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Pro-cannabis lobby questions research link to schizophrenia March 25, 2005 stuff.co.nz
A lobby group advocating the legalisation of cannabis, Norml, says it rejects research that shows some New Zealand teens who used the drug have an increased risk of developing schizophrenia.
Norml spokesman Chris Fowlie said he was surprised that such a "sweeping generalisation" could be made from a small sample in the study, reported this week in New Scientist magazine.
A study of 759 people born in Dunedin in 1972-73, reported a 10 per cent higher chance of suffering the symptoms of schizophrenia among those who had smoked cannabis three or more times by the age of fifteen, compared to those who had not. The team concluded that there was a vulnerable minority of teenagers for whom cannabis is harmful.
Epidemiologist Mary Cannon of the Royal College of Surgeons in Ireland, based in Dublin, who helped carry out the New Zealand study, said her research team believed that cannabis was part of the cause of schizophrenia.
In the New Zealand study, the number of people who had smoked dope on three occasions by the age of 15 was just 29, and only three went on to develop psychosis, but Dr Cannon's research team recently re-analysed the data from the study to check genetic predisposition to schizophrenia.
The gene they investigated, called COMT, encodes an enzyme (catechol-O-methyl transferase) that breaks down a signalling chemical in the brain called dopamine. COMT comes in two forms, one of which is marginally more common in people with schizophrenia and is thought to be a risk factor for the disease. The team found that in New Zealanders with two copies of the "normal" version of COMT, smoking cannabis had little effect on their mental health. In people with one normal and one "bad" form of the gene, smoking cannabis slightly increased their risk of psychosis.
For people with two copies of the bad gene, cannabis spelled trouble: smoking it as a teenager increased their likelihood of developing psychosis by a factor of 10.
But Mr Fowlie said the small number of people in the study who went on to suffer schizophrenic symptoms raised questions.
"It is simply wrong to claim any sort of result from such a small number of people," he said.
"In the real world, we can see that cannabis use rates are increasing throughout western society, and yet rates of mental illness are falling.
"Using cannabis is normal, with 74 per cent of 21-year-olds in a Christchurch study admitting to using cannabis, and yet we can see they're not all going insane."