Co-exposure of cannabinoids with amphetamines and biological, behavioural and health outcomes: a scoping review of animal and human studies

Rationale The growing prevalence of psychostimulant (including amphetamine) use and associated health harms, with limited treatment options, present a global challenge. There is an increasing availability and medical applications of cannabinoids, and growing interest in their therapeutic potential for addictive disorders. Objectives The objective of this study is to review available data regarding cannabis/cannabinoid co-use or exposure on amphetamine-related outcomes. Methods Towards the present scoping review, we systematically searched four databases (Medline, Web-of-Science, CINAHL Plus and PsycInfo) using cannabis/cannabinoid and amphetamine text-terms identifying peer-reviewed, English-language studies published in 2000–2020 involving multiple methods approaches among both human and animal study samples, assessing the association of co-use/administration of cannabis/cannabinoids products with non-medical amphetamines on biological, behavioural or health outcomes. Results Twenty-five articles were included. Pre-clinical studies (n = 15) found mostly protective effects of single or repeated cannabinoids administration on rodents in amphetamine addiction models, amphetamine-induced models of human mental disorders (e.g. schizophrenia) and amphetamine-induced neurotoxicity. Human studies (n = 10) were more heterogeneously designed (e.g. cross-sectional, case–control, longitudinal) and assessed natural ongoing cannabis and methamphetamine use or dependence, showing mostly enhanced harms in a diversity of outcomes (e.g. mental health, methamphetamine use, cognition). Conclusions While human studies suggest cannabis use as an adverse risk factor among non-medical amphetamine users, pre-clinical studies suggest therapeutic potential of cannabinoids, especially cannabidiol, to alleviate amphetamine addiction and harms, including treatment outcomes. Given increasing psychostimulant harms but lack of care options, rigorous, high-quality design studies should aim to translate and investigate pre-clinical study results for potential therapeutic benefits of cannabinoids for amphetamine use/abuse in human subjects. Supplementary Information The online version contains supplementary material available at 10.1007/s00213-021-05960-2.


Introduction
A 0.5% (approximately 27 million people) of the world's adult population use amphetamines, a central nervous system stimulant. Of these users, approximately 6.6 million (86/100,000 population) are estimated to be dependent, rendering amphetamine substances to be among the most used illicit drugs globally (Degenhardt et al. 2013;Peacock et al. 2018;UNODC 2020). Extra-medical amphetamine use, especially methamphetamine (a more potent structural form (Lake and Quirk 1984)), is associated with elevated risks for both mortality and morbidity outcomes, including acute poisoning, mental health (e.g. depression, psychosis, suicidal behaviours) and neurocognitive impairments, cardiovascular problems, injury and violence (Cumming et al. 2020;Marshall and Werb 2010;Marshall et al. 2011;McKetin et al. 2019;Wood et al. 2006). Amphetamine dependence was responsible for about 0.6% (approximately 326,000 deaths) of global all-cause mortality in 2017, entailing a mortality risk among people with amphetamine dependence up to six times higher than the general population (Farrell et al. 2019). Changes in illegal drug production patterns suggest that amphetamines supply and use have been globally increasing. Global methamphetamine seizures increased by 16% in 2015/16; despite seizures occurring mainly in North America and Asia, a 50% increase of countries reporting seizures has been observed in the last decade. The majority (90%) of dismantled methamphetamine laboratories are located in North America, with the USA harbouring most of these (3,036 or 82%) worldwide in 2017 -mostly supplying the local market (i.e. "kitchen lab") in contrast with large-scale, export-oriented laboratories in Mexico and East/ South-East Asia (Farrell et al. 2019;UNODC 2020).
