Smoking marijuana can increase your risk of developing a psychotic illness by more than 40 per cent, authors conclude in a controversial study in this week's edition of The Lancet.

The study suggests that even occasional pot use could raise the risk of psychosis, a category of mental disorders that includes schizophrenia. The authors say the findings underline the need to remind marijuana users of the long-term risks.

Dr. Theresa Moore, University of Bristol, and Dr. Stanley Zammit, Cardiff University, and colleagues conducted a meta-analysis of 35 studies, dated up to 2006, to see whether there was evidence to connect cannabis use to mental health disorders.

They found that those who smoked pot were 41 per cent more likely to develop a psychotic illness than those who had never used the drug.

The most frequent pot smokers were more than twice as likely to develop a psychotic illness than non-smokers -- a 50 to 200 per cent higher chance.

Nevertheless, the overall risk for all marijuana smokers remains very low, the authors note.

The researchers say governments should now work to dispel the misconception that marijuana is a benign drug.

"We have described a consistent association between cannabis use and psychotic symptoms, including disabling psychotic disorders," the authors say.

"We believe that there is now enough evidence to inform people that using cannabis could increase their risk of developing a psychotic illness later in life."

The 2007 World Drug Report issued by the United Nations Office on Drugs and Crime found that 16.8 per cent of Canadians between the ages of 15 and 64 reported using marijuana at least once in the past year -- four times more than the global average of 3.8 per cent.

Previous research has highlighted the link between marijuana and the risk of schizophrenia-like symptoms, such as paranoia and hallucinations. Some scientists have speculated that cannabis could cause psychosis because it interrupts important neurotransmitters, such as dopamine. That can interfere with the brain's communication systems.

The researchers on this latest study concede that they can't prove that marijuana itself increases the risk of psychosis, or whether those who choose to smoke marijuana have certain personality traits the pre-dispose them to developing psychosis.

It's also possible that pre-existing mental conditions lead many to both marijuana use and psychoses.

The Associated Press reports that two of the authors of this study were invited to sit as experts on the Advisory Council on the Misuse of Drugs Cannabis Review in 2005. Several authors reported being paid to attend drug company-sponsored meetings related to marijuana, and one received consulting fees from companies that make antipsychotic medications.