News archive 2009

Skunk ‘poses greatest risk of psychosis’

01 Dec 2009, PR 261/09

Researchers at the Institute of Psychiatry at King’s 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 King’s, 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'.

Public education vital

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 per cent Δ9-THC and less than 1.5 per cent 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 per cent 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.'



Notes to editors

The authors
The authors of 'High-potency cannabis and the risk of psychosis' are 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, and it can be accessed here:  http://www.rcpsych.ac.uk/publications/journals/bjpinfo1.aspx

King's College London
King's College London is one of the top 25 universities in the world (Times Higher Education 2009) and the fourth oldest in England. A research-led university based in the heart of London, King's has more than 21,000 students from nearly 140 countries, and more than 5,700 employees. King's is in the second phase of a £1 billion redevelopment programme which is transforming its estate.

King's has an outstanding reputation for providing world-class teaching and cutting-edge research. In the 2008 Research Assessment Exercise for British universities, 23 departments were ranked in the top quartile of British universities; over half of our academic staff work in departments that are in the top 10 per cent in the UK in their field and can thus be classed as world leading. The College is in the top seven UK universities for research earnings and has an overall annual income of nearly £450 million.

King's has a particularly distinguished reputation in the humanities, law, the sciences (including a wide range of health areas such as psychiatry, medicine and dentistry) and social sciences including international affairs. It has played a major role in many of the advances that have shaped modern life, such as the discovery of the structure of DNA and research that led to the development of radio, television, mobile phones and radar. It is the largest centre for the education of healthcare professionals in Europe; no university has more Medical Research Council Centres.

King's College London and Guy's and St Thomas', King's College Hospital and South London and Maudsley NHS Foundation Trusts are part of King's Health Partners. King's Health Partners Academic Health Sciences Centre (AHSC) is a pioneering global collaboration between one of the world's leading research-led universities and three of London's most successful NHS Foundation Trusts, including leading teaching hospitals and comprehensive mental health services. For more information, visit: www.kingshealthpartners.org.



Further information
Louise Pratt
Institute of Psychiatry
Email: louise.a.pratt@kcl.ac.uk
Tel: +44 20 7848 5378

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