Cannabis link to psychosis

Very heavy use of cannabis could be a cause of psychosis, according to a leading psychiatrist who believes that society should think carefully about the potential consequences of its increasing use. Robin Murray, professor of psychiatry at the Institute of Psychiatry and consultant psychiatrist at the Maudsley hospital in London, says that in the last 18 months, there has been increasing evidence that cannabis causes serious mental illness. In particular, a Dutch study of 4,000 people from the general population found that those taking large amounts of cannabis were almost seven times more likely to have psychotic symptoms three years later.
“This research must not be ignored,” said Prof Murray, speaking at the annual general meeting of the Royal College of Psychiatrists in Edinburgh. Writing in the Guardian last August, Prof Murray said he had been surprised that the discussion around cannabis had skirted around the issue of psychosis. Psychiatrists had known for 150 years that very heavy consumption of cannabis could cause hallucinations and delusions.
“This was thought to be very 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 regular daily consumers of cannabis than the general population.”

In the UK, he said, people with schizophrenia are about twice as likely to smoke cannabis. The reason appears to be the effect that the drugs have on chemicals in the brain. “In schizophrenia the hallucinations and delusions result from an excess of a brain chemical called dopamine. All the drugs which are known to cause psychosis – amphetamine, cocaine and cannabis -increase the release of dopamine in the brain.” Cannabis had been the downfall of many a promising student, he suggested. “Like any practising psychiatrist, I have often listened to the distraught parents of a young man diagnosed with schizophrenia tell me that as a child their son was very bright and had no obvious psychological problems. Then in his mid-teens his grades began falling. He started complaining that his friends were against him and that people were talking about him behind his back.“After several years of increasingly bizarre behaviour he dropped out of school job or university; he was admitted to a psychiatric unit overwhelmed by paranoid fears and persecution by voices. The parents tell me that, at some point, their son was heavily dependent on cannabis.”
It used to be thought that the high numbers of psychotic patients taking cannabis could be explained because they used it to alleviate their symptoms. The recent studies, however have looked at large populations without mental illness and studied the numbers of cannabis takers within them who have developed psychosis.
Source: The Guardian July 2003

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