The Priory hospital is offering therapy to children as young as 12 damaged by cannabis abuse.

Doctors at the clinic in Roehampton have set up a new unit which will treat teenagers with psychiatric problems related to drug and alcohol abuse. This includes 12-18 year-olds suffering psychological effects after smoking super-strength cannabis.

The move came amid mounting concern as figures showed more than 500 people - adults and children - are being treated in hospital every week for mental health problems connected with cannabis use.

It will fuel debate over whether cannabis should be reclassified as a class B drug, after the controversial downgrading to class C four years ago.

The Priory's treatment clinic will give NHS-funded therapy to children as young as 12 with cannabis-related paranoia and schizophrenia.

The number of teenagers being treated for cannabis addiction is now nearly 10,000. Forty per cent of 11- to 15-year-olds who have smoked cannabis have been diagnosed with a mental disorder.

Experts blame the rise on the emergence of skunk, the extra-potent strain of the drug. Cannabis has until now been regarded as a "soft" drug which scores relatively low on the harm scale. But there is now growing scientific evidence that increased availability of skunk is behind the rise in cases of teenage paranoia and schizophrenia.

Ministers are now planning a U-turn on cannabis classification in response to warnings from experts.

Government drugs advisors are carrying out a review but it was revealed this week that Prime Minister Gordon Brown has made it to clear he intends to reclassify cannabis as class B.

A spokeswoman for The Priory said that the link between cannabis use and schizophrenia was now well established. She said: "We know that adolescents who smoke cannabis just 50 times before the age of 18 treble their chances of developing schizophrenia."

Research by the Institute of Psychiatry has revealed that some people are genetically "wired" to be more sensitive to the effects of cannabis.

Professor Robin Murray, an expert on drug addiction, warned that up to a quarter of the British population may be at risk of developing psychiatric problems related to cannabis abuse.

He said: "The fact is that cannabis is now a lot stronger than it used to be."

Figures out today showed that 16,685 adults were treated by English hospital trusts after abusing cannabis in 2006-07. The number of children treated for using cannabis rose from 8,014 in 2005-06 to 9,259 last year.

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