Friday, 4 December 2009

Skunk ‘poses greatest risk of psychosis’

RCPsych - Press Release

Embargoed until 01 December 2009
People who smoke skunk, the most potent form of cannabis available in UK, are almost seven times more likely to develop psychotic illnesses like schizophrenia than those who use traditional hash, according to new research.

The study, published in the December issue of the British Journal of Psychiatry, was carried out by psychiatrists and researchers at the Institute of Psychiatry, King’s College London (KCL).

The team collected information on cannabis use from 280 people attending South London & Maudsley NHS Foundation Trust 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.

Moreover, 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: “Patients experiencing their first episode of psychosis were not more likely to have ever taken cannabis or to have started doing so earlier than the control group. However, psychosis was associated with more frequent and longer use of cannabis. Our most striking finding is that patients with a first episode of psychosis preferentially used high-potency cannabis preparations of the 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.”

The study was funded by the Maudsley Charitable Fund and the National Institute for Health Research specialist Biomedical Research Centre for Mental Health at South London & Maudsley NHS Foundation Trust and KCL.

For further information, please contact Liz Fox or Deborah Hart in the Communications Department.
Telephone: 020 7235 2351 Extensions. 298 or 127


Di Forti M, Morgan C, Dazzan P, Pariante C, Mondelli V, Reis Marques T, Handley R, Luzi S, Russo M, Paparelli A, Butt A, Stilo SA, Wiffen B, Powell J and Murray RM (2009) High-potency cannabis and the risk of psychosis, British Journal of Psychiatry, 195: 488-491

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