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'Skunk-like' cannabis associated with 24% of new psychosis cases

Scientists have found that 24% of all new cases of psychosis are associated with the use of high potency ‘skunk-like’ cannabis.  In addition, the risk of psychosis is three times higher for potent ‘skunk-like’ cannabis users and five times higher for those who use it every day, according to research from the Institute of Psychiatry, Psychology & Neuroscience (IoPPN) at King’s College London, published today in Lancet Psychiatry.

The findings, based on a study of nearly 800 people aged 18-65 in South London, have major implications for prevention of cannabis-associated psychosis as well as developing new treatments. 

“Compared with those who had never tried cannabis, users of high potency ‘skunk-like’ cannabis had a threefold increase in risk of psychosis,” said Dr Marta Di Forti from the IoPPN, King’s College London, and lead author on the research. “The risk to those who use every day was even higher; a fivefold increase compared to people who never use. 

“The results show that psychosis risk in cannabis users depends on both the frequency of use and cannabis potency.  The use of hash was not associated with increased risk of psychosis,” she added.

Sir Robin Murray, Professor of Psychiatric Research at the IoPPN at King’s and senior researcher on the study stated: “It is now well known that use of cannabis increases the risk of psychosis. However, sceptics still claim that this is not an important cause of schizophrenia-like psychosis. This paper suggests that we could prevent almost one quarter of cases of psychosis if no-one smoked high potency cannabis. This could save young patients a lot of suffering and the NHS a lot of money.”

Between 2005 and 2011, researchers worked with 410 patients aged 18-65 who reported a first episode of psychosis at the South London and Maudsley NHS Foundation Trust. A further 370 healthy participants from the same area of South London were included as controls. A main finding was that the frequency of use and cannabis potency, which are often overlooked when determining how harmful the drug can be, are essential factors in the mental health effects on users. These factors are not sufficiently considered by doctors.

“As with smoking tobacco and drinking alcohol you need a clear public message,” said Dr Di Forti. “When a GP or psychiatrist asks if a patient uses cannabis it’s not helpful; it’s like asking whether someone drinks. As with alcohol, the relevant questions are how often and what type of cannabis. This gives more information about whether the user is at risk of mental health problems; awareness needs to increase for this to happen.”

This project has been carried out with the support of: 1. The GAP (Genetic and Psychosis) and PUMP study teams, Institute of Psychiatry, Psychology & Neuroscience, King’s College London; 2. The South London and Maudsley (SLaM) NHS Foundation Trust; 3. Funding was provided by the National Institute for Health Research (NIHR) Maudsley Biomedical Research Centre, SLaM and the Institute of Psychiatry, Psychology & Neuroscience at King’s College London; The Psychiatry Research Trust, the Maudsley Charity Research Fund and by the European Community's Seventh Framework Program under grant agreement No. HEALTH-F2-2009-241909 (Project EUGEI).

Paper reference: Di Forti, M. et al. ‘Proportion of patients in south London with first-episode psychosis attributable to use of high potency cannabis: a case-control study’ published in Lancet Psychiatry DOI:

For further information contact Tom Bragg, Press Officer at IoPPN, King’s College London, on +44(0)2078485377 or email