Researchers at the Institute of Psychiatry compared the pattern of cannabis use between 112 patients with first episode psychosis and 75 healthy people screened for psychosis, both groups from South East London.
They found that people who had a psychotic episode were twice as likely to have used cannabis for longer and three times more likely to have used it every day.
Skunk is an extra strong cannabis grown in hothouse conditions. The percentage of THC – the molecule that causes psychosis – it contains has increased by up to three times since 1995.
Researcher Dr Marta Di Forti warned: “It would be naïve to say that smoking a joint is safe as we do not have enough data to reach such a conclusion.”
Findings were presented this week at the Annual Meeting of the Royal College of Psychiatrists, held at Imperial College, London.