are also associated with mitochondrial membranes in several cell types. Activation of these subcellular receptor pools tremendously impacts cell bioenergetic status, resulting in important behavioral and physiological alterations [52][53][54][55][56][57].

Tongue
Several studies found the expression of both CB 1 and CB 2 receptors in the human tongue [58]. Immunohistochemical positive CB 1 and CB 2 immunoreactivity throughout the full thickness of the epithelium has been found in the epithelial cells of the tongue and in circumvallate and fungiform papillae [59]. Moreover, both CB 2 and TRPV 1 receptors have been described in epithelial cells adjacent to taste buds and in the basal layers of tongue epithelium [60][61][62].
However, how cannabinoids are involved in tongue functions is still unclear. To date, the elegant series of studies performed by Yoshida and colleagues showed that administration of both exogenous agonists and endogenous cannabinoids increases gustatory nerve responses to sweeteners, as well as behavioral responses to sweet-bitter mixtures, and electrophysiological responses of taste receptor cells to sweet compounds [60,61,[63][64][65][66][67][68][69]. Interestingly, genetic and pharmacological receptor blockades highlight an exclusive role of CB 1 receptors in the aforementioned cannabinoid effects [60,61]. The pathophysiological status of the tongue has been recently associated with cannabinoid receptor expression levels. Indeed, several pieces of evidence found a higher expression of both CB 1 and CB 2 receptors in patients suffering from mobile tongue squamous cell carcinoma (SCC) [70]. Moreover, higher levels of TRPV 1 and CB 2 are also associated with a reduction in CB 1 expression levels, which have been described in the epithelial cells of the tongue from patients with burning mouth syndrome [59]. These last observations are in line with the role of cannabinoid receptors in cancer [71] and inflammation [72], which will be treated in the next session.

Salivary Glands
Salivary glands express both CB 1 and CB 2 receptors with specific patterns [73][74][75][76][77][78][79][80][81][82][83][84][85][86]. CB 1 receptors have been detected in the major salivary glands, however their expression was not observed in the acinous cells but were restricted to the striated duct cells near to the apical membrane [87]. CB 2 receptors instead have been visualized mainly in myoepithelial cells surrounding the acini, where the production and release of saliva takes place, as well as in neurons of ganglia from the secretory ducts ( Figure 3) [88]. Cannabinoid receptor expression in salivary glands has been shown to be under the control of several factors, including food quantity and quality and noradrenergic tone [74,88]. For instance, in the submandibular gland, basolateral membranes of ductal cells primarily express CB 1 which, however, is also found in the serous cells of mixed acini according to dietary status [88]. Several pieces of evidence from the Elverdin lab pointed out a negative action of both CB 1 and CB 2 receptor activation in the regulation of saliva secretion [89][90][91][92][93], which might explain the dry mouth sensation always experienced by heavy cannabis users [11]. These sets of findings were supported by another study showing that endogenous cannabinoid anandamide, by activating CB 1 receptors expressed in rat parotid glands, triggers cAMP accumulation. This results in amylase release with subsequent Na + -K + -ATPase inhibition and impacts upon salivary gland functions [73].

Pulp Tissue
Although in dental pulp tissues only few reports succeed in the detection of CB 1 receptor expression, several reports pinpoint out a therapeutical role of cannabinoids in this oral tissue. Indeed, CB 1 receptors have been found at the pulp-dentin border, especially located on the nerve terminals impinging into the dental pulp tissue, and this pattern of expression was maintained in nerve fibers of symptomatic painful dental pulp [94]. Given the well-known role of neurotransmitter suppressors in basically all kinds of transmission [95], together with the presence of CB 1 receptors on these nerve terminals cannabinoids might represent a good therapeutic target for diseases with dental pain. Another target of cannabinoid-based medicine in the dental pulp might be dentin repair/regeneration. Indeed, functional CB 1 receptors have also been reported in human odontoblasts [96]. Cannabinoid treatment of rat odontoblasts has been shown to promote the formation of "reparative dentin" by modulating extracellular Ca 2+ entry [97], which might be the mechanism for CB 1 -mediated dental pulp tissue repair via the matrix metalloproteinase-2 activation in dental pulp cells [98][99][100][101][102].

