But weaker forms such as hash don't carry the same risk of mental illness

Super-strength strains of cannabis are responsible for up to a quarter of new cases of psychotic mental illness, 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 suffer a psychotic episode than those who have never tried it.

The study, leaked to The Mail on Sunday ahead of its publication, is set to reignite the 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.

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Scientists are warning that super-strength strains of cannabis are responsible for up to a quarter of new cases of psychotic mental illness (file image)

According to Crime Survey figures for England and Wales, over a million youngsters aged 16 to 24 smoke cannabis. Regular users are most at risk, prompting experts to warn that youngsters need to be aware of the dangers of skunk, which has been specially cultivated to be four times as strong as the cannabis smoked by previous generations.

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

They will reveal there is a key difference between potent skunk strains and '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 people 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 part in the new study. 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 research appears to show a striking difference between the effects of skunk and the weaker form of cannabis, hash resin, revealing that hash seemed not to add to a person's risk of psychosis – even if smoked daily

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

This was almost double the previous highest estimate of psychiatric cases linked to the drug – 13 per cent – from a 2002 Dutch study.

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.'

But the research appears to show a striking difference between the effects of skunk and the weaker form of cannabis, hash resin.

It will reveal that hash seemed not to add to a person's risk of psychosis – even if smoked daily.

Skunk is shorthand for around 100 strains of cannabis that contain a high proportion of tetrahydrocannabinol (THC), the drug's primary psychoactive compound. But the levels of another compound, cannabidiol – which may have anti-psychotic effects – are the reverse, high in hash and virtually zero in skunk.

The researchers speculate this could be due to the differing chemical make-up of the two forms: 'The presence of cannabidiol [in hash] might explain our results, which showed that hash users do not have any increase in risk of psychotic disorders compared with non-users.'

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.'