We analyzed the association of age at onset of psychosis(AOP) with having a history of cannabis use in patients with a first episode of non-affective psychosis(FENAP) and investigated the impact on the AOP of exposure to cannabis in adolescence, compared with young adulthood, and of the additional exposure to cocaine.

We recruited 112 consecutive patients with a FENAP. CIDI was used to assess drug use and to define the age at onset of heaviest use(AOHU) of a drug, as the age when drug was used the most for each patient. The effect of cannabis and cocaine AOHU on AOP was explored through Kruskal-Wallis and Mann-Whitney tests, and logistic regression. Sex-adjusted cumulative hazard curves and Cox regression models were used to compare the AOP of patients with and without a history of cannabis use, or associated cocaine use.

AOP was significantly associated with the use of cannabis, independently of sex, use of cocaine, tobacco smoking or excessive alcohol consumption. There was a dose-response relationship between cannabis AOHU and AOP: the earlier the AOHU the earlier the AOP. Hazard curves showed that patients with a history of cannabis use had a higher hazard of having a first episode psychosis than the rest of the patients (sex-adjusted log rank χ2=23.43,df=1, p< 0.001). Their respective median AOP (25th, 75th percentiles) were 23.5 (21,28) and 33.5 years (27,45) (for log-transformed AOP, t=5.6, df=110, p< 0.001).

Our results are in favor of a catalytic role for cannabis use in onset of psychosis.