Periodontal Tissue
In periodontal tissues, several reports have suggested a role for both CB 1 and CB 2 receptors in pathological conditions, such as inflammation and wound healing [103][104][105]. Indeed, CB 1 are expressed at a significantly higher level than CB 2 receptors in both epithelium and periodontal ligaments (PDL) in periodontal tissues from healthy subjects. Furthermore, there is a switch in receptor expression (downregulation of CB 1 and overexpression of CB 2 receptor) within the PDL following bacterial inflammation. On the other hand, sterile inflammation strongly increases CB 1 and CB 2 expression in the PDL, but not in the alveolar bone nor in the cementum [103].

Oral Mucosa
At a histological level, oral mucosa is made by a stratified squamous epithelium and underlying connective tissues. Although no direct report on cannabinoid receptor expression in oral mucosa has yet been provided, CB 1 and CB 2 have been shown to be functionally expressed by skin epithelial cells, suggesting a putative role in modulating several cellular functions in the mucosa epithelium [116]. Indeed CB 1 and CB 2 receptor activation exerts opposite effects on human epidermal keratinocyte proliferation and differentiation [117][118][119][120][121][122][123][124][125][126][127]. As previously mentioned, CB 1 , CB 2 and TRPV 1 receptors are indeed identified in the connective tissue from the lamina propria layer from the oral mucosa especially on salivary glands, blood vessels, nerve endings, and immune cells belonging to this tissue [59]. However, there is to date a poor scientific description of cannabinoid receptor expression in the oral mucosa, an issue that will need to be addressed since oral mucosa is the first line of tissue interacting with cannabinoids during marijuana consumption. Thus, exploring the physiological and pathophysiological role of cannabinoids on oral mucosal health and diseases might represent the way to improve cannabis-based medicine or mitigate side effects of cannabis recreational consumption. The aim of the present investigation was to evaluate the cannabinoids and their biological effects through a systematic review of the literature.

Patient and Public Involvement
The present investigation evaluated the effects of cannabinoids on oral health associated with recreational using and therapeutic purposes through a systematic review of the literature.
No patients have been involved in the present study, while no investigational ethical considerations are associated with the present paper.

Search Strategy
The study PICO question has been summarized in Table 1, and the scope of the present investigation was to evaluate the effectiveness of cannabinoids derived adjuvant for the treatment of different diseases of the oral cavity such as: dry mouth, tooth caries, periodontal and gingival diseases, oral hygiene maintenance, oral cancer and oral tissue diseases. The paper search and selection was conducted independently by two expert reviewers (F.I. and F.L.), and a Boolean database search has been conducted in the Pubmed (MEDLINE) and EMBASE electronic databases without any time limitations. The key words search indicators are presented in Table 2: (cannabinoids AND dry mouth); (cannabinoids AND caries); (cannabinoids AND periodontal diseases); (cannabinoids AND oral hygiene); (cannabinoids AND oral cancer); (cannabinoids AND oral tissue diseases). Moreover, a manual paper search was conducted to improve the article pool; the duplicates were removed after the title evaluation. The abstracts were manually evaluated to perform an initial screening of the articles identified and the final selection was performed with the full text of the papers in order to conduct the eligibility for the qualitative analysis. At the end of the process, the papers selected were categorized according to the reference data, year of publication, type of the study, patients treated, test and control group treatments, follow-up, and study effectiveness.

Inclusion and Exclusion Criteria
For the present investigation, for the qualitative analysis full-length articles written in English language were considered, as well as literature reviews and meta-analyses, randomized and non-randomized clinical trials, case reports and case series. The exclusion criteria for the evaluations were: editorial letters, book chapters and conference proceedings.

Study Selection
The full texts were recorded and evaluated for all the papers included in the present systematic review. Each one was studied independently according to the inclusion and exclusion criteria mentioned above. The majority of the papers were in the English language; we only choose the ones in which the drilling technique was performed following the guidelines of the burst producer. The minimum follow up period was set to three weeks.

Data Extraction
For the qualitative synthesis of the studies included, the following data were considered: the drug description, the design of the study, the experimental model, the administration protocol, and the effectiveness of the study.

Articles Selection Process
The entire article identification, initial screening, eligibility assessment criteria and qualitative analysis processes are described in Figure 4. The initial screening process retrieved a total of 276 articles. The papers identified were merged, and after the initial screening a total of 162 articles were excluded. The eligibility assessment was performed and a total of 59 manuscripts were excluded from the articles pool: 53 off topic papers, 3 book chapters, 1 editorial letter and 2 congress proceedings. A total of 31 articles were selected for the qualitative synthesis.

