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'Skunk’ causes a quarter of new cases of psychosis

Doctors call for more education about the mental health risks of using high potency cannabis

Mark Gould

Monday, 16 February 2015

A new study suggests that almost a quarter of new cases of psychosis result from use of high potency ‘skunk-like’ cannabis. The research*, to be published later this week in The Lancet Psychiatry, suggests that the risk of psychosis is three times higher for users of potent ‘skunk-like’ cannabis than for non-users.

The study of 780 people was carried out by King's College London's Institute of Psychiatry, Psychology and Neuroscience. The research was carried out over several years, comparing 410 patients aged 18-65 who reported a first episode of psychosis at a south London psychiatric hospital with 370 healthy participants within the same age range from the same area of London.

Scientists found the risk of psychosis was five times higher for those who use 'skunk' every day compared with non-users. They also concluded the use of hash, a milder form of the drug, was not associated with increased risk of psychosis.

Skunk contains more Tetrahydrocannabinol (THC) - the main psychoactive ingredient - than other types of cannabis. Unlike skunk, hashish - cannabis resin - contains substantial quantities of another chemical called cannabidiol or CBD and research suggests this can act as an antidote to the THC, counteracting psychotic side effects.

The findings have prompted calls for more education of young people as to the risks of drug use and the long-term effects on their health.

"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, lead author on the research.

She added: "The results show that psychosis risk in cannabis users depends on both the frequency of use and cannabis potency."

Dr Di Forti told the BBC Radio 4's Today programme that the availability of skunk-like cannabis was becoming more widespread.

"In London, it's very difficult to find anything else," she said.

"There were lots of reports from police across the UK saying we have become a great producer of skunk. And not only do we use it locally but we export, so this is a Made in England product."

Someone suffering from psychosis would often be "extremely paranoid and become very suspicious" about the people around them, she added.

She has called for "a clear public message" to cannabis users, comparable to medical advice on alcohol and tobacco. GPs should be encouraged to ask how often and what type of cannabis patients use, she added.

Sir Robin Murray, professor of psychiatric research at King's, commented: "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."

Mark Winstanley, CEO of Rethink Mental Illness said: “People often think of cannabis as a safe or harmless drug, but this study clearly shows that smoking ‘skunk’ greatly increases your chances of developing serious mental health problems. Reclassifying cannabis isn’t the answer. What we really need to see is more education about the risks of using the drug, especially for younger people, who are particularly vulnerable. Essentially, smoking cannabis is like playing a very real game of Russian roulette with your mental health.”

A Home Office spokesman said the findings backed up the government's approach: "Drugs such as cannabis are illegal because scientific and medical evidence demonstrates they are harmful.

"This report serves to emphasise how they can destroy lives and communities."


* Marta Di Forti, et al. Proportion of patients in south London with first-episode psychosis attributable to use of high potency cannabis: a case-control study. Lancet Psychiatry 2015. doi: 10.1016/S2215-0366(14)00117-5

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