Strong cannabis causes one in four cases of psychosis: Users three times more likely to have an episode than those who have never tried it

Scientists at King’s College London say youngsters must be told of risks
Study will add weight to calls for a tougher stance on cannabis users
More than 1 million 16-24-year-olds in England and Wales smoke the drug
Those who use weaker 'hash' over potent 'skunk' less affected by episodes

Published: 19:27 EST, 15 February 2015 | Updated: 04:17 EST, 16 February 2015

Concern: As many as a quarter of new cases of psychotic mental illness can be blamed on super-strength strains of cannabis

As many as a quarter of new cases of psychotic mental illness can be blamed on super-strength strains of cannabis, scientists will warn this week.

The potent form of the drug – known as ‘skunk’ – is so powerful that users are three times more likely to have a psychotic episode than those who have never tried it.

The study, leaked to The Mail on Sunday, will reignite debate around Britain’s drug laws – and will add weight to calls for a tougher stance towards those caught dealing or in possession of cannabis.

According to Crime Survey figures for England and Wales, more than a million youngsters aged 16 to 24 smoke cannabis.

Regular users are most at risk of a psychotic episode, prompting experts to warn that youngsters need to be aware of the dangers of skunk, which has been cultivated to be four times as strong as cannabis smoked by previous generations.

The researchers, led by a team at the Institute of Psychiatry at King’s College London, conclude there is an ‘urgent need… to inform young people about the risks of high-potency cannabis’, despite a worldwide trend towards relaxing drug laws.

They will reveal there is a key difference between potent skunk strains and ordinary ‘hash’. Those who used these ‘weaker’ forms did not seem to suffer the same increase in risks.

Psychosis is defined as a form of mental illness where people experience delusions, hallucinations, or both at the same time.

Associated with conditions such as schizophrenia and bipolar disorder, some victims are so badly affected that they end up committing suicide or seriously harming others because they believe they are being ordered to do so by voices in their heads.

The findings will add substance to a 2012 report by the Schizophrenia Commission, which recommended the need for ‘warnings about the risks of cannabis’ to mental health.

That report was chaired by schizophrenia expert Professor Sir Robin Murray, who also played a key role in the new study.

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Prevalent: According to Crime Survey figures for England and Wales, more than a million youngsters aged 16 to 24 smoke cannabis

It looked at cannabis use in two groups, each containing about 400 people, from 2005 to 2011. Those in the first group had all suffered ‘first-episode psychosis’ – a diagnosed first occurrence of the disorder.

The second group were volunteers who agreed to answer questions about themselves – including on cannabis use and mental health history.

Some had suffered psychosis, others not. They were not told the nature of the project. The academics found those in the first group were more likely to smoke cannabis daily – and to smoke skunk – than those in the second.

The researchers say: ‘Skunk use alone was responsible for 24 per cent of adults presenting with first-episode psychosis to the psychiatric services in South London.’

Michael Ellis (above), a Tory member of the Home Affairs Select Committee, said the 'study illustrates that those in government and the police must be careful to send out the right message' about cannabis

The latest research, to be published in The Lancet, concludes: ‘People who used cannabis or skunk every day were roughly three times more likely to have a diagnosis of a psychotic disorder than were those who never used cannabis.’

Michael Ellis, a Tory member of the Home Affairs Select Committee, said: ‘This powerful new study illustrates that those in government and the police must be careful to send out the right message.

‘Cannabis isn’t a harmless drug: it can ruin lives.’

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