Cur rently, there are no pharmacotherapeutic options approved for the treatment of stimulant use disorder, including amphetamines, at any of the main stages of treatment (e.g. withdrawal, relapse prevention) (Brandt et al. 2020;Castells et al. 2016;Morley et al. 2017;Pérez-Mañá et al. 2013;Ronsley et al. 2020;Siefried et al. 2020;Tardelli et al. 2020). The range of evidence-based, non-pharmacotherapeutic treatment or targeted prevention measures is overall highly limited, although select cognitive-behavioural interventions, including contingency management approaches, have shown some promise (Cumming et al. 2016;Fischer et al. 2015a;Knapp et al. 2007;Lee and Rawson 2008;Minozzi et al. 2016;Shearer 2007). This situation underscores a need for the examination of potentially effective therapeutic strategies or interventions for psychostimulant and specifically problematic amphetamine use or use disorders (Fischer et al. 2015b;Kogan and Mechoulam 2007;Shorter et al. 2015). Recently, there has been increased focus on the endocannabinoid system as a possible target for treating drug dependence, craving or withdrawal (Chye et al. 2019;Manzanares et al. 2018;Parsons and Hurd 2015;Spanagel 2020), with a growing number of studies examining the therapeutic effects of cannabinoid agents for addictive disorders (Batalla et al. 2019;Calpe-López et al. 2019;Prud'homme et al. 2015;Turna et al. 2019).
The endocannabinoid system (ECS) is a central biological system composed of at least two endocannabinoids (neurotransmitters; anandamide and 2-arachidoyl glycerol) that bind to cannabinoid receptors expressed throughout the mammalian vertebrate central and peripheral nervous systems. The ECS is centrally involved in regulating essential neural, physiological and cognitive processes, including movement control and motor coordination, learning and memory, mood, stress response and motivation, the immune system, appetite, addictive behaviours and pain modulation, among others (Cabral et al. 2015;Kruk-Slomka et al. 2017;Lu and Mackie 2021;Mechoulam and Parker 2013). The ECS is also involved in key brain structures (e.g. prefrontal, limbic, striatal) and neurotransmitter systems (e.g. serotonin, dopamine) essential in the reward system and drug addiction. Of special interest is the role of CB1 receptors, expressed in brain regions involved in decision-making, withdrawal and relapse, and therefore, essential for modulating the rewarding effects of drugs (Chye et al. 2019;Maldonado et al. 2006;Manzanares et al. 2018;Parsons and Hurd 2015;Spanagel 2020). Moreover, there is increasing evidence and practice of medical usage for some indications (e.g. neuropathic pain, chemotherapy-induced nausea -for reviews see (Abrams 2018;Crippa et al. 2018;NASEM 2017)) and legal availability of cannabinoids products in many countries. Related, a purified cannabis-derived prescription form of cannabinoid has been recently approved in Europe and the USA for selected medical usage indications (GlobeNewswire 2020(GlobeNewswire , 2021Marcu 2020). Possibly beneficial effects or properties from cannabinoids agents specifically for addictive behaviours related to various substance groups (e.g. opioids, alcohol, nicotine, other stimulants) have been suggested, but no conclusive (e.g. clinical trialbased) evidence or reviews for amphetamine use and related problems presently exist (Abrams 2018;Prud'homme et al. 2015). On this basis, we sought to comprehensively review the existing, multidisciplinary studies and outcome data that have investigated the associations of cannabinoid exposure on amphetamine use and adverse outcomes in different study populations.

Study aim and scope
We conducted a scoping review which aimed to identify and summarize data on possible associations of natural or experimental co-administration of cannabinoids on amphetamine use-related biological, behavioural or health outcomes from different methodological or disciplinary approaches (e.g. pre-clinical, human experimental, observational studies). Cannabinoids were defined to include all-natural cannabis product forms or compositions (e.g. cannabidiol [CBD], tetrahydrocannabinol [THC]) and amphetamine products referred to both amphetamines and the structurally similar methamphetamines. Both cannabinoids and amphetamines were considered irrespective of way of use (e.g. smoke, oral ingestion) or user status (e.g. dependence); while cannabis was considered regardless of the intended purpose of use (e.g. medical, non-medical); only extra-medical use of amphetamines was considered. Considering the over-time evolution in amphetamine-type drug use, types of cannabis products available and the fact that the potential of cannabinoids for addiction-related outcomes has only been investigated more recently (Fischer et al. 2015b), we limited our search to studies published since 2000 with a goal of providing a comprehensive review of relatively recent studies and data.

