Does Cannabis Cause Psychosis?

Cannabis use in Vermont youth is extremely high with about a quarter of 11th and 12th graders having used it in the last 30 days, according to the Vermont Youth Risk Behavior Survey.  While cannabis is still considered to be relatively harmless by many, the health risks associated with its use are becoming increasingly apparent, especially as the THC potency has been increasing.  

As described in a recent edition of Psychiatric Times, one such danger that has been noticed by researchers is psychosis and schizophrenia. The amount of increased risk varies by study, but a 40% increased risk among users which rises to 200% and more for those who smoke regularly has been found in many reports.   Given that the base rate of psychotic illness is not that low, this increased risk translates into a large number of youth who can progress to severe disability. These symptoms often don’t remit when the cannabis use stops.  The risk may be higher for individuals who begin smoking in adolescence and has been found to be related to specific genetic vulnerabilities, as documented in the well known study from New Zealand that showed that cannabis users who had the Val/Val or Val/Met genotype at the Val158Met functional polymorphism of the COMT gene were at especially high risk.  Many of the studies have some methodological complications, however, that limit firmer conclusions.

A recent meta-analysis also published a surprising finding that individuals with schizophrenia had improved cognitive functioning if they had a history of a cannabis use disorder.  However, some have argued that the cannabis use group contains people who would not have developed schizophrenia otherwise and may have a less severe course than those the non-user group who came to their diagnosis through other means. 

Will the prospect of possibly developing schizophrenia deter some adolescents from using cannabis?  While many will likely remain unconvinced, this risk should certainly be part of an important conversation to have with our adolescent patients. 


Caspi A, Moffitt TE, Cannon M, et al. Moderation of the effect of adolescent-onset cannabis use on adult psychosis by a functional polymorphism in the catechol-O-methyltransferase gene: longitudinal evidence of a gene X environment interaction. Biol Psychiatry. 2005;57:1117-1127.

Rabin RA, Zakzanis KK, George TP. The effects of cannabis use on neurocognition in schizophrenia: a meta-analysis. Schizophr Res. 2011;128:111-116.

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