Rates of cannabis use and misuse are particularly high in those with psychotic disorders, especially those with early onset of these illnesses. For instance, Koskinen and colleagues (50) report a mean lifetime rate of 27.1% for cannabis use disorders in a meta-analysis of schizophrenia patients while another systematic meta-analysis (51) revealed that cannabis users, on average, had an onset of psychotic illness 2.7 years prior to non-users. Prospective studies have been remarkably informative as well, showing that the pooled (meta-analysis) odds of developing psychosis in cannabis users is 2.1 (52). The association is particularly strong in those who start to use cannabis at an early age – for example, Arseneault and colleagues (53) found that rates of schizophreniform disorder in those who had used cannabis by age 15 was more than threefold greater than in others.
