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PARIS - The widest-yet investigation into cannabis and mental health says individuals who use marijuana increase their risk of developing a psychotic illness by more than 40 per cent.
Reporting in Saturday's issue of The Lancet, the doctors call on health supremos to warn young people about the risk to their mind from a drug that many today may dismiss as harmless and recreational.
Their conclusions are founded on a review of 35 published studies that explored the frequency of schizophrenia, delusions, hallucinations, thought disorder and other psychotic illnesses among marijuana users.
Those who had used the drug were 41 per cent likelier to experience an episode of this kind than people who never smoked, according to the paper.
The review also found that the risk increased relative to usage. The most frequent users were twice as likely to have these symptoms compared with non-users.
The study also looked at the risk of depression, anxiety and other emotional states, but declared the evidence for any link to cannabis was unclear.
The authors say that, despite their best efforts, they cannot rule out the possibility that their study was affected by "confounding factors" that typically dog research into cannabis.
The biggest of these problems is whether cannabis can in fact be blamed for causing mental ill-health -- or whether cannabis smokers with these problems were already unwell before they used the drug.
But the review says the weight of evidence has now come down clearly in favour of warning youngsters, the biggest users of cannabis, that the drug can lead to mental illness.
"Despite the inevitable uncertainty, policymakers need to provide the public with advice about this widely-used drug," it declares.
"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."
It stresses that the risk of schizophrenia and other chronic psychotic disorders, even in people who use cannabis regularly, is statistically low, with a less-than one-in-33 possibility in the course of a lifetime.
Even so, "cannabis use can be expected to have a substantial effect on psychotic disorders at a population level because exposure to this drug is so common," it argues.
In the case of Britain, about 40 per cent of young adults and adolescents have used cannabis, according to figures cited in the study. By extrapolation, around 14 per cent of cases of psychotic episodes among young British adults would be avoided if cannabis were not consumed, the paper contends.
The research is led by Theresa Moore of the University of Bristol, western England, and Stanley Zammit of Cardiff University, Wales.
Seeking to get a long-term perspective, their review excluded "transient" effects -- that is, the immediate effect on the mind when the drug was being smoked -- and looked only at cases that had been diagnosed as psychotic by an expert.
In addition, they ruled out studies that covered people with addictions or previous mental problems, medicinal use of cannabis and prison populations.
The authors admit, though, that it may be impossible to establish for sure whether cannabis causes psychosis on the basis of current methods.
The big problem for researchers is that cannabis is illegal, which means that the strength and dose of active ingredient varies, in contrast for instance to the well-known dose of nicotine in tobacco, which is legally regulated.
Testing cannabis on the mind would run into huge legal and ethical hurdles.
So researchers are mainly left with the option of comparing groups of cannabis users against groups of non-users -- with the hope that "confounding factors" do not undermine the exercise. - AFP/fa
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