Search strategy
The systematic search strategy applied was originally developed for MEDLINE through an iterative process by the co-authors; the final search strategy was revised and adjusted using the Peer Review Electronic Search Strategies (PRESS) checklist (McGowan et al. 2016) and translated to other databases. Four databases were accessed: MEDLINE (PubMed), Web of Science (core collection), CINAHL plus and PsycInfo. The search strategy focused on related cannabis/cannabinoid and amphetamine text words based on MeSH indexing terms (e.g. marijuana, cannabis, CBD, THC, amphetamine, methamphetamine). Given the broad scope and high heterogeneity/interdisciplinarity of study data specifically for outcomes comprised by our scoping review and the intent to comprehensively capture and document these, we did not limit the search by specific outcome terms to avoid a potential restriction, or loss of relevant studies or data. Rather, we left the search for, and identification of, any relevant outcomes as defined by the scope of the review to the subsequent title/abstract and content screening. The full strategy details are presented as supplementary material. Complementary search strategies included comprehensive hand-searching of the reference lists of included studies. The results from the multiple databases were collated and deduplicated using the Systematic Review Assistant-Deduplication Module (SRA-DM) (Rathbone et al. 2015) and then uploaded to Endnote (v.X9.2), where all screening management tasks were performed.

Study screening and selection
Study screening and selection were based on the following inclusion criteria: (1) original articles based on primary study data; (2) studies examining the direct/immediate, recent (i.e. prior 3 months in humans or same life-phase in animals [e.g. adults, adolescents]), or lifetime (of active users) co-use or co-exposure of cannabinoid products with amphetamine products in human or animal populations; (3) reporting biological, behavioural or health-related measures or outcomes (e.g. brain activity/function, craving, drug use/dependence); (4) with measures or outcomes available that somehow comparatively attributed to amphetamineonly use or groups versus amphetamine and cannabinoid co-use or co-exposure; and (5) in English-language studies published since 2000 to 28 October 2020. Following deduplication, titles and abstracts were screened by one lead reviewer (DDB), with potentially unclear or ambiguous cases identified and arbitrated towards consensus together with a second reviewer (BF), followed by full-text review of studies identified for possible inclusion by the same procedure. We excluded studies that did not meet the above criteria, as well as reviews, commentaries, case reports and case-control studies that mixed amphetamine-or cannabinoid-related data with other non-target drug use in study groups. To assess the eligibility of case-control studies for inclusion, we verified the exclusive presence of data for amphetamine use with/without cannabinoid use groups and attributable outcomes through information from the studies' methods and results sections. Abstracts or full studies excluded predominantly did not match the content-or outcome-specific criteria for the review (e.g. no clear cannabis/ amphetamine comparison, no current cannabis use) or could not be accessed. The attached flowchart (Fig. 1) presents the main process steps for the present review.

Data presentation
Given the aim of a comprehensive, scoping review comprising evidence from multi-disciplinary perspectives, both quantitative and qualitative types of studies in different population/samples (e.g. animal, human experimental, treatment, community) were included. For results documentation, we used a descriptive approach, inductively grouping studies principally by type of basic study population and methods/approach, further dividing into study sub-cluster, and narratively summarizing the essential study scope, measures and results. As is common for a scoping review, no risk of bias or quality assessment (Arksey and O'Malley 2005) and no meta-analyses (also due to the heterogeneity of study designs and outcomes) were conducted.