Cannabinoids Drugs for the Treatment of Dry Mouth
A total of four studies were included about cannabinoid use and dry mouth disease. Darling et al. reported the only cross-sectional study conducted on 300 patients that reported cannabinoids consumption by smoking (Table 3) [129]. The subjects included reported nicotinic stomatitis in a total of four cannabis consumers but not smokers. A higher incidence of leukoedema and dry mouth was evident in cannabis users compared to the control groups. The other studies were conducted on animals: two papers on rat models [89,92] and one article on pigs [88]. Pirino et al. evaluated the cannabinoid receptor expressions CB 1 and CB 2 after a dietary supplement administration on 32 pigs, reporting an influence of the expression of salivary ducts and secretion of the mandibular glands related to endocannabinoids activity (Table 3).

Cannabinoids and Dental Caries
A total of three studies were included about the topic of cannabinoids and dental caries. Two articles reported a clinical study on humans: a case report [130] and a retrospective cohort trial [131]. Grafton et al. [130] reported a clinical report of a low compliance of a marijuana smoker that submitted to a tooth extraction procedure with a high incidence of dental caries. Ditmyer et al. [131] reported through a retrospective cohort study on 66,941 subjects an increase of the prevalence and severity of dental caries in patients that declared tobacco/marijuana administration. In vitro, Liu et al. [104] reported that delta-9tetrahydrocannabinol (THC) promoted periodontal cell adhesion and migration in wound tissue healing (Table 4).

Cannabinoids and Periodontal Diseases
A total of 10 articles were included about the topic of cannabinoids and periodontal diseases: two clinical studies, three studies only in vitro, one study only in vivo on rats and three articles with both in vitro/in vivo on rats. Thomson et al. [132] reported in patients affected by periodontitis that the cannabis smoking may be a risk factor for periodontal disease independent from the tobacco use, while Shariff et al. [133] showed that cannabis smoking was correlated to deeper probing depths, increased clinical attachment loss and higher risk for severe periodontitis. Nogueira-Filho et al. [134] reported on rats that cannabis smoke exposure may impact alveolar bones by increasing bone loss, while in other studies the administration of synthetic cannabinoid derived molecules such as anandamide (AEA)/2-arachidonoylglycerol (2-AG)+ AM251, AM630 and HU-308 seems to be correlated with an increased activity and proliferation of human gingival fibroblasts, a lower bone loss by the inhibition of the RANK/RANKL expression, and antiinflammatory and osteoprotective effects on the oral tissue in vivo [107,[135][136][137]. In studies conducted on human periodontal fibroblasts (HPLF) and human gingival fibroblasts, the cannabinoids exhibited a strong inhibition of pro-inflammatory molecules such as LPS, TNF-α, and IL-1β expression [106][107][108] (Table 5).

Cannabinoids and Oral/Neck Cancer
A total of 13 articles were included about the topic of cannabinoids and oral/neck cancer development: three literature reviews [138][139][140], four in vitro studies [141][142][143][144], one case series, and five case-control and cohort studies [145][146][147][148][149]. The studies [138,145,[147][148][149] that evaluated marijuana consumption reported that the smoking habitude has been correlated to a carcinogen induction with no completely clarified chemical and physical pathogenesis, while Rosenblatt et al. [146] demonstrated a similar oral cancer incidence between test and control with no cannabis smoke evidence. The studies [141][142][143][144] that considered cannabinoids supplements in vitro reported a capability to inhibit the growth of different cancer cells lineages, including aggressive and chemotherapy-resistant variants of lung cancers (Table 6).

Cannabis and Oral Tissue Diseases
A total of two studies were included for the qualitative synthesis: a literature review [11] and a cross-sectional study on humans [129]. Versteeg et al. [11] reported that the cannabis smoking habit has been correlated with an increased incidence of xerostomia, leukoedema and a higher prevalence of Candida albicans infections. Darling et al. [129] reported a high incidence of nicotinic stomatitis associated with cannabis consumers with no tobacco use.

Cannabis Consumption and Effect on Oral Health
Cannabis abuse has always been known to impact on proper oral health status. Several compounds assume that cannabis smoke will possibly put cannabis users to a higher risk of dry mouth, dental caries, soft tissue disease, poor oral hygiene, periodontal disease and even oral cancer by changing the physiology of the oral environment ( Figure 5). On the other hand, cannabis might represent a good pain management tool for dental anesthesia as well as post-operative management.      Figure 5. Oral pathologies and disease involved with cannabinoid exposure and abuse.

