Psychotic episodes are on the rise

Taking any version of the drug on a daily basis was found to triple the chances of developing psychosis. But using high-potency cannabis, known as "skunk", which contains high levels of the compound THC, raised the risk five-fold. An estimated one in three new cases of severe mental disorder in London were linked to skunk use, the research found.

Scientists looked at the drug use history of 901 patients who had experienced a psychotic episode, and 1,237 healthy individuals. Patients were twice as likely to report using skunk compared to healthy people, with almost 40 percent of the patients admitting to using high-potency cannabis. Lead study author Dr Marta Di Forti, of King's College London, said: "Our findings are consistent with previous studies showing that the use of cannabis with a high concentration of THC has more harmful effects on mental health than the use of weaker forms. As the legal status of cannabis changes in many countries and states, and as we consider the medicinal properties of some types of cannabis, it is of vital public health importance that we also consider the potential adverse effects that are associated with daily cannabis use, especially high potency varieties."

The study was the first to analyse the impact of cannabis on population rates of psychosis. In London, high potency cannabis with an average THC content of 14 percent accounts for 94 percent of the market. Researchers said if high potency cannabis were no longer available, the incidence of psychosis in London would be expected to drop from 45 to 31 cases per 100,000 people per year. Dr Adrian James, of the Royal College of Psychiatrists, said: "This is a good quality study and the results need to be taken seriously. "Cannabis carries severe health risks and users have a higher chance of developing psychosis." The findings were published in mental health journal The Lancet Psychiatry.

Potent cannabis adds to the risks