"The study re-analysed the results from 20,000 patients with schizophrenia or other psychotic illnesses from 83 previous studies. The study used meta-analysis a modern statistical method to show that an earlier onset of severe mental illness among substance users is a result of cannabis use, and cannot be explained by other factors such as alcohol use," Dr Large said.
"Results of this study are conclusive and clarify previously conflicting evidence of a relationship between cannabis use and the earlier onset of a psychotic illness, with evidence supporting the theory that cannabis use plays a causal role in the development of psychosis in some patients."
Dr Large said there was a high prevalence of substance use among individuals treated in mental health settings, and patients with schizophrenia were more likely to use substances than members of the wider community.
"The results of this study provide strong evidence that stopping or reducing cannabis use could delay or even prevent some cases of psychosis.
"The study raises the question of whether those substance users would still have gone on to develop psychosis a few years later.
"However, even if the onset of psychosis were inevitable, an extra two or three years of psychosis-free functioning could allow many patients to achieve important developmental milestones of late adolescence and early adulthood that could lower long-term disability arising from psychotic disorders," Dr Large said.
"The results of this study confirm the need for an ongoing public health warning about the potentially harmful effects of cannabis."
|Contact: Steve Offner|
University of New South Wales