Robin Murray: Teenage schizophrenia is the issue, not legality

For 150 years it has been known that acute intoxication with cann-abis can induce hallucinations and delusions. However, this was thought to be a transient effect which usually rapidly resolved itself. Then in the late 1980s and 1990s psychiatrists like me began to see growing numbers of young people with schizophrenia who were taking large amounts of cannabis.

We first thought that they were self-medicating in an attempt to ameliorate their anxiety and paranoia. However, since their families often told us that cannabis seemed to exacerbate the symptoms, we decided to examine this. When we followed up 119 young people who had been diagnosed with schizophrenia, we discovered that far from being helped by cannabis, those who continued to take the drug four years later were three times more likely still to be hallucinating and deluded than non-consumers.

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If cannabis could make schizophrenia worse, could it have caused the psychosis in the first place? The only way to decide is to question large numbers of healthy people about their cannabis habits and follow them up to see whether the cannabis consumers are more likely to develop psychosis. Eight studies have now reported that those who consistently take large amounts of cannabis have an increased risk of later developing schizophrenia-like psychosis.

It is estimated that at least 10 per cent of all people with schizophrenia in the UK would not have developed the illness if they had not smoked cannabis, so there are about 25,000 individuals whose lives have been ruined by cannabis.

Why are we seeing so many cases of cannabis-induced schizophrenia? A UN report in 2006 suggested three reasons.

First, the consumption of cannabis climbed steadily across Europe over the past four decades to reach a peak about 2002.

Second, high-potency cannabis preparations are more widely available. Traditional 1960s herbal cannabis contained about 2-3 per cent of the active ingredient tetrahydrocannabinol (THC); but today's skunk varieties may contain 15 or 20 per cent THC and new resin preparations have up to 30 per cent. Skunk is to old-fashioned hash as is whisky to lager. You can become alcoholic by just drinking lager; but you have to drink a lot more lager than whisky. Similarly, you can go psychotic if you smoke enough traditional marijuana, but you have to consume a lot more for a lot longer than with skunk.

Third, the age of starting cannabis use has been steadily lowering. It is now commonly taken at 15 and some of the patients I see started at 12 or 13 years.

Of course, most cannabis smokers never come to any harm, just as the vast majority of drinkers don't get liver disease. It is simply that the more you take the greater the risk.

The frequency of cannabis consumption and the resultant psychosis in the UK is among the highest in Europe. I am not convinced, however, that the exact classification of cannabis is of much relevance. The Government's mistake was rather to give the impression that cannabis was harmless and that there was no link with psychosis.

Charles Clarke, the then Home Secretary, realised the error in 2005 and promised education and research. The benefits of education about the risks can be seen in the US where cannabis consumption has fallen.

The UK education campaign was largely invisible, and there is still no significant government-funded research into cannabis-induced schizophrenia. Indeed to judge by the relative amounts of parliamentary time devoted to fox-hunting and psychosis since 1997, MPs have been much more interested in the mental health of foxes than of its young citizens.

Robin M Murray is professor of psychiatry and consultant at the Maudsley Hospital, London