Is there really a link between cannabis and psychosis?

As a consultant psychiatrist working in the Maudsley Hospital, which serves the Brixton area, I have been surprised that in all the recent discussions about cannabis, there has been virtually no mention of the drug's relationship to psychosis.

Psychiatrists have known for 150 years that heavy consumption of cannabis can produce hallucinations. This was thought to be rare and transient until the 1980s when, as cannabis consumption rose across Europe and the USA, it became apparent that people with chronic psychotic illnesses were more likely to be daily consumers of cannabis. Here in Britain, for example, people with schizophrenia do not take more alcohol, heroin, or ecstasy than the rest of us - but they are twice as likely to smoke cannabis regularly.

Since people with schizophrenia have a miserable life, most psychiatrists initially thought that if the odd spliff brought them some pleasure, what was the harm? Then, in the mid-90s, a Dutch psychiatrist named Don Lintzen from the University Clinic in Amsterdam noted that people with schizophrenia who used a lot of cannabis had a much worse outcome than those who didn't. This was confirmed by other studies, including a four-year follow-up at the Maudsley Hospital. Those who continued to smoke cannabis were three times more likely to develop a chronic illness than those who didn't.

Why does cannabis exacerbate psychosis? In schizophrenia, the hallucinations result from an excess of a brain chemical called dopamine. All the drugs that cause psychosis - amphetamines, cocaine and cannabis - increase the release of dopamine in the brain. In this way, they are distinct from illicit drugs such as heroin or morphine, which do not make psychosis worse.

The distraught parents of a young man diagnosed with schizophrenia tell me that their son was a very bright child with no obvious psychological problems. Then, in his mid-teens, his school grades deteriorated and he seemed to have trouble thinking clearly. He complained that people were talking about him behind his back.

After years of increasingly bizarre behaviour, he dropped out of school, job and university, and was finally admitted to a psychiatric unit, overwhelmed by paranoid fears and persecution by voices. The parents tell me that, at some point during this downward spiral, they realised their son was dependent on cannabis. The National Schizophrenia Fellowship (Rethink) is full of parents who see cannabis as the cause of their son's or, less commonly, daughter's madness.

Psychiatrists began to wonder if cannabis could actually cause psychosis as well as make established psychosis worse. A famous study interviewed 50,000 conscripts into the Swedish Army about their drug consumption and followed them up. Those who were heavy consumers of cannabis at 18 were six times more likely to be diagnosed with schizophrenia over the next 15 years than those did not take it.

This year, Dutch epidemiologist Jim Van Os published the results of his study, in which 7,500 people were interviewed about their drug consumption and followed up for three years. Once again, regular consumers of cannabis were more likely to develop psychosis than those who didn't. Two other studies with similar findings are in progress.

It is perhaps surprising that it took the professionals so long to reach this conclusion. For example, it is widely accepted in Jamaica that too much ganja can cause paranoia. Several famous Rastafarians spent their last years incarcerated in Bellevue, the squalid mental hospital in Kingston, among them the legendary ska trombonist, Don Drummond.

Cannabis is now one of the biggest problems on in-patient psychiatric wards in England's major cities. It is common at Maudsley for those making progress to relapse suddenly. The explanation comes when a urine sample tests positive for cannabis. The same effect has been shown at Yale Medical School, where volunteers were given THC - the major active ingredient of cannabis - by injection. Psychotic symptoms could be produced in normal subjects, and people with schizophrenia had a brief exacerbation of their psychosis.

So will reclassifying cannabis cause more people to become psychotic? The incidence of schizophrenia in south London has doubled since the 1960s; the use of cannabis and cocaine could be a factor. The increase in the prevalence and the deteriorating outcomes of schizophrenia due to cannabis use is the main reason why psychiatric services in London are in such a mess.

Any public debate on cannabis needs to take account of the risks as well as the pleasure. Pro-marijuana campaigners claim, extrapolating from their Saturday-night joint, that cannabis is totally safe. Yet they would be unlikely to claim that a bottle of vodka a day is healthy on the basis of sharing a bottle of Chablis over dinner.

No drugs that alter brain chemistry are totally safe. Just as some who drink heavily become alcoholic, so a minority of those who smoke cannabis daily go psychotic. Society has to balance the enjoyment that the majority get from cannabis with the harm it causes to a vulnerable few.

· Robin Murray is professor of psychiatry at the Institute of Psychiatry, and consultant psychiatrist at the Maudsley Hospital.

Thanks to guardian.co.uk who have provided this article. View the original here.