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Cannabis: the final word

A risk to mental health, or a boon for people with MS? Both, says Barbara Lantin


Barbara Lantin

Telegraph UK




Why can't people make up their minds about cannabis? A recent issue of a tabloid newspaper carried news of an Australian study that showed that four out of five people with severe schizophrenia had been regular marijuana smokers in their youth. A few pages later, it included an article extolling the virtues of a cannabis-derived medicine for multiple sclerosis (MS) patients.


The Home Office insists that cannabis "is still a controlled drug, and possession, production and supply are still illegal", but last week, it published proposals that mean anybody carrying enough to make up to 500 joints is likely to escape trafficking charges. What are we to make of these apparent contradictions?


The 'good' news about cannabis...


Last month, the Government announced that Sativex - an oral spray derived from cannabis that is licensed in Canada but not yet in the UK - could be prescribed on a "named patient" basis for pain relief in patients with MS. Sativex, which is the only medicine in the world derived from the cannabis plant, works by influencing the way pain messages are transmitted through the body.


"It's not that patients get high and stop caring about their pain," emphasises Mark Rogerson, spokesman for the manufacturers, GW Pharmaceuticals. "A person taking a normal dose will receive only a fraction of the tetrahydrocannabinol (THC) - the active ingredient in cannabis that causes a high - of a recreational user."


A recent trial in the journal Neurology showed that Sativex was significantly better than a placebo at reducing pain and sleep disturbances in MS patients.


... and the 'bad'


Two pieces of research published last week have added to the growing body of evidence that, when smoked by vulnerable, young people, cannabis can lead to serious mental illness.


A Danish study in the British Journal of Psychiatry found that almost half of patients treated for a cannabis-related mental disorder go on to develop a schizophrenic illness. People who had used the drug developed schizophrenia earlier than those with the illness who had not smoked marijuana.


The researchers emphasised that the study did not show that cannabis caused psychosis, because factors such as heredity, other drug use and socio-economic status had not been taken into account. However, an American study using sophisticated imaging techniques found similar abnormalities in the brains of adolescents with schizophrenia, and those who use cannabis daily, but no such abnormalities in healthy teenagers.


"These findings suggest that, in addition to interfering with normal brain development, heavy marijuana use in adolescents may also lead to an earlier onset of schizophrenia in individuals who are genetically predisposed to the disorder," says Dr Sanjiv Kumra, assistant professor of psychiatry at the Albert Einstein College of Medicine, New York, who worked on the study.


According to Robin Murray, professor of psychiatry at the Institute of Psychiatry in London, one person in four has the genes that make them susceptible to developing cannabis-induced psychosis.


Are patients who take cannabis-derived drugs at risk?


"There is a distinct difference between using a drug therapeutically, under medical supervision, and taking it recreationally," says Prof Murray. "Cannabis is a mixture of substances; when it is used in medicine, the aim is to have less of the hallucinogenic components, and more of those that have an effect on muscle tension.


"Also, if you smoke a joint, you get high levels of these components in the blood, and they decrease quite rapidly; whereas in pharmaceutical use, you have slow absorption, and levels remain fairly constant, so the psychotropic effects are likely to be fewer."


According to one survey, around 16 per cent of people with MS smoke cannabis to help relieve their symptoms. However, because of their age, this group is less likely to be at risk of schizophrenia. "Psychosis is a disorder of youth," says Prof Murray.


According to Rogerson of GW Pharmaceuticals: "We have found no evidence that Sativex causes psychosis. Such side effects as there are - and no drug is without them - are generally mild, reversible and well tolerated. There may be a temporary intoxication-like reaction, and, for this reason, we have always excluded people with serious mental illness from our trials."


What is the Government's attitude to cannabis?


Having downgraded the drug from class-B to class-C, the Government has asked its advisory council on the misuse of drugs to review the possible links between cannabis and mental illness. Its report is due by the end of the year, but leaks suggest that it will recommend no change in the law.


"I don't care about the classification of cannabis," says Prof Murray. "But the Government has made a major error in accompanying the reclassification with reassurance that the drug is not harmful. What is needed now is education. The Americans are tackling it very well. What is our Government doing?"