Cannabis use trebles the chance of schizophrenia

TEENAGERS who regularly smoke cannabis are trebling their chances of becoming schizophrenic later in life, an expert warned yesterday.

Dr Stanley Zammit said the terrifying statistic applies to those who use the drug at least 50 times before the age of 18.

Youngsters who smoke it just ten times are still twice as likely to suffer from the mental illness.

The warning follows repeated studies highlighting the dangers of cannabis – which was recently downgraded to a Class C drug, the least serious category. Teenagers are particularly vulnerable because their brains are still developing.

Dr Zammit, from Cardiff University’s psychological medicine department, made the claim in a speech to the BUPA Foundation. ‘ From a public health point of view, these findings are potentially hugely important,’ he said.

‘ If cannabis does indeed increase the risk of developing schizophrenia, then you could potentially prevent hundreds of people getting the mental illness if cannabis was not available in the population.’ His research is based on data from a separate study of 50,000 people in Sweden.

The data – which covered a 27year period, allowed him to compare the amount of cannabis smoked before the age of 18 with rates of schizophrenia in later life.

Dr Zammit’s research, originally published in the British Medical Journal in 2002, concluded that a person’s chances of developing schizophrenia rose in line with their cannabis use.

Dr Zammit said his research showed that those who used the drug more than ten times before they were 18 had double the risk of schizophrenia compared to people who had never touched it.

And among those who used it at least 50 times prior to their 18th birthday, the risk tripled.

' We found the more cannabis people used the more likely it was they would experience some kind of mental illness, such as schizophrenia,' he said.

Dr Zammit said, however, that not all heavy users would develop mental illness. He added that he plans to conduct further studies into why some are more susceptible than others.

Dr Andrew Vallance- Owen, vice chairman of the BUPA Foundation, said Dr Zammit's work was a key piece of research that ' sheds light on cannabis and a mental illness that blights the lives of one in one hundred of us.' He added: ‘ The findings are thought-provoking not only for those who suffer from schizophrenia, but for everyone, and will spark widespread debate.’

Dr Zammit, has now been awarded £10,000 in recognition of his work and to allow him to conduct further research into the potential dangers of cannabis use.

Previous studies have shown that, in susceptible teenagers, cannabis raises the risk of a range of mental problems including psychosis and depression.

Earlier this year, doctors warned that children as young as 11 are suffering personality changes caused by the drug, particularly those who use a potent variety called skunk.

j.wheldon@dailymail.co.uk