Prospective cohort study of cannabis use, predisposition for psychosis, and psychotic symptoms in young people

Objective: To investigate the relation between cannabis use mid psychotic symptoms in individuals with above average predisposition for psychosis who first used cannabis during adolescence. Design: Analysis of prospective data from a population based sample. Assessment of substance use, predisposition for psychosis, and psychotic symptoms was based on standardised personal interviews at baseline and at follow tip four years later. Participants: 2437 young people (aged 14 to 24 years) with and without predisposition for psychosis. Main outcome: measure Psychotic symptoms at follow up as a function of cannabis use mid predisposition for psychosis at baseline. Results: After adjustment for age, sex, socioeconomic status, urbanicity childhood trauma, predisposition for psychosis at baseline, and use of other drugs, tobacco, and alcohol, cannabis use at baseline increased the cumulative incidence of psychotic symptoms at follow up four years later (adjusted odds ratio 1.67, 95% confidence interval 1.13 to 2.46). The effect of cannabis use was much stronger in those with any predisposition for psychosis at baseline (23.8% ad. Listed difference in risk, 95% confidence interval 7.9 to 39.7, P = 0.003) than in those without (5.6%, 0.4 to 10.8, P = 0.033). The risk difference in the "predisposition" group was significantly greater than the risk difference in the "no predisposition" group (test for interaction 18.2%, 1.6 to 34.8, P = 0.032). There was a close-response relation with increasing., frequency of cannabis use. Predisposition for psychosis at baseline did not significantly predict cannabis use four years later (adjusted odds ratio 1.42, 95% confidence interval 0.88 to 2.31). Conclusion: Cannabis use moderately increases the risk of psychotic symptoms in young people but has a much stronger effect ill those with evidence of predisposition for psychosis.