Cannabis use appears to be associated with an earlier onset of psychotic illness, according to a meta-analysis of previously published studies posted online today that will appear in the June print issue of Archives of General Psychiatry, one of the JAMA/Archives journals.

More than 16 million Americans use cannabis on a regular basis, most of whom began using this and other drugs during their teenage years, according to background information in the article. "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," the authors write.

Matthew Large, B.Sc. (Med.), M.B.B.S., F.R.A.N.Z.C.P., of Prince of Wales Hospital, New South Wales, Australia, and colleagues identified 83 studies involving 8,167 participants who used cannabis or other substances and 14,352 individuals who did not. All of the studies compared the age at onset of psychosis between those two groups.

The meta-analysis found that individuals who used cannabis developed psychosis about 2.7 years younger than those who did not use cannabis. Those who used any type of substance developed psychosis about two years younger, whereas the use of alcohol only was not associated with the age at onset of psychosis.

"A number of hypotheses have been proposed to explain the association between cannabis use and schizophrenia, including the following: (1) that cannabis use is a causal factor for schizophrenia; (2) that cannabis use precipitates psychosis in vulnerable people; (3) that cannabis use exacerbates symptoms of schizophrenia; and (4) that people with schizophrenia are more likely to use cannabis," the authors write. The current findings support the view that cannabis use precipitates schizophrenia and other psychotic disorders, perhaps through an interaction between genetic and environmental disorders or by disrupting brain development, they note.

"The results of this study 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," the authors conclude. "The results of this study confirm the need for a renewed public health warning about the potential for cannabis use to bring on psychotic illness."