Characteristics of included studies
Overall, 6,705 article citations were identified from the database searches which, after deduplication, resulted in a total of 3,237 potentially relevant abstracts that were screened. From these, 48 abstracts were selected for full-text review, with the final 25 articles included in the present review ( Fig. 1). Eighteen (> 70%) of the studies were published 2010 onward. Articles broadly ranged across animal-based experimental models (n = 15) and human population (n = 10) studies. Animal studies involved rodents (rats or mice) experiments of THC or CBD (single or repeat) administration compared to controls (i.e. vehicle administration). On the basis of the data, studies were divided into three clusters: non-contingent models of addiction (e.g. conditioned place preference and sensitization experiments) (n = 6), contingent models of addiction (drug self-administration experiments) (n = 3) and amphetamine-induced human mental health models and neurotoxicity experiments (n = 6). Human population studies comprised methamphetamine and cannabis users or dependents within small or larger population sample. Study designs were cross-sectional (e.g. case-control, experimental intervention), prospective and longitudinal with biological, psycho-behavioural and mental health-or addiction-related outcomes, organized into three topical study clusters: brain structure/metabolism (n = 3), neuropsychological function or psychopathology symptoms (n = 3) and methamphetamine use patterns (n = 4). See Tables 1 and 2 for animal and human studies summaries.
Overall, studies with non-contingent models of amphetamine/methamphetamine exposure showed a protective potential of CBD by reducing amphetamine reward and risk of relapse and facilitating related memory extinction. CBD was also shown to reduce neuroinflammatory factors in the brain, though it also enhanced the same factors in one specific (sleep-deprivation stress) condition. While THC was found to potentiate the extinction of amphetamine-related memory, it also showed no or counter-protective effects, specifically in vulnerable drug animals, as measured by methamphetamine sensitization.
Overall, contingent models of amphetamine/methamphetamine addiction studies generally showed a relapse-preventive potential of CBD and THC by reducing the motivation to seek and consume methamphetamine, while one study showed THC to increase amphetamine-induced hyperlocomotion but without affecting its self-administration.
Overall, for amphetamine-induced mental health, there was a CBD-related protective effect for amphetamine/ methamphetamine-induced animal models of psychosis but not mania, suggesting antipsychotic-like action. Also, CBD and THC protected against amphetamine/methamphetamineinduced brain damage.
Overall, brain structure and metabolism studies showed that cannabinoids co-use with methamphetamines is associated with neurotoxicity and abnormal brain function.
Overall, two studies suggested that cannabis co-use was associated with increased methamphetamine-related harm by increasing neurocognitive impairment and psychotic symptoms, while one found no exacerbation of cannabis exposure on methamphetamine-related neurotoxicity measures.

Methamphetamine use patterns (n = 4)
Two US-based studies assessed different factors associated with methamphetamine use, including cannabis use. A crosssectional study with 18-34-year-old methamphetamine-users (at least 1 prior-year use instance) using data from the population-representative National Survey of Drug Use and Health (NSDUH, 2015/18) found that self-reported past-year cannabis use strongly increased the risk of past-year methamphetamine use (n = 912) (aOR: 5.51, 95% CI [4.22,7.19]) (Yockey et al. 2020). A prospective study among a convenience sample of rural area college students (18-25 years old), based on a self-report electronic questionnaire containing methamphetamine (n = 112) and cannabis (n = 1,087) users, found methamphetamine-lifetime use at baseline and methamphetamine-use at 6-month follow-up (controlled by lifetime-methamphetamine use) to be positively associated with lifetime cannabis use (b = 1.03, p < 0.001; b = 0.60, p < 0.001), although 89% of cannabis users reported having never used methamphetamine at 6-month follow-up (Simons et al. 2008).
Two Canada-based descriptive studies explored the influence of cannabis use on the use of other drugs (including methamphetamine) from the participants' perspectives. A study with anonymous self-reported survey-data from a sample of medical cannabis dispensary users (n = 404) reported cannabis use as a substitute for illicit drugs (36.1%), including methamphetamine (n = 7), due to perceived lesser withdrawal, fewer side effects and better symptom management characteristics ascribed to cannabis use (Lucas et al. 2013). In-depth qualitative interviews with street-involved young people (16-26 years old, n = 56) and youth-focused care providers (n = 13) reported that most drug users engaged in daily cannabis use at the same time as they used other substances (including methamphetamine), and several participants described cannabis use as a form to alleviate harms from other drugs' use. Respondents also perceived cannabis use as being more effective than psychopharmaceutical-assisted treatments for mental health and substance use treatment, and few participants reported harms related to the intensive-cannabis use (Paul et al. 2020).
