People who smoke the extra-strong type of cannabis known as skunk are 18 times more likely to develop psychosis than those who use its milder forms, research has shown.
The study, carried out at the Institute of Psychiatry and presented this week to delegates at the Annual Meeting of the Royal College of Psychiatrists, looked at the links between cannabis use and psychosis.
Psychiatrists found that people who had had a psychotic episode were twice as likely to use cannabis, three times more likely to use it every day - and 18 times more likely to use skunk, as opposed to the milder grass or resin.
Skunk has higher concentrations of the active ingredient delta-9 tetrahydrocannabinol (THC), which is linked to psychosis, than hash.
Dr Marta di Forti said: "Of those people coming to us with first episode psychosis, more than 80% had used skunk. Among those acting as controls, 76% had used hash. My message is that people who develop first episode psychosis are significantly more likely to use skunk than hash."
The super-strength drug now comprises three quarters of the cannabis market in the UK, according to Home Office figures.
Richard Colwill, spokesman for the mental health charity Sane, said: "This study is a welcome contribution to our understanding of the links between cannabis abuse and psychosis, and we hope it provides further insight into why such a significant proportion of people with serious mental illness have used the drug."