People who smoke the more powerful kind of cannabis known as skunk are 18 times more likely to develop psychosis than those who smoke milder forms, according to research by psychiatrists.

Early results from a study presented to the annual meeting of the Royal College of Psychiatrists suggest that skunk poses significantly greater dangers to mental health than the traditional kinds of cannabis, such as hash.

Dr Marta Di Forti, from the Institute of Psychiatry, in London, said research with south-east London patients involved 112 who had suffered psychosis after smoking cannabis and 75 cannabis smokers who had not. Those who had experienced a psychotic episode were twice as likely to have used cannabis for longer, three times as likely to be using it every day and 18 times as likely to be taking skunk.

Di Forti said that although the sample was small, the work was part of a much bigger study led by Robin Murray, a professor at the institute. This arm of the study was also investigating people's genetic predisposition to developing psychosis.

The importance of the findings, Di Forti said, was that for the first time, questions similar to those lung cancer specialists would ask of smokers were being presented to the drug users - such as whether they had the occasional cigarette with a cup of coffee with their peers or whether they smoked three packets a day.

"The problem is the long-term effect - to establish that we need to differentiate between people who have over a long period used cannabis of high potency and people who only used it once or twice."

The potency of cannabis has increased over the past 10 years, with more concentrated forms now on sale. In 1995, skunk had 6% THC (Delta-9-tetrahydrocannabinol) - the chemical which is held responsible for the psychotic symptoms. Now, according to a recent study, skunk contains 16-18% THC and it contains little or no CBD (cannabidiol), a molecule that appears to counteract the damaging effects of THC. Hash, on the other hand, has much less THC and an almost equal amount of CBD.