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.”