Cannabis use accelerates the onset of psychotic illnesses, according to a new study published in the Archives of General Psychiatry today. Meta-analysis of data collected over 30 years, from over 20,000 patients, concludes the “age at onset of psychosis for cannabis users was 2.7 years younger than for nonusers”.

Lead author Dr Mathew Large from the University of New South Wales School of Psychiatry and Sydney’s Prince of Wales Hospital says the findings add more weight in understanding the link between mental illness and cannabis use, particularly in adolescents.

“I am surprised by the findings,” he told Crikey. “Younger people that smoke cannabis will accelerate the onset of psychotic illnesses.”

According to the National Cannabis Prevention and Information Centre, cannabis is the most commonly used illicit drug in Australia. The latest statistics reveal one in three Australians have reportedly used the drug in their lifetime, with 18% of secondary school students having used cannabis.

“An extra two or three years of psychosis-free functioning could allow many patients to achieve the important developmental milestones of late adolescence and early adulthood that could lower the long-term disability arising from psychotic disorders,” the paper states.

While three years doesn’t sound like a long period of time, Dr Large says psychotic disorders such as schizophrenia create a “high level of disability”.

“Schizophrenia comes on early in life — three years can make the difference between completing school or not,” he said. “[The findings are] important because schizophrenia is such a bad disorder to have — you hear voices, you see things and you believe things that are not real. It makes it difficult to have relationships, work and finish school.”

The paper draws on published and non-published work over the last 30 years to raise the question of “whether those substance users would still have gone on to develop psychosis a few years later”. “The results of this study provide strong evidence that reducing cannabis use could delay or even prevent some cases of psychosis,” the paper states.

“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.”

Professor Jan Copeland, Director of the National Cannabis Prevention and Information Centre, says the findings will provide a framework to deal more effectively with the issue.

“It is very important new evidence of the risks associated with cannabis use, particularly in adolescents,” she told Crikey, advocating the evidence be used in campaigns against the use of the drug. “It is clear and precise information that should be incorporated into general education and programs targeted at high risk people.”

Professor Copeland identified high-risk people to be adolescents not doing well at school, children in the criminal justice system, Aboriginal and Torres Strait Islanders and children with a personal and family history of mental illnesses. According to the Australian Bureau of Statistics’ most recent figures, the rates of mental illnesses are higher for men aged 16-34 and women aged 16-24 compared with older age groups.

Dr Large says the most important question the study raised was what to do with the findings. Rather than campaigns in Australia targeting older people, programs should focus on the relationship between parents and children.

“Most people at risk are under the age of 16,” he said. “Programs should target parents and their attitudes towards marijuana.”

But Keven Kellehear, chairperson of the Management Committee of the Mental Health Services Conference of Australia and New Zealand, and lecturer at the University of Technology in Sydney, says we are yet to find out if anyone benefits from the use of cannabis.

“There is some literature that comes from accounts of the use of cannabis by consumers who report how it helps them to manage symptoms and experiences of psychosis and other mental health problems,” he told Crikey. “But of course the definitive research is yet to find who will benefit and who not benefit from the use of cannabis. The earlier the age, the most likely the development of psychotic illness and more likely the development of more serious disability.”

In July 2006, the Council of Australian Governments endorsed a five-year national action plan on mental health. The Commonwealth is implementing 17 methods to improve services for people with mental illnesses, their families and carers under the plan.