Overall, two studies on methamphetamine use patterns showed cannabis use to be associated with increased methamphetamine use; conversely, two self-reports suggested cannabis use to alleviate harms from, or to substitute for, methamphetamine use.

Discussion
Animal studies retrieved in this review showed mostly protective effects of both CBD and THC upon amphetamine exposure in models used to investigate several phases of amphetamine addiction (e.g. drug-seeking, relapse, extinction) (Kuhn et al. 2019;Spanagel 2017), and neurobiological markers (e.g. neurotoxicity, brain damage). While almost all CBD-based studies disclosed protective results, including for alleviating amphetamine-induced psychosis, some studies with THC-agents reported no effect and one showed a counter-protective effect, suggesting a facilitated progression of THC towards amphetamine use in vulnerable individuals (i.e. prior high-locomotion responsive animals).
The endocannabinoid system has been a topic of growing interest as a possible target for treating craving and addiction given its relationship to reward pathways, and substances acting on this system have been suggested as possible candidates for the pharmacotherapeutic treatment of substance use disorders (Batalla et al. 2019;Chye et al. 2019;Manzanares et al. 2018;Parsons and Hurd 2015;Prud'homme et al. 2015;Spanagel 2020). Although the full mechanisms of action of CBD are yet to be determined, CBD has been reported to act as a negative allosteric modulator of the CB 1 receptor and is unlikely to produce a rewarding effect (e.g. voluntary self-administration) (Ibeas Bih et al. 2015;Laprairie et al. 2015;Spanagel 2020). CBD also exerts its actions through other circuitry relevant to addictive disorder (e.g. 5HT1a, α7-nicotinic receptors), and some of its pharmacological effects such as reversing amphetamine-induced neuroinflammation and cognitive deficits, as well as alleviating mental disorder comorbidities (e.g. anxiety, depression) can be implied as a therapeutic benefit for amphetamine addiction (Calpe-López et al. 2019;Manzanares et al. 2018). In contrast, THC features substantial differences in its pharmacological actions that may account for the more heterogeneous results. By acting as an agonist of the CB 1 receptor, THC produces a dose-sensitive rewarding effect, with increasing doses shifting to negative, instead of positive, reinforcement (Spanagel 2020). Moreover, the differences in results observed in THC studies, ranging from protective to counterprotective effects, indicate a possible benefit from THC to amphetamine addiction that may depend on experimental factors (e.g. dose, length and timing of administration).
Overall, pre-clinical studies identified in this review aggregate evidence that phyto-cannabinoids, especially CBD, may have protective potential both for amphetamine addiction in several phases (e.g. relapse, abstinence) and for other amphetamine-related consequences (e.g. neurotoxicity). Also, CBD showed some protective effects for amphetamine-induced models of psychosis. Amphetamine-induced sensitization is a useful animal model for assessing human mental health conditions (e.g. schizophrenia, mania) that relies on the behavioural and neurochemical, including toxic, effects of amphetamine (Featherstone et al. 2007;Young et al. 2011). Moreover, investigating CBD's potential on the amphetamine-induced model of mental health can provide insights into behavioural and neuroprotective factors that may be used translationally not only for specific mental illness populations, but also to amphetamine users. For instance, some of the known psychiatric (e.g. acute mania or psychotic) symptoms of amphetamine exposure observed in different populations (Anand et al. 2000;Curran et al. 2004;McKetin et al. 2013;Strakowski and Sax 1998) could be reduced by CBD-related actions.
