Cannabis and Neuropsychiatry, 2: The Longitudinal Risk of Psychosis as an Adverse Outcome.

Psychosis is one of the most serious among the adverse effects associated with cannabis use. The association between cannabis use and psychosis has been variously explored in a series of recent meta-analyses. The results of these meta-analyses show that persons who develop psychosis experience onset of psychosis about 2-3 years earlier if they are cannabis users; this effect is not observed with alcohol or other substance use. Higher levels of cannabis use are associated with greater risk of psychosis. Current cannabis abuse or dependence (but not past use or lower levels of current use) increases the risk of transition into psychosis in persons at ultrahigh risk of psychosis. About a third of patients with first-episode psychosis are cannabis users, and, at follow-up, about half of these users are found to continue their cannabis use. Continued cannabis use (in those who are treated after developing psychosis) is associated with higher risk of relapse into psychosis, and discontinuation of cannabis use reduces the risk of relapse to that in cannabis nonusers. Finally, persons with psychosis who continue to use cannabis have more severe positive symptoms and poorer levels of functioning. Because experimental studies in humans show that cannabinoids and cannabis can induce psychotic symptoms, it is reasonable to assume that the epidemiologic data indicate a causal effect of cannabis in anticipating, triggering, or exacerbating psychosis in vulnerable individuals and in worsening the course and outcome of the illness in those who continue to use the substance. Given the public health implications of these findings, the trend to legalize medical marijuana must be viewed with concern, and efforts are necessary to educate patients and the public about the serious mental and physical health risks associated with cannabis use and abuse.