Cannabis use linked to earlier psychosis onset

Cannabis use linked to earlier psychosis onset

By Springer
July 26, 2011
MedWire News: Cannabis use is associated with an earlier onset of psychosis, conclude US and Australian scientists in findings that support the notion that cannabis has a causal association with the development of psychosis in some patients.

While it is clear that there is a link between substance use, particularly cannabis, and psychotic illness, some researchers have argued that the link could be explained by confounding variables such as demographic factors.

To investigate further, Matthew Large, from Prince of Wales Hospital in Randwick, New South Wales, Australia, and colleagues searched multiple literature databases for peer-reviewed publications reporting age at onset of psychosis in substance-using and nonsubstance-using groups.

Substances included cannabis, alcohol, and other psychoactive drugs/chemicals; tobacco was not included. In all, 443 articles were identified, of which 83, comprising a total of 8167 substance-using patients and 14,352 nonsubstance-using patients, met the inclusion criteria.
A meta-analysis of the studies revealed that the age at onset of psychosis was 2.7 years lower among cannabis users, and 2.0 years lower among nonspecified substance users, than in nonsubstance-using controls.

The mean age of the substance-using patients across all samples was 1.73 years younger than in controls. The pooled effect size of substance use on psychosis onset was greater in heavy or continuous users than in those who were lighter users or had stopped use, at -2.72 years versus -2.07 years, respectively, compared with controls. However, this difference was not statistically significant.

On both meta-regression and multiple meta-regression, a higher proportion of cannabis users in substance-using groups was associated with a greater negative effect size, suggesting an earlier mean age at onset of psychosis.

There was no association between psychosis age at onset and alcohol use.
The team writes in the Archives of General Psychiatry: "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."

MedWire (www.medwire-news.md ) is an independent clinical news service provided by Springer Healthcare Limited. © Springer Healthcare Ltd; 2011
By Liam Davenport