In contrast, studies in human mostly showed counter-protective effects or no effect on the co-use of cannabis with amphetamine for several outcomes, including brain (e.g. MRS, MRI), mental health, cognition (e.g. neuropsychological performance, psychotic symptoms) and amphetamine use. Several factors may account for the observed discrepancies with the human-based results and the growing body of literature suggesting a therapeutic potential of cannabinoids for addictive, including psychostimulant-and amphetaminerelated behaviours (Abrams 2018;Prud'homme et al. 2015). First, these studies are highly heterogeneous, with fundamental differences in design (e.g. case-control, longitudinal, prospective), cannabis use pattern (e.g. use or dependence), outcomes measured and population studied (e.g. college students, drug dependents). One major difference between the pre-clinical and human studies that may account to the discrepant results is the cannabinoid/cannabis exposure pattern. While all human studies assessed cannabis and amphetamines co-used within a naturalistic form of use and/ or dependence, with no studies assessing the direct administration (e.g. experimental setting) of cannabis or cannabis products (e.g. CBD, THC, Sativex®) on amphetamine users, pre-clinical studies did not evaluate cannabinoids within an abuse model (e.g. THC self-administration learning, conditioned place preference).
Given the non-clinical nature, the human studies included did not control other relevant factors on addiction, such as drug (cannabinoid)-related conditioned response (e.g. eliciting drug desire, triggering relapse) and social interactions (Bardo et al. 2013;Kenny et al. 2003;O'Brien et al. 1998;Pelloux et al. 2019). In addition to the common pathway (i.e. inhalation) that cannabis and amphetamine use may share, the ongoing, naturalistic use of cannabis by participants, often taken as co-dependence, may entail a poly-substance use scenario, which is commonly associated with more cooccurring and/or severe problems (e.g. psychological distress, anxiety, depression) among users (Connor et al. 2013(Connor et al. , 2014. Also, it could be possible that amphetamine users with co-morbid conditions use cannabis in an attempt to self-regulate or self-treat comorbid mental health or cognitive problems experienced. Some qualitative reports, mostly focusing on crack cocaine but also methamphetamine use, highlight intentional cannabis use patterns towards alleviation of psychostimulant-related problems (Andrade et al. 2011;Gonçalves and Nappo 2015;Paul et al. 2020).
On this basis, the absence of more controlled studies (e.g. clinical trials) for psychostimulants, and for amphetamines specifically, still represents a major gap in the field; conducting such studies would allow for a better translation of the pre-clinical findings into human/clinical setting. Despite these translational limitations, some of the human studies in this review suggest that cannabis use, especially when involving subjects with substance use disorder/dependence, may entail increased risks for adverse outcomes or co-morbidities among users, which highlights the importance of considering potential cannabis use-related risks as much as its therapeutic potential among amphetamine users.
Two descriptive studies in human samples reported cannabis use reducing amphetamine use-related harms and detrimental effects, including one qualitative in-depth selfreport study. Similar results have been reported more frequently for the co-use of cannabis with crack cocaine, and some observers have already proposed for cannabis application use a pragmatic, substitutive or therapeutic option for other harmful drug use to be further investigated (Andrade et al. 2011;Gonçalves and Nappo 2015;Lau et al. 2015;Socías et al. 2017). Different dynamics in the context of drug use, motivational aspects or subpopulation specificities could be involved in these contrasting, protective results. However, while descriptive designs with qualitative approaches are relevant to investigate specific populations (Patton 2015), the identified studies are limited in nature since they rely on participants' (or others') subjective perception/beliefs and lack control measures for generalization (e.g. control group, standardized measures, biased sample), and, as such, conclusions are limited. At best, these results, together with the mostly enhanced harm outcomes in human study populations described, suggest that any possible benefits from cannabis use regarding amphetamines addiction is likely strongly dependent on patterns of cannabis use (e.g. timing, motivation, dependence).
