Cannabis and mental illness : a review
Darby J. E. Lowe · Julia D. Sasiadek · Alexandria S. Coles · Tony P. George
European Archives of Psychiatry and Clinical Neuroscience, 2018, 1-14.
Doi : 10.1007/s00406-018-0970-7
With the increasing push to legalize cannabis in Western nations, there is a need to gage the potential impact of this policy change on vulnerable populations, such as those with mental illness, including schizophrenia, mood, and anxiety disorders. This is particularly important as there are strong motives in these individuals to seek short-term reward (e.g., “getting high”). Nonetheless, data to support the beneficial effects of cannabis use in psychiatric populations are limited, and potential harms in patients with psychotic and mood disorders have been increasingly documented. This article reviews the effects of cannabis in people with mental illness. Then, we provide a reconciliation of the addiction vulnerability and allostatic hypotheses to explain co-morbidity addiction in mentally ill cannabis users, as well as to further aid in developing a rational framework for the assessment and treatment of problematic cannabis use in these patients.
Keywords : Cannabis · Addiction · Therapeutics · Harms · Schizophrenia · Mood disorders · Anxiety disorders · Post- traumatic stress disorder · Legalization
In recent years, there has been a shift surrounding societal and legal perspectives on cannabis. In 2017, past-year rates of cannabis use in Canada were approximately 43% in individuals 16–24 years , and 18% in individuals over 25 years . Moreover, there has been an increasing legalization of cannabis throughout the United States, as well as nationwide legalization in Canada as of 17 October 2018. The trend in legalization coincides with heightened acceptance, reduced perception of risk, and an increase in cannabis use in both adults and adolescents [23, 112, 130]. Based on the experience from commercial tobacco and the few examples of legalized recreational cannabis use (e.g., American states, such as Colorado and Washington, and in Uruguay), there is strong evidence that cannabis prevalence may increase [66, 121], which would have a disproportionate burden on those with mental illness . Moreover, there will be clear societal benefits of cannabis legalization, such as shifting law enforcement away from minor crimes such as possession, which will benefit people with mental illness. Thus, from both a policy and clinical perspective, it is vital to examine the epidemiological and scientific trends of cannabis use alongside a projected growing user population.
Cannabis contains various cannabinoids that compose the Cannabis Sativa or Indica plant, such as cannabidiol (CBD) and delta-9-tetrahydrocannabinol (THC), which have been a primary clinical focus for research and clinical interests. CBD and THC both induce clinical effects through their influence on the endogenous endocannabinoid system; how- ever, the observable impact of these pharmacological com- ponents differs significantly. The psychoactive component of cannabis involves effects produced primarily by THC’s partial agonistic effects on cannabinoid-1 receptors (CB1R’s), which generates the “high” that users feel. In contrast, CBD has been shown in the early studies to exhibit potential ther- apeutic effects (e.g. antipsychotic, anxiolytic, anti-craving pro-cognitive, and neuroprotective effects), as it appears to have differential, and somewhat opposing, pharmacological effects compared to THC . Recreational cannabis, both unregulated and regulated, is prepared with various com- binations of THC and CBD. Overtime, however, the most common cannabis preparations have come to consist of high THC and lower CBD potency . Due to the complex pharmacological makeup of cannabis products, as well as the diffuse distribution of cannabinoid receptors throughout the brain that influence a variety of neurotransmitters, the clinical effects range from euphoria and relaxation to panic anxiety and psychosis.
Problematic cannabis use [e.g., cannabis use disorder (CUD)] is much higher in individuals with mental illness, including schizophrenia, mood and anxiety disorders, personality disorders, and post-traumatic stress disorder, compared to the general population [14, 17, 22, 67, 90]. Psychiatric symptoms have been shown to predict not only problematic cannabis use, but also the perception of cannabis as harmless [11, 131]. In addition, as cannabis use becomes more clinically severe, a stronger correlation has been described between a diagnosis of CUD and concur- rent psychiatric disorder . Despite this high prevalence, well-controlled studies have suggested more harms than therapeutic benefits from recreational cannabis use in these populations; however, further research is needed.
This review will examine the paradox between the high prevalence of cannabis use in mentally ill populations and evidence for the therapeutic potential of cannabis. To under- stand this paradox, the addiction vulnerability  and self- medication  hypotheses will be used as pillars to guide the analysis in determining whether cannabis (and its constituents) has true therapeutic potential versus whether its effects are more harmful.
A review of the effects of cannabis in mental illness
There are several lines of evidence suggesting either thera- peutic potential or harmful effects associated with cannabis use in psychiatric populations. In this section, we provide an overview of the current scientific literature on the effects of cannabis across schizophrenia, mood disorders, anxiety disorders, and post-traumatic stress disorder (PTSD). A recent report by the National Academies of Sciences  assessed the potential therapeutic effects and harms of can- nabis in detail, concluding that there are many gaps of which remain across the scientific literature. To understand what the evidence currently suggests about both the acute and long-term effects of cannabis in mental illness, high-quality studies (see Table 1) will be discussed. Search strategies for this review included PubMED and PSYCHinfo to select appropriate studies for the analysis.