A protester smokes marijuana on Parliament Hill in 2003. ((Jonathan Hayward/Canadian Press))

Smoking pot mayincrease the risk for the type of psychosis commonly associated with schizophrenia, and the risk increases with each puff, a new British study reports.

Occasional smokers were 40 per cent more likely to suffer from psychosis, while dedicated tokers who used marijuana daily or weekly increased their risk by 50 to 200 per cent.

According tothe findings, however, the risk of developing psychoses remains low. About five in 1,000 people have schizophrenia.

Marijuana is one of the most commonly used drugs in the world. According to the 2007 UN World Drug Report, 16.8 per cent of Canadians aged 15 through 64, approximately five million people,smoked pot or used another marijuana product in 2006.

"The available evidence now suggests that cannabis is not as harmless as many people think," said Dr. Stanley Zammit, one of the authors of the study, which will be published Friday in the medical journal the Lancet.

Zammit and his colleagues examined 35 studies that tracked tens of thousands of people for periods ranging from one to 27 years to examine the effect of marijuana on mental health.

The researchers looked for psychotic illnesses as well as cognitive disorders including delusions and hallucinations, bipolar disorder, depression, anxiety, neuroses and suicidal tendencies.

Zammit said the research couldn't prove that marijuana use itself increases the risk of psychosis, as there could be something else about marijuana users, "like their tendency to use other drugs or certain personality traits, that could be causing the psychoses."

Wende Wood, drug use and drug information pharmacist with the Centre for Addiction and Mental Health in Toronto, agrees. Shesays chronic pot use can push people predisposed to psychosis over the edge.

"There needs to be several factors involved, and marijuana might be one of them that might trigger it earlier," Wood told CBC News. "We're talking about people thinking unusual thoughts, hearing voices, hearing things that are not true, that are different from the rest of society."

Scientists think it is biologically possible that marijuana could cause psychoses because it interrupts neurotransmitters, such as the chemical messenger dopamine, which can interfere with the brain's communication systems.

With files from the Associated Press