Research carried out on New Zealand teenagers has confirmed cannabis use is a serious risk factor for schizophrenia.
http://www.stuff.co.nz/stuff/print/0,1478,3226588a10,00.html
“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,” a Dutch researcher, Jim van Os, said in the latest New Scientist magazine, due to be published next Saturday.
Dr van Os, a psychiatrist at the University of Maastricht, investigates the effect of marijuana on people’s brains – particularly adolescents’ brains. He and other researchers have been building a scientific case that, for some teenagers, smoking cannabis leads to serious mental health problems in later life, including schizophrenia.
Dr van Os said claims that marijuana is responsible for up to 13 per cent of schizophrenia cases in the Netherlands. He said the figure will only increase because cannabis use among teenagers was increasing in many countries, the age at first use was falling and the strength of cannabis was rising.
Part of the research is based on a study that followed 759 people born in Dunedin, New Zealand, in 1972 and 1973 and was reported in the British Medical Journal.
After carefully controlling for self-medication and other confounding factors, 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 by the time they were 26 – they had a 10 per cent chance compared with 3 per cent for the general population.
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 says Murray, 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 New Zealand study. “Cannabis is part of the cause of schizophrenia.”
And late last year Dr van Os and his team brought out further results following 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, 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 age 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 had a 50 per cent risk. And the more cannabis they smoked, and the earlier they smoked it, the worse the outcome.
In the Netherlands, the findings 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 recently 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.
The New Scientist reported that, compared with substances like heroin and crack cocaine, cannabis was seen by many people as relatively harmless, and in the UK marijuana was downgraded from a class B to a class C drug last year, meaning people caught with small quantities were not usually arrested.
New Scientist reported one research avenue that may shed further light on the matter was to look at whether genes are involved.
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 it.
This time, they added in in another variable – 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 results were crystal clear.
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.
But 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.
The results have not yet been published, and Dr Cannon warned that they 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.
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