Cannabis use linked to early onset of psychosis

Debate has long raged about the link between cannabis and psychotic illnesses such as schizophrenia - with some claiming that the drug causes it and others that people with mental illnesses may just be more likely to be drug users.

Now a new study by scientists at the University of New South Wales in Australia has set out to answer this question by reviewing the available evidence.

"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 wrote.

The results suggests that people with mental illness who also use cannabis develop the disease at an earlier age that other patients who abstain.

In fact, cannabis users 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.

Commenting on the results, the authors state that "this study provides strong evidence that reducing cannabis use could delay or even prevent some cases of psychosis".

They also note that the evidence suggests that cannabis use "precipitates schizophrenia and other psychotic disorders, perhaps through an interaction between genetic and environmental disorders or by disrupting brain development."

In conclusion, they call for "a renewed public health warning about the potential for cannabis use to bring on psychotic illness."

The results of the study are published in the Archives of General Psychiatry.

This article was published on Tue 8 February 2011