Smoking powerful skunk cannabis triples the risk of suffering a serious psychotic episode, scientists have found.
In the population studied by the researchers in south London, where cannabis use is widespread, the drug is linked to one quarter of all new cases of psychosis, the team found.
The findings add to a compelling body of evidence that smoking strong cannabis “tilts the odds” towards a person developing psychosis, which leads to schizophrenia in about half of cases.
The study found that those who smoked skunk every day had five times the normal risk of experiencing extended episodes in which they heard voices, suffered delusions or demonstrated erratic behaviour.
Sir Robin Murray, professor of psychiatric research at King’s College London, said there was now clear evidence for a causal link between smoking strong cannabis and the risk of mental illness. “The argument initially was that the people who are going to smoke cannabis are a bit odd anyway,” he said. “In south London, two-thirds of people have used cannabis and it seems unlikely that two-thirds of people are abnormal.”
Between 2005 and 2011, the scientists worked with 410 patients, aged 18-65, who presented at south London hospitals with a first episode of psychosis and had shown symptoms such as hearing voices or suffering delusions for at least a month. A further 370 healthy participants from the same area were included as controls.
The study, published in the journal Lancet Psychiatry, showed that cannabis potency and frequency of use were strongly linked to the risk of developing mental health problems – factors that the authors claim are often overlooked by doctors.
Those who reported smoking milder forms of the drug, such as hash, did not appear to be at increased risk, for instance. Murray said that, in line with this finding, he recommends hash to patients who are struggling to give up smoking altogether.
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