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Page last updated at 15:44 GMT, Wednesday, 11 May 2005 16:44 UK

In 2005, Newsnight looked at evidence that some young people using cannabis are putting themselves at risk of serious mental health problems. Susan Watts sought out psychiatrists and genetics experts in Europe for answers.

Susan Watts
by Susan Watts
Science Editor, Newsnight

What's worrying doctors and psychiatrists is that cannabis users are starting younger, when the pathways of the brain are still forming.

An image of a joint being lit
Many experts do not think re-classifying cannabis is a solution

Britain has the highest rate of heavy cannabis use among 15 year-olds in Europe.

For years, clinicians have noted cannabis use in the case histories of people with psychosis - a catch-all phrase for mental health problems ranging from delusional episodes and hallucinations to schizophrenia.

But is the cannabis a cause? In landmark studies over the past two years, Robin Murray and colleagues at London's Institute of Psychiatry have found that the risks are considerably higher for those who start young.

Super-strong varieties

Another clue may lie in today's stronger forms of cannabis - so-called "Skunk" varieties. The Dutch are traditionally at ease with cannabis, so new trends in its production and use often surface there first.

Clare Gerada
In the last 10 years we've seen a stream of people with problems relating to cannabis misuse
Clare Gerada
South London GP
At the coffee-shops that sell cannabis they talk openly about these new super-strong varieties - grown to boost the psychoactive part of the plant.

According to psychiatrist Jim Van Os, from the University of Maastricht, it's relatively easy for under-18 year-olds to get hold of the stronger "Skunk" cannabis, despite a ban on sales to this age group. And it's the combination of their youth and the potency of what they're taking that's alarmed him.

His team's just come up with a third intriguing part of the puzzle - results that suggest our genes may play a role in making some of us more vulnerable to the effects of cannabis than others.

South London GP Clare Gerada is convinced there's a silent trend emerging.

"15 years ago there was absolutely nobody presenting with cannabis problems at all," she says. "It just wasn't on the agenda, whereas over the last, say, 10 years I've seen a steady stream of people coming to see me with problems relating to cannabis misuse - feelings of anxiety... paranoid feelings."

But sceptics say it's too early to draw firm conclusions. John Macleod of Birmingham University told Newsnight:

An image of a square in Maastricht
In Maastricht it's relatively easy for under-18s to acquire "Skunk".
"Cannabis and psychosis may have common antecedents, related to adversity in early life - possibly related to nutritional factors, infection factors or factors that we just don't know about. And these may lead to psychosis and may also lead to an association between cannabis use and psychosis, but it doesn't follow that cannabis use causes psychosis."

Professor Macleod recently reviewed the published science for the Department of Health.

Pause for thought

But the latest from the psychiatrists - particularly the hints of a genetic element - have caused a pause for thought at Government level.

After re-classifying cannabis from Class B to the softer Class C category just last year, the Home Office has had to ask for fresh advice after the work at the Institute of Psychiatry and Maastricht came to the fore.

But it seems a knee-jerk response - such as re-classifying cannabis back into Class B status - wouldn't be a welcome move among the experts. None of the scientists we spoke to saw this as the solution.

Instead, the concern was that we do all we can to dissuade young teenagers from getting started...

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