Recent studies, which were unavailable to the Commons Home Affairs Select Committee when they last considered drugs policy 18 months ago, have highlighted a greater link between cannabis use and psychosis.

Janet Dean, the Labour MP for Burton and a committee member, promised to raise reports in The Times on the growing concern among psychiatrists about the use of cannabis by young people.

The committee endorsed David Blunkett’s decision to reclassify cannabis from a class B to a class C drug, which comes into force on January 29.

But since then Robin Murray, head of psychiatry at the Institute of Psychiatry, told The Times that inner-city psychiatric services were nearing a crisis point, with up to 80 per cent of all new psychotic cases reporting a history of cannabis use.

Professor Murray said that recent studies showed that those who used cannabis in their teens were up to seven times more likely to develop psychosis, delusions or manic depression.

He said: “Unfortunately, there were no experts in psychosis on the committees that advised the Government. That is not a criticism; at the time, no one thought there should have been. Since then, there have been at least four studies that show the use of cannabis, particularly in young people, can significantly increase the likelihood of the onset of psychosis.”

Ms Dean said she would draw The Times’ article to the attention of the committee at its meeting today.

She said: “I think it is important and I will draw the article to the attention of the committee.

“I would hope that the Government bear that in mind and perhaps we ought to ask them if they are taking the recent reports into account.”

“We said (in 2002) that we were aware that cannabis can be harmful but the reason we supported the Home Secretary’s proposals was to do with the credibility of the message on drugs, particularly for young people.

“Ideally of course we do not want people to start taking drugs at all.”

Last night John Denham, the new committee chairman, said that it could re-visit its decision but only after a chance to see the impact of the change in government policy.

Mr Denham, a former Minister of State at both the Health Department and the Home Office who quit the Government over the Iraq war, said: “There has been a mood in the committee to re-visit reports and major government initiatives some time after they are implemented. It is possible in a year’s time that the committee will return to the drugs issue.”

He added: “Neither the Home Affairs select committee nor the Home Secretary tried to make their own assessment of the medical evidence (on cannabis). It is the advisory committee on medicines that has provided the scientific support to the Government and it has always been their recommendation that it should be categorised as a C.”

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