Does accumulating exposure to illicit drugs bring forward the age at onset in schizophrenia?

The article reports a study which was conducted to test the hypothesis that consumption of amphetamine and/or cocaine in addition to cannabis would lead to an earlier age at onset of schizophrenia than that seen for cannabis consumption alone. The results confirmed the hypothesis. These findings suggest that additional consumption of each substance predicted an earlier age at onset by approximately 1 additional year.