Dry Mouth
Cannabis use can lead to xerostomia by reducing salivary flow. Dry mouth associated with cannabis abuse is reported to be similar to the one after cigarette smoking, and in most subjects dry mouths appear immediately after cannabis use [129]. Cannabis use has always been associated with dry mouth and hypo-salivation via a CB 1 /CB 2 receptormediated THC effect on the salivary glands cholinergic transmission [89,92]. THC has also been shown to importantly reduce submandibular salivary flow induced by electrical stimulation in dogs [150]. These findings may help to better understand the mechanisms of reduced saliva production, which eventually lead cannabis smokers to xerostomia.

Caries
Amongst the main dental complication of cannabis use, an increased incidence of caries has frequently been reported. This is probably mediated by several factors, which might include less saliva production, poor oral hygiene and higher plaque scores. Indeed, cannabis smokers have been shown to present a higher number of DMF teeth scores with a greater accumulation of plaque [130]. Another study, after correcting some confounding factors such as exposure to second-hand smoke, gender and race/ethnicity, reported an increased prevalence and severity of dental caries among marijuana users [131]. However, one has to also take into account the potential beneficial roles of cannabinoids on dental pulp diseases and regeneration/repair [104,[106][107][108][109], which will be discussed in the next section.

Periodontal Diseases
To date, a potential link between cannabis use and periodontal disease is supported only by a limited and inconsistent literature background. Some studies tend to suggest chronic cannabis use as a potential risk factor for periodontal diseases including gingival leukoplakia, gingival hyperplasia, alveolar bone loss and gingivitis [132]. Additionally, a US Survey supports an incidence of more severe periodontitis associated with recreational cannabis use [133]. Higher bone loss and lower bone density were associated with marijuana smoke inhalation (MSI) in rats following ligature-induced periodontitis [134] with, however, no significant histological differences.
On the other hand, no association between cannabis smoking and periodontitis was found in another groups of studies. For example, no significant associations between cannabis use and periodontitis have been found in adolescent populations [151]. Moreover, in mice with ligature-induced periodontitis, cannabinoids have been shown to protect them from periodontal diseases, as CBD/THC injection strongly reduced pro-inflammatory cytokine levels and PMN cell motility as well as less furcation bone loss [137].
Several pieces of evidence against the causative effects of cannabinoids on periodontal disease are given by the well-known role of the endocannabinoid system in periodontal healing, as mentioned previously. Cannabinoids, by activating CB 1 /CB 2 receptors, promote the proliferation of gingival fibroblasts in periodontal healing [107], and methanandamide and HU308, selective CB 1 and CB 2 receptor agonists, are able to dampen LPS-induced periodontitis in vitro and in vivo [135,136], especially by attenuating alveolar bone loss and increased inflammatory mediator. Moreover, administration of CBD inhibited RANK/RANKL expression resulting in a diminished bone resorption and pro-inflammatory cytokine in the periodontal tissue [137]. Thus, these findings highlight different receptor and molecular mechanisms on periodontal disease, which are all in support of an anti-inflammatory and protective effects of cannabinoids.
Multiple factors and research designs might explain the conflicting findings for the link between cannabis use and periodontal disease. First, patients presented several risk factors apart from cannabis use such as age, systemic health, concurrent tobacco smoking and oral hygiene. Second, individuals had different amounts, frequencies, duration, and modes of administration of cannabis use. Third, the effects of cannabis use on oral tissues and oral health have been described only in limited reports; thus, more well-designed studies will be needed to address these issues.

Oral Hygiene
Cannabis abusers, as well as cigarette smokers, normally have poor oral hygiene and higher plaque scores, increasing the likelihood of caries and periodontal disease [152]. Unfortunately, it is difficult to determine whether neglect of oral hygiene and failure to seek regular preventative dental care might be the causes directly linking cannabis use to oral uncleanliness. One study showed that increasing amounts of drug used was not associated with a lower oral hygiene index, or decayed, missing and filled teeth (DMF-T) [129]. As cannabis users often also abuse tobacco and alcohol, this relationship is of course hard to disentangle.

