Cannabis is the most widely used addictive substance after tobacco and alcohol. The 2009 National Survey on Drug Use and Health reported that more than 16 million Americans use cannabis on a regular basis, most of whom started using cannabis and other drugs during their teenage years. There is little doubt about the existence of an association between substance use and psychotic illness. National mental health surveys have repeatedly found more substance use, especially cannabis use, among people with a diagnosis of a psychotic disorder. There is a high prevalence of substance use among individuals treated in mental health settings, and patients with schizophrenia are more likely to use substances than members of the wider community. Prospective birth cohort and population studies suggest that the association between cannabis use and later psychosis might be causal, a conclusion supported by studies showing that cannabis use is associated with an earlier age at onset of psychotic disorders, particularly schizophrenia.
Not all researchers agree that the association between cannabis use and earlier age at onset is causal. Sevy et al argue that the association between cannabis use and earlier age at onset could be explained by demographic variables, including lower socioeconomic status and the proportion of male cannabis users. Wade has suggested that the apparent association between earlier age at onset and cannabis use might simply be owing to older patients with first-episode psychosis being less likely to use cannabis.
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Copyright: 2011 American Medical Association, February 2011