The Association of Cannabis Use with Onset of Psychosis: Still Controversial

A number of studies have found an association between cannabis use and earlier onset of psychosis, but this relationship is controversial. This meta-analysis combined data from 83 peer-reviewed English-language publications that reported substance use and age at onset of psychosis. These studies included 131 samples comprised of 8167 substance-using and 14,352 non-substance-using persons.

• Age at onset of psychosis was 2.7 years younger for people with cannabis use (whether cannabis users also used alcohol was not reported) and 2.0 years younger for people with unspecified substance use compared with those who had no substance use.

• Alcohol use alone was not associated with age at onset of psychosis.

• No statistical evidence was found for publication bias.

Comments:

This pooled analysis presents evidence for an association between cannabis use and earlier onset of psychotic illness. The association with other substance use (but not alcohol) raises the possibility that people with a propensity to develop psychosis are more likely to use substances like cannabis, perhaps to “self-treat” preclinical symptoms. Cannabis use also produces neurocognitive symptoms, such as transient hallucinations or paranoia, whose presence might lead to earlier detection of nascent psychosis. Thus, this study cannot settle the causal question of whether cannabis use precipitates psychosis in genetically predisposed young people.

Peter D. Friedmann, MD, MPH

Reference:

Large M, Sharma S, Compton MT, et al. Cannabis use and earlier onset of psychosis: a systematic meta-analysis. Arch Gen Psychiatry. 2011;68(6):555–561.