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Impact of Cannabis and Other Drugs on Age at Onset of Psychosis

J Clin Psychiatry 2008;69(7):1210-1216

Objective: The aim of this study was to investigate the relationship between age and cannabis use in patients with a first psychotic episode, and to analyze the mediating effect of comorbid use of other drugs and sex on age at onset of psychosis.

Method: All consenting patients (aged 15 to 65 years) with a first psychotic episode needing inpatient psychiatric treatment during a 2-year period between February 1997 and January 1999 were considered, confirming a total of 131 patients. Subjects were interviewed using the Structured Clinical Interview for DSM-IV Axis I Disorders, and clinical and demographic data were collected. We used general linear models with age at onset as the response variable and survival Cox models to confirm the results. Both a multivariate linear model and the corresponding Cox model were fitted with a covariate that summarizes the most significant contributors that seemed to decrease age at onset.

Results: Regarding the effect of cannabis use, a significant gradual reduction on age at onset was found as dependence on cannabis increased, consisting in a decrement of 7, 8.5, and 12 years for users, abusers, and dependents, respectively, with respect to nonusers (p = .004, p < .001, and p < .001, respectively). Multivariate analysis showed a clear effect of cannabis use on age at onset, which was not explained by the use of other drugs or by gender. The finding was similar in the youngest patients, suggesting that this effect was not due to chance.

Conclusion: The major contribution of this investigation is the independent and strong link between cannabis use and early age at onset of psychosis, and the slight or nonexistent effect of sex and comorbid substance abuse in this variable. These results point to cannabis as a dangerous drug in young people at risk of developing psychosis.