Aim:
This review analyzes the relationship between cannabis use and psychotic, bipolar, depressive and anxiety disorders, as well as suicide. It summarizes epidemiological evidence from cross-sectional and long-term prospective studies and considers possible etiological mechanisms.
Methods:
Systematic reviews and methodologically sound studies in the field (from inception to February 2019) were identified through a comprehensive search of Medline, PsychINFO, and Embase and summarized using a narrative synthesis.
Results:
Consistent evidence, from both observational and experimental studies, has confirmed the important role of cannabis use in the onset and persistence of psychotic disorders.
The effect size is related to the degree of cannabis use, with a higher risk of early cannabis use and the use of high potency strains and synthetic cannabinoids . Accumulating evidence suggests that frequent cannabis use also increases the risk of mania and suicide. However, the effect on depression is less clear and findings on anxiety are contradictory with only a few methodologically sound studies.
Additionally, the relationship with common mental disorders may imply reverse causality , as depression and anxiety are reported to lead to increased cannabis use in some studies.
Pathogenetic mechanisms focus on the effect of tetrahydrocannabinol (THC, the main psychoactive ingredient in cannabis) interacting with genetic predisposition and perhaps other environmental risk factors.
Cannabidiol (CBD), the other important ingredient in traditional cannabis, enhances the psychogenic effects of THC but is absent from the high-potency strains that are increasingly available.
Conclusions:
The evidence that heavy use of high-THC/low-CBD types of cannabis increases the risk of psychosis is strong enough to warrant public health education.
The evidence for similar but smaller effects in mania and suicide is growing, but is unconvincing for depression and anxiety. There is much current interest in the possibility that CBD may be therapeutically useful.
