Abstract
Adults suffering from schizophrenia have high rates of comorbid cannabis use, but it is not known whether adolescent cannabis use acts as a risk factor for developing psychosis subsequently. We were able to examine this issue in the Dunedin Multidisciplinary Health and Development Study. In this birth cohort of 26-year olds from New Zealand, past-year schizophreniform disorder was assessed using standardised DSM-IV interviews, self-reports of cannabis use were collected at ages 15 and 18, and self-reports of psychotic symptoms Were abtained at age 11 before cannabis use began. We found that study members who had used cannabis frequently during adolescence had elevated schizophrenic symptoms as adults, even after taking into account childhood psychotic symptoms. However, only those who had begun frequent use by age 15 had a significantly elevated risk of schizophreniform disorder (OR=4.5; 95% CI= 1.1 - 18.2). After controlling for childhood psychotic symptoms this risk remained high (OR 3.1; 95% CI 0.7-13.3). Our study therefore found that adolescent cannabis use is associated with an increased risk of schizophreniform disorder and schizophrenia symptoms in adulthood. Our study adds 3 new pieces of evidence to the literature ( 1 ) cannabis use is associated with an increased risk for experiencing schizophrenia symptoms, even after controlling for psychotic symptoms preceding the onset of cannabis use, thereby ruling out the possibility that early cannabis use is consequent to early emerging psychosis. (2) Early cannabis use (before age 15) confers the greatest risk for schizophrenia outcomes (3) the risk is specific to cannabis use rather than use of other drugs, and cannabis use specifically predicts schizophreniform outcomes and not depressive disorder.