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Skunk 'poses greatest risk of psychosis'

1 December 2009

Researchers at the Institute of Psychiatry at King’s College London (KCL) have found that people who smoke skunk, the most potent form of cannabis available in UK, are almost seven times more likely to develop psychotic illnesses than those who use traditional cannabis resin (hash) or grass.

The study, funded by the Maudsley Charitable Fund and the National Institute for Health Research specialist Biomedical Research Centre for Mental Health at South London and Maudsley NHS Foundation Trust (SLaM) and KCL, is published today in the British Journal of Psychiatry.

The team collected information on cannabis use from 280 people attending SLaM with their first episode of psychosis. A control group of 174 healthy people from the local area was also studied. There was no significant difference between the two groups in whether they had ever used cannabis or their age at first use, however, the patients with psychosis were twice as likely to have used cannabis for longer than five years, and over six times more likely to use it every day. Among those who had used cannabis, patients with psychosis were almost seven times more likely to use skunk than the control subjects.

Psychiatrist and lead researcher Dr Marta Di Forti said: “In both groups a high proportion had used cannabis at some point in their lives and they were both likely to have started early in adolescence, however, psychosis was associated with more frequent and longer use. Our most striking finding was that among those who had used cannabis and developed psychosis, the type of cannabis which was preferentially used was the high-potency skunk variety.”

The researchers believe the high level of delta-9 tetrahydrocannabinol (Δ9-THC) found in skunk is to blame. The two main constituents of cannabis are Δ9-THC and cannabidiol. Δ9-THC is the main psychoactive ingredient, and in experiments has been shown to produce psychotic symptoms such as hallucinations and delusions. Cannabidiol does not induce these symptoms and seems to have anti-psychotic properties – possibly counteracting the effects of THC.

In south-east London, where the study was carried out, the skunk variety of cannabis contains 12-18% Δ9-THC and less than 1.5% cannabidiol. In contrast, resin (hash), which was preferred by cannabis users in the study’s control group, has an average Δ9-THC of 3.4% and a similar proportion of cannabidiol.

Dr Di Forti concluded: “Our study is the first to demonstrate that the risk of psychosis is much greater among people who are frequent cannabis users, especially among those using skunk, rather than among occasional users of traditional hash. It is not surprising that those who use skunk daily have the highest risk of all, because skunk has the highest concentration of Δ9-THC and a relative lack of cannabidiol with its protective effect.”

She added: “Unfortunately, skunk is displacing traditional cannabis preparations in many countries, and the availability of skunk on the UK ‘street’ market has steadily increased over the past six years. Public education about the risks of heavy use of high-potency cannabis is vital.”

A copy of the paper can be accessed:


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