Background and objectives: Observational studies have reported earlier onset of psychosis in schizophrenic patients with a history of cannabis use. Earlier age of onset of schizophrenia has been associated with a poorer outcome. We aimed to examine whether cannabis use determined an earlier onset of schizophrenia in a sample of first episode patients, in an area with one of Europe's highest rates of cannabis use.
Methods: 116 subjects with first episode psychosis and subsequent diagnosis of schizophrenia (after a 12-month follow-up) were included Age at first antipsychotic treatment (A1T) was used as proxy for age of psychosis onset, and acted as dependent variable for the statistical analysis. Cannabis use was evaluated retrospectively, and divided into three groups according to peak frequency (never, sporadic/frequent, daily).
Results: 46 (39.7%) subjects had never used cannabis, 23 (19.9%) had done so sporadically/frequently, and 47 (40.5%) daily. A1T differed between the three groups (mean, in years and [SD]: 27.0 [4.94]; 25.7 [4.44] and 24.5 [4.36]; p=0.033) and diminished as cannabis use increased (linear tendency; p=0.009). Post-hoc analysis showed that cannabis use (irrespective of frequency) was significantly associated with decrease in A1T (p=0.033), as shown by the first contrast [1 -1/2 -1/2]. Post-hoc contrast showed that cannabis users had a significantly lower age of onset of psychosis (mean decrease, in years: 1.93; CI (confidence interval) 95%: 0.17-3.70; p=0.033).
Conclusions: Cannabis use was significantly associated with a decrease in age of onset of schizophrenia. Age of onset of the disease correlated with frequency of cannabis use.