The meta analysis of research into cannabis smoking and psychosis may at last convince some politicians and doctors who have persisted in promoting a libertarian approach to the use of cannabis to change their mind.

The Lancet paper gives irrefutable evidence that there is an obvious and proven association between cannabis smoking and psychotic breakdown, presumably whether the symptoms are predominantly those of schizophrenia or bipolar disorder. Medical opinion is now likely to follow the teaching of Robin Murray, Professor of Psychiatry at the Institute of Psychiatry in London, one of the acknowledged international experts on psychotic diseases who co-edited Marijuana and Madness published by Cambridge University Press three years ago.

The initial study on this subject was published in the 1960s. In 1972 the first key survey on the effect of cannabis on psychotic breakdown studied the effect of cannabis smoking on American servicemen in Europe over a 15-year period. Since then there have been more than 75 other significant research projects that have shown an association between the drug and psychosis.

Probably the most important and convincing previous paper on the link between cannabis and psychosis was published in 1987 in Sweden, a country famed for its liberal approach to life. The Swedish paper studied 45,000 conscripts and their drug-taking habits. It demonstrated that those who smoked cannabis on more than 50 occasions were six times more likely to develop schizophrenia than those who had never used the drug.

Even when every possible confounding factor had been taken into account, cannabis was still found to be responsible for an unacceptably high number of cases of psychotic disease. The only possible debate after this mass of evidence had been published was whether cannabis uncovered psychoses that might otherwise have remained hidden, or was capable of inducing it in those who were not genetically preconditioned to be vulnerable to factors that could result in a psychotic breakdown.

The Lancet study still leaves unanswered the extent to which cannabis affects the personality of smokers, short of actually causing psychotic breakdown.

Many believe that in young people it has a disastrous effect on academic performance, motivation, judgment, ability to argue rationally and can leave cannabis takers abnormally and excessively introspective so that they are preoccupied with their own psyche and physique.

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