Oral Cancer
Although still unclear, an association between marijuana use and oral cancer has been recently proposed. Indeed, cannabis smoke increases the possibility of developing oral cancer, since it contains similar carcinogens as in tobacco. Some studies indicate that cannabis use increases oral premalignant lesions such as leukoplakia and erythroplakia, especially on the anterior floor of the mouth and the tongue [129,138]. Cannabis smoking has also been suggested to be a possible cause of tongue carcinoma [138,145,146], and marijuana smokers have been found with epithelial dysplasia in the buccal mucosa [129]. A strong association between cannabis use and head and neck cancer has also been reported among younger patients [145,147]. Furthermore, frequent, forever and long duration marijuana use increases significantly the possibility of developing oropharyngeal cancer [147].
However, other studies failed to associate cannabis use to head and neck cancer [139,148,149,153]. Moreover, a case-control study with strict control for confounding factors, such as birth year, education, sex, cigarette and alcohol consumption, showed no association between oral squamous cell carcinoma before and after cannabis consumption [146], indicating that conflicting results may be due to different methods used, and a lack of quality research. Targeting the cannabinoid system represents a potential therapeutic target in the treatment of several types of cancer [140]. Cannabinoid agonists prevent cancer cell progression, reducing tumor growth and metastasis in at least in two ways: by inhibiting cancer cell proliferation and/or inducing autophagy and cell apoptosis [143,144] by suppressing cancer cell migration [141,142]. Thus, the potential of therapeutic targeting of cannabinoid receptors in oral cancers should not be neglected.

Other Oral Tissue Diseases
Cannabis smoking may also result in lesions in the oral soft tissue. Stomatitis with leukoedema and hyperkeratosis are often found in the buccal mucosa of cannabis smokers, probably resulting from the high temperature of the smoke or the specific chemicals inhaled [129]. Moreover, due to their poor oral/denture hygiene and nutritional deficiency, heavy cannabis users are also more prone to Candida albicans infections [11].
3.14. Potential Therapeutic Application of Cannabinoids on Oral Health As mentioned before, its anti-oxidant, anti-inflammatory and analgesic properties have allowed CBD to be proposed as a therapeutic and safe drug for use in oral mucositis [154], Thus, this recent proposition of CBD use in dentistry will surely open the way to studies on the use of cannabinoids in oral mucositis and other oral mucosal diseases caused by oxidative stress, chemotherapy, or radiotherapy.
There are many considerations of the role of marijuana's effect with dental anesthesia, especially as a pain management tool for surgical analgesia as well as post-operative management. In a study done by Holdcroft et al., capsules of THC and CBD were given to patients following major operations [155]. Pain relief and mood, measured by eight assessments trough a visual scale, showed that these capsules reduced demands and extended the lag time for rescue analgesia (morphine) in patients; the optimal dosage, to avoid dose-related side effects such as dizziness and sedation, was ten milligram [155]. This and other studies showed morphine-sparing effects of cannabis, which are crucial as opioid compounds have high abuse potential and fatal risks [156], indicating the potential use of marijuana as an analgesic alternative with positive future implications for the dental field.

Conclusions
Although there is a long history of cannabis use, the knowledge of the effects of cannabis on human health has only been enriched in recent decades. The discovery of synthetic cannabinoids, cannabinoid receptors and the endocannabinoid system has paved the way for better understanding of several effects of cannabis on the human brain and body. Given the present and future increase in health issues related to recently legalized cannabinoid consumption, it is mandatory for oral healthcare providers and dentists to know and understand both the adverse and beneficial oral effects of cannabis. It is critical for oral healthcare providers to be aware of a patient's status, to recognize the potential risks, and to seek the best treatment options.
The most common way of consuming cannabis, marijuana smoking, has several direct and indirect deleterious effects on oral cavities; however, the evidence linking cannabis to oral/dental diseases is contradictory and at best limited. This is often related to different personal risk factors, as well as the lack of details in marijuana usage information.
Innovative compounds active on selective cannabinoids receptors could be useful for the treatment of numerous systemic disease and novel implications in several pathologies.
Well-designed research controlling for confounding factors are needed in the future, and more basic and clinical research should be designed to understand the mechanisms of action of cannabis. This will allow us to precisely target the systemic and oral effects in a more specific manner, by developing synthetic agonists, antagonists and more general modulators of the endocannabinoid system. This will largely benefit patients by developing new therapeutic approaches to increase treatment efficacy and to reduce the side effects.  Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: All experimental data to support the findings of this study are available contacting the corresponding author upon request.