The research, published in the Lancet Psychiatry journal, found that skunk – with a THC level of more than 10 per cent (AP Photo/David Zalubowski, File)

Powerful ‘skunk’ cannabis flooding the streets of Britain increases the risk of psychosis five-fold, a major study reveals.

The problem is so widespread that nearly a third of psychosis cases in London are caused by the drug, researchers found.

They warned that 94 per cent of all cannabis available on the streets of the capital is now in the form of skunk.

It is cultivated to have super-high levels of the psychoactive ingredient THC, making it up to ten times more powerful than the ‘weed’ and ‘hashish’ common 20 to 30 years ago.

Researchers from King’s College London studied 2,100 people in 11 cities in Europe and South America in the biggest study of its kind.

They found that the link with psychotic disorders such as schizophrenia and paranoid delusion was strongest in London and Amsterdam – the two cities where high-potency cannabis is most commonly available.

Voices for legalisation of cannabis have been growing in recent months, buoyed by the Government’s decision to permit limited use for medical treatment. The researchers warned against following the lead of Canada and the American states of Colorado and California, where legalisation has seen the potency increase.

And they said that even medicinal cannabis oil – available in the UK for a very limited number of people – should come with a warning of psychosis as a possible side effect.

Professor Sir Robin Murray, one of the researchers, said: ‘If you are going to legalise, unless you want to pay for a lot more psychiatric beds and a lot more psychiatrists then you need to devise a system in a way that will not increase the consumption and will not increase the potency. Because that is what has happened in the US states where there has been legalisation for recreational use.

‘The critical question is whether medicinal use remains medicinal. The problem in California and Canada was that medicinal use became a synonym for recreational use.

‘You could go on the internet and tell a doctor, “I have headaches, I have back pain, I feel better if I have cannabis”. The main reason they legalised it was to try to control the amount of so-called medicinal use there, hoping that there would be a decrease in the use.’ There was not a risk of that in the UK ‘at present’ because cannabis oil is strictly controlled.

The research, published in the Lancet Psychiatry journal, found that skunk – with a THC level of more than 10 per cent – increased the odds of psychosis 4.8-fold in a person who smoked every day compared with someone who never used the drug. Using it more than once a week was less dangerous, but still increased the risk 1.6-fold.

Low-strength cannabis – with a THC level below 10 per cent – increased the odds of psychosis 2.2-fold if used daily and 1.4-fold if used more than once a week.

Study leader Dr Marta Di Forti said the effect of skunk on mental health is so high that in cities where it is widely available it has a huge impact on numbers diagnosed with psychotic disorders.

If skunk was taken off the streets of London, new cases of psychosis would drop 30 per cent, from 46 to 32 cases per 100,000 people, she said.

This was second only to Amsterdam, which would see a 50 per cent fall. In Cambridge, the only other British city to take part in the study, 8 per cent of psychosis cases were attributed to strong cannabis.

Dr Di Forti said even low-THC cannabis oil, used for epilepsy and MS, should come with a warning of possible mental health effects. The research comes after a Lancet study said cannabis is responsible for 60,000 cases of depression in young people in Britain.

Psychosis is a much rarer condition than depression, so the numbers affected will be far smaller, but the consequences are generally far more serious.

Voices are growing in Britain for cannabis to be legalised. Former deputy prime minister Sir Nick Clegg says making mild forms legally available would stop people using skunk.

Even the Royal College of Psychiatrists is reviewing its position to consider the view that decriminalisation would give the government power to regulate its strength and generate taxes.

Daily Mail