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This Is Your Brain on Drugs: The Marijuana-Psychosis Connection Revisited

In this German population-based study, cannabis use clearly increased the risk for any psychotic symptom.

Some studies have suggested cannabis use as a risk factor for psychosis, but these studies failed to control for predisposition to psychosis and other confounders. In a German population-based study, researchers used personal interviews at baseline and 4 years later to examine the effects of marijuana use in 2437 young subjects (age range, 14-24). Prevalence of lifetime cannabis use (at least 5 times) was 13% at baseline and 15% at follow-up. Lifetime incidence of one psychotic symptom (based on the Composite International Diagnostic Interview) at follow-up was 17% and incidence of at least two symptoms was 7%. Researchers adjusted results for self-reported psychoticism and paranoia scores at baseline ("psychotic predisposition"), demographics, head-trauma history, and use of other drugs and alcohol.

In a logistic-regression analysis, cannabis use significantly increased risk for any psychotic symptom, with a clear dose-response effect. Psychotic predisposition significantly increased this risk, which was greater in subjects with at least two psychosis symptoms than in those with one symptom. The population-attributable risk (proportion of cases that could be avoided by eliminating the risk factor) was 6% overall and 14% for participants with psychotic predisposition.

Comment: Cannabis use clearly increased the risk for any psychotic symptom; having more symptoms, psychotic predisposition, or more frequent cannabis use strengthened this association. These findings are consistent with cannabis effects on increasing dopamine release in the frontal lobe, with the increase in cannabinoid receptors in schizophrenic brains, and with increased levels of endogenous cannabinoids in the spinal fluid of schizophrenic individuals. Given the minimal level of psychotic symptoms examined in this study, its findings may pertain more to schizophrenic-spectrum and atypical-psychotic conditions than to more narrowly defined schizophrenia.

— Peter Roy-Byrne, MD

Published in Journal Watch Psychiatry January 27, 2005

Citation(s):

Henquet C et al. Prospective cohort study of cannabis use, predisposition for psychosis, and psychotic symptoms in young people. BMJ 2005 Jan 1; 330:11. http://bmj.bmjjournals.com/cgi/content/full/330/7481/11

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