Powerful ‘skunk’ cannabis flooding the streets of Britain increases the risk of psychosis five-fold, a major study has found.
The problem is so widespread that an astonishing 30 per cent of all cases of psychosis in London are caused by the drug, researchers found.
They warned that 94 per cent of all cannabis available on the streets of the UK capital is now of the skunk form.
It is cultivated to have super-high levels of psychoactive THC – making it up to 10 times more powerful than the ‘weed’ and ‘hashish’ more common 20 or 30 years ago.
As much as 94 per cent of all cannabis available from drug dealers in the UK is now skunk, according to the researchers. This form of the drug tends to have higher amounts of THC, the chemical which makes people ‘high’ (stock image)
The King’s College London researchers studied more than 2,100 people in 11 cities in Europe and South America, in the biggest study of its kind ever undertaken.
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 US states of Colorado and California, where legalisation has seen the potency of cannabis increase even further.
And they said that even medicinal cannabis oil – which is 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 on the paper, 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.’
He added: ‘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 and control the amount of so-called medicinal use that is being used there, hoping that there would be a decrease in the use.’
He said there was not a risk of that in the UK ‘at present’ because cannabis oil is so strictly controlled.
But be said politicians here should be very cautious and closely monitor the situation on the other side of the Atlantic.
‘Studying animals is very expensive – you have to give them cannabis, you have to feed them, you have to have cages for them,’ Sir Robin joked.
‘But North Americans come free.’
The research team, whose work is published in the Lancet Psychiatry journal, found that skunk – which they defined as having a THC level of more than 10 per cent – increased the odds of psychosis 4.8-fold if smoked every day, compared to 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 – that which has a THC level of less than 10 per cent – increased the odds of psychosis 2.2 times if used daily and 1.4 times 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 of people diagnosed with psychotic disorders.
She said 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.
This was second only to Amsterdam, where the eradication of skunk would cut psychosis 50 per cent, from 38 to 19 cases per 100,000.
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 cannabis oil used for epilepsy and MS – which has very low levels of THC – should come with a warning of possible mental health effects until it is proven that there is no risk.
‘There is no such thing as a medicine which doesn’t come with a side effect,’ she said.
‘That doesn’t stop me prescribing them.
‘But… we should include risk of psychosis in the side-effect profile, both so people that prescribe it can monitor it effectively, and so that people who take it know what to look out for.’
The research comes after another major Lancet study last month concluded that cannabis is responsible for 60,000 cases of depression among 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 legalisation.
Sir Nick Clegg, the former deputy prime minister, has repeatedly claimed making mild forms of the drug legally available would stop people using skunk.
The All-Party Parliamentary Group on Drug Policy Reform – led by Baroness Molly Meacher – wants sick people to be allowed to grow their own cannabis under licence.
And even the Royal College of Psychiatrists – which for years has warned against legalisation – is currently reviewing its position to take into account the view that decriminalisation would give the government power to both regulate its strength and generate tax from its sale.
Dr Adrian James, registrar of the Royal College of Psychiatrists, said last night: ‘Cannabis carries severe health risks and users have a higher chance of developing psychosis.’
But he added: ‘Because of these risks, a good drugs strategy should focus on preventing and reducing harm, not on diverting people to the criminal justice system.’
Despite agreeing to the limited prescription of medicinal cannabis last year, Theresa May and the Government remain opposed to legalisation.