Cannabis use and age at onset of schizophrenia.

Objective: The purpose of the study was to assess the independent influences of gender and cannabis use on milestones of early course in schizophrenia.

Method: In this population-based, first-contact incidence study conducted in The Hague, the Netherlands, patients (N=133) were interviewed with the Comprehensive Assessment of Symptoms and History, and key informants were interviewed with the Instrument for the Retrospective Assessment of the Onset of Schizophrenia. Milestones of early course were 1) first social and/or occupational dysfunction, 2) first psychotic episode, and 3) first negative symptoms.

Results: Male patients were significantly younger than female patients at first social and/or occupational dysfunction, first psychotic episode, and first negative symptoms. Cannabis-using patients were significantly younger at these milestones than were patients who did not use cannabis. Multivariate analyses showed that cannabis use, but not gender, made an independent contribution to the prediction of age at first psychotic episode: male cannabis users were a mean of 6.9 years younger at illness onset than male nonusers. In contrast, age at first social and/or occupational dysfunction and the risk of developing negative symptoms before the first contact with a physician for treatment of possible psychotic disorder were predicted by gender, but not by cannabis use.

Conclusions: The results indicate a strong association between use of cannabis and earlier age at first psychotic episode in male schizophrenia patients. Additional studies examining this possibly causal relationship are needed.