Cannabis users show earlier onset of psychoses

Users of cannabis and other psychoactive substances have an earlier onset of psychotic illness compared to nonusers, according to a meta-analysis of 83 studies published online in the Archives of General Psychiatry.

The 2009 National Survey on Drug Use and Health found that more than 16 million Americans use cannabis on a regular basis, and most users began using cannabis and other drugs during their teenage years.

Australian researchers identified 83 studies in which age at onset of psychosis and prior substance use was reported. In these studies, 8,167 individuals reported a history of cannabis or other substance use; 14,352 individuals reported no such use.

Age at onset of psychosis was 2.70 years earlier among the cannabis users and 2.00 years earlier among those reporting unspecified substance use compared with those who did not report substance use. Age at onset of psychosis was not affected by the use of alcohol only.

The researchers state a number of hypotheses proposed to explain the link between cannabis use and schizophrenia, noting that their study supports “the view that cannabis use precipitates schizophrenia and other psychotic disorders, perhaps by an interaction between genetic and environmental factors . . . or by disrupting brain development.”

They add that their findings “provide strong evidence that reducing cannabis use could delay or even prevent some cases of psychosis. Reducing the use of cannabis could be one of the few ways of altering the outcome of the illness because earlier onset of schizophrenia is associated with a worse prognosis and because other factors associated with age at onset, such as family history and sex, cannot be changed.”

A public health warning about the potential for cannabis use to influence psychotic illness is needed, they believe.

Large M, Sharma S, Compton MT, Slade T, Nielssen O. Cannabis use and earlier onset of psychosis: a systematic meta-analysis. Arch Gen Psychiatry. 2011. Epub ahead of print.