Cannabis use as a teenager increases the risk of developing schizophrenia in later life, a study using research on New Zealand youths has revealed.
A report in the British Medical Journal showed those who used cannabis as a teenager had a 10 per cent chance of developing psychosis by the age of 26. The general public have a 3 per cent risk.
The conclusions were based on a study by the Institute of Psychiatry in London of 759 people born in Dunedin, New Zealand, between 1972 and 1973.
That report was used and supported by Dutch researcher Dr Jim van Os, who drew similar conclusions from a study of youths in Munich, Germany, to be published in the New Scientist next Saturday.
After carefully controlling for self-medication and other confounding factors, the British researchers found that those who had smoked cannabis three times or more before the age of 15 were much more likely to suffer symptoms of schizophrenia.
The team concluded that there was a vulnerable minority of teenagers for whom cannabis is harmful.
"We're not saying that cannabis is the major cause of schizophrenia," said Robin Murray of the Institute of Psychiatry in London, who led the study. "But it's a risk factor."
"I don't think we can deny it any longer," said epidemiologist Mary Cannon of the Royal College of Surgeons in Ireland, based in Dublin, who helped carry out the research in New Zealand. "Cannabis is part of the cause of schizophrenia."
Dr van Os and his team from the University of Maastricht have brought out further results on a group of nearly 2500 14 to 24-year-olds living in and around Munich, Germany, over four years.
They found that, overall, smoking cannabis as an adolescent moderately raised the risk of developing signs of psychosis later on in their lives, from 16 per cent to 25 per cent.
But when they focused on individuals who were known to be susceptible to psychosis - those who were showing signs of disturbed thought processes by the age of 11 - they found a much stronger link.
Susceptible individuals who avoided cannabis had a 25 per cent chance of developing psychosis. Susceptible individuals who smoked it faced double the risk - a 50 per cent chance. And the more cannabis they smoked, and the earlier they smoked it, the worse the outcome.
"There is a small but significant minority of people who have a predisposition to psychosis and who would be well advised to steer clear of cannabis," Dr van Os said.
Compared with substances like heroin and crack cocaine, cannabis has been seen by many people as relatively harmless.
Dr van Os claims that cannabis use is responsible for up to 13 per cent of schizophrenia cases in the Netherlands.
He said the figure would rise because use among teenagers was increasing in many countries, the age at first use was falling and the strength of cannabis was rising.
In Britain the drug was downgraded from a class B to a class C drug last year, meaning people caught with small quantities deemed to be for personal use were not usually arrested.
These findings may change that view. In the Netherlands, they have fuelled a growing clamour for reform of the laws regulating drug use.
In Britain, Dr van Os's findings have been seized upon by politicians, tabloid newspapers and mental-health lobby groups who want drug laws tightened up.
A mental-health charity, Sane, has called for the reclassification of cannabis to be reversed. And the British Government has acknowledged the link in its strongest terms yet, when it said in a press release that cannabis was an "important causal factor" in mental illness.
Dr Cannon warned that the studies needed replicating, but even so she said: "This is a very large effect, similar to the size of smoking and lung cancer. This is a very significant finding."
What should be done about it, however, remained an open question. Dr van Os told New Scientist that teenagers with a personal or family history of mental illness should be urged to steer clear of the drug. He also advocated legal changes: Governments should focus on keeping cannabis out of the hands of teenagers.