THE Government is to review its decision to downgrade cannabis after mounting scientific evidence that the drug could be more harmful than thought.

Charles Clarke ordered the Advisory Council on the Misuse of Drugs last night to review its conclusion that high cannabis use was not associated with health problems.

The council’s findings were the basis for a Home Office decision to downgrade cannabis from a Class B drug to Class C from January 2004, which meant that possession was no longer an arrestable offence.

In a letter to Professor Sir Michael Rawlins, the council chairman, the Home Secretary noted that two recent studies had linked cannabis with increased mental health problems.

One, by Professor Jim van Os, of Maastricht University, in 2004, concluded: “Cannabis use moderately increases the risk of psychotic symptoms in young people but has a much stronger effect in those with . . . predisposition for psychosis.”

The study of 2,437 people aged between 14 and 24 found that half of those who were psychologically vulnerable and smoked cannabis developed psychotic symptoms over a four-year period. This was twice the rate among those who did not use cannabis.

In his letter, Mr Clarke implies that the findings have emerged since cannabis was reclassified. The two studies that he refers to are new, but both authors have been publishing similar findings for several years.

The second study is by Professor David Fergusson, of the University of Otago, who collected data over 25 years on a group of 1,055 people born in 1977. At the ages of 18, 21 and 25 they were questioned about their use of cannabis.

He concluded that, even when all possible confounding factors were taken into account, “there was a clear increase in rates of psychotic symptoms after the start of regular use, with daily users of cannabis having rates over 150 per cent those of non-users”.

In the journal Addiction, Professor Fergusson wrote: “These findings add to a growing body of evidence from different sources, all of which suggest that heavy use of cannabis may lead to increased risk of psychotic symptoms.”

The advisory council has resisted pleas from the medical profession to reconsider its opinion in the light of such research. But Mr Clarke said that he could no longer ignore the evidence.

He also asked the council to examine Dutch proposals for a higher classification of strong variants of cannabis, known as “skunk”. The Home Office said that the council would be expected to start a review at its meeting on May 19 and to report by early 2006.

Mr Clarke’s decision was broadly welcomed last night, although some commentators questioned the timing in the run-up to a general election.Professor Robin Murray, a consultant psychiatrist at the Institute of Psychiatry at King’s College London, said: “Anybody who knows anything about this subject will be pleased.

“The council’s original decision was based on research conducted in 2001, but there have been six studies since then showing a clear link between prolonged cannabis use and psychosis. The problem with the earlier report was that the council took evidence from psychiatrists who knew about addiction, but not psychiatrists, who know about psychosis.”

David Davis, the Shadow Home Secretary, described the review as “a humiliating recognition of the failure of a central plank of Labour’s drugs policy”.

He added: “The latest psychological evidence shows that cannabis is a serious threat to the health of young people and a gateway to harder drugs.”

But Brian Paddick, Deputy Assistant Commissioner of the Metroplitan Police, architect of the experiment that led to reclassification, was sceptical about giving stronger cannabis a higher classification.

“It would be difficult to ask operational police officers to make a decision on the street as to what sort of cannabis a person had on them,” he said.

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