Using cannabis even once can significantly increase a person's risk of suffering from a psychotic illness later in life, according to research published this week in the British medical journal The Lancet.
Researchers found that people who had used marijuana at least once were 41 per cent more likely to experience psychosis, including schizophrenia, hallucinations and hearing voices, than those who had never used the drug.
Frequent pot smokers - which are defined as daily or weekly users of marijuana - had a 50 to 200 per cent greater likelihood.
The government-funded research was published with an accompanying editorial that concluded: "There is now sufficient evidence to warn young people that cannabis use will increase their risk of psychosis."
Stanley Zammit of Cardiff University, one of the researchers, cautioned that the findings do not imply that marijuana causes psychosis. "Uncertainty is inevitable, but we think the evidence is strong enough that people should be made aware that if they use cannabis they have this increased risk of having a psychotic episode," he said. "Especially if they use it on a regular basis."
He said, however, that most people who smoke pot would not develop psychosis, estimating that less than 3 per cent of the general population exhibits psychotic behaviour.
The report was based on results from seven previous studies on the long-term effects of marijuana use. The authors said people diagnosed with mental illness prior to their participation were excluded from their research.
The findings come on the heels of a United Nations study that showed Canada leads the industrialized world in pot smoking per capita.
Alan Young, a criminal law professor at York University's Osgoode Hall who has led efforts to liberalize Canada's marijuana laws, said evidence suggests that marijuana may trigger mental illness in users who are predisposed to it, but questioned whether researchers have ever shown a causal link between the two.
"If that were the case, after 40 years of heavy marijuana consumption in the Western world, we should be seeing an epidemic of schizophrenia," Mr. Young said. "That's what makes these claims so hollow."
But Patrick White, chair of psychiatry at the University of Alberta who opposes legalizing marijuana, called the causal-link argument "a moot point."
"Marijuana certainly is a precipitating factor in the onset of an acute or chronic psychosis, particularly in that population who is predisposed to develop a psychosis," Dr. White said.
The research comes at a time when marijuana is under heightened scrutiny in Britain.
Nine federal public officials have confessed in recent days to smoking pot in their past. The "cannabis chorus line," as it was dubbed, followed news that the government would conduct a review of cannabis to determine whether it should be upgraded to a more dangerous drug. The work published in The Lancet was commissioned by the Department of Health, but is not linked to the impending review.
The editorial called for an intensive public awareness campaign and treatment for young abusers. "In the public debate, cannabis has been considered a more or less harmless drug ...," the journal reported. "However, the potential long-term hazardous effects of cannabis with regard to psychosis seem to have been overlooked, and there is a need to warn the public of these dangers."
Jodie Emery, editor of Cannabis Culture magazine in Vancouver, dismissed the research as "reefer madness sweeping the U.K." and pointed to studies that have shown marijuana to have beneficial properties.
"Even if it does cause problems for some people, we shouldn't be lying to the public, saying it's going to drive you crazy," said Ms. Emery, 22, a self-proclaimed daily marijuana user. "The people who have not gone crazy will point out that that's a lie."