Effects of cannabis use on age at onset in schizophrenia and bipolar disorder

Cannabis use may decrease age at onset in both schizophrenia and bipolar disorder, given the evidence for substantial phenotypic and genetic overlap between both disorders.

766 patients, aged 16 to 65 years, were assessed with the Composite International Diagnostic Interview (CIDI) for substance abuse/use. 676 subjects were diagnosed with schizophrenia and 90 subjects with bipolar disorder. The influence of cannabis use on age at onset in both schizophrenia and bipolar disorder was examined using regression analysis.

Cannabis and other substance use was more frequent in patients with schizophrenia compared to the bipolar group. Both cannabis use and a schizophrenia diagnosis predicted earlier age at onset. There was a significant interaction between cannabis use and diagnosis, cannabis having a greater effect in bipolar patients. Age at onset in users of cannabis was comparable in both diagnostic groups whereas bipolar non-users were significantly older than schizophrenia non-users at onset.

Cannabis use may decrease age at onset in both schizophrenia and bipolar patients and reduce the effect of diagnosis. This is consistent with the view that cannabis use may unmask a pre-existing genetic liability that is partly shared between patients with schizophrenia and bipolar disorder.