Although the limitations of the studies in humans showing some protective effects related to cannabinoids use (i.e. less frequently and/or less controlled than studies finding counter-protective effects), they reinforce, together with the pre-clinical studies' results, the necessity to conduct controlled studies on cannabinoids and their potential to alleviate amphetamine addiction or amphetamine-related deficits. Overall, in addition (or due) to the non-specific cannabinoid's, especially CBD, mechanisms, the therapeutic potential for substance use disorder may crucially vary according to the substance. For example, when considering other types of psychostimulants, two randomized controlled trials with repeated CBD administration on cocaine or crack cocaine users found no differences in cue-induced craving or relapse risk (Meneses-Gaya et al. 2020;Mongeau-Pérusse et al. 2021); conversely, controlled trials found a protective action of CBD for other drug classes (e.g. cannabinoids, opioids), either by reducing cue-induced craving or drug consumption (Freeman et al. 2020;Hurd et al. 2019;Morgan et al. 2013). For the specific case of amphetamines, there are different pathways in which CBD may have therapeutic effects for addiction-related outcomes. CBD agonist actions on serotoninergic and dopaminergic receptors could play a relevant role, given that the agonist action on 5-HT1a receptors can inhibit behaviours related to addiction, and partial dopaminergic agonists have been shown to attenuate stimulants self-administration in pre-clinical models (Calpe-López et al. 2019;Müller and Homberg 2015). Nevertheless, while much emphasis has been given on whether cannabinoids may help with problematic substance use-related craving and relapse, a main role for cannabinoid products may also be able to decrease deleterious biological impact (e.g. neuronal toxicity, psychiatric symptoms) of amphetamines and other drugs, as has been demonstrated by several of the pre-clinical studies included in this review (Castelli et al. 2014;Karimi-Haghighi et al. 2020;Prud'homme et al. 2015;Razavi et al. 2020;Valvassori et al. 2011).
The present review's limited search and selection approach, including English-language studies only, number of databases, exclusive reliance on peer-reviewed (i.e. no grey literature) studies, may imply limitations of the present work. Furthermore, no meta-analyses of the study data were done given the heterogeneous approach and designs. In conclusion, data from pre-clinical studies suggest potentially protective effect of THC and CBD regarding amphetamine (or methamphetamine)-related addiction behaviour and impairments, while human studies, despite some preliminary protective results limited to specific populations, suggest greater morbidity between the co-use of cannabis and amphetamines in a range of (e.g. brain, cognitive, health) outcomes but mostly assessing natural cannabis use or abuse. Given recent evidence of increasing psychostimulant use and harms (e.g. in North America and elsewhere (Farrell et al. 2019;Fischer et al. 2021;Strickland et al. 2019)) and substantive pre-clinical evidence of cannabinoids therapeutic potential for addiction (Chye et al. 2019;Parsons and Hurd 2015), the dearth of systematic, rigorous (e.g. controlled clinical) studies assessing the potentially beneficial effects of cannabinoids on amphetamine-related health or behavioural outcomes should urgently be addressed.
Author contribution BF developed the concept for the study. DDB developed the review strategy and led the data collection and writing for the paper. DDB and LOM extracted, collated and analysed the data. All co-authors contributed to data/results interpretation and provided substantial intellectual content towards iterative drafts and revisions of the paper. All authors approved the final version of the paper submitted.
Funding Prof. Fischer acknowledges support from the endowed Hugh Green Foundation Chair in Addiction Research, Faculty of Medical and Health Sciences, University of Auckland; he furthermore reports research grants and contract funding from public only (e.g. public funding, government) agencies. Didier Jutras-Aswad holds a scholar award from the Fonds de recherche du Québec en Santé and has received investigational product from Insys Therapeutics for a clinical trial funded by the Canadian Institutes of Health Research; he furthermore reports research grants and contract funding from public (e.g. public funding, government) agencies.

Declarations
Ethics aproval This article is based on previously conducted studies and published data and does not involve any studies with human participants or animals performed by any of the authors.

Conflict of interest The authors declare no competing interest.
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