The risks of heavy cannabis for mental health are serious enough to warrant global public health campaigns, according to international drugs experts who said young people were particularly vulnerable.
The warning from scientists in the UK, US, Europe and Australia reflects a growing consensus that frequent use of the drug can increase the risk of psychosis in vulnerable people, and comes as the UN prepares to convene a special session on the global drugs problem for the first time since 1998. The meeting in New York next week aims to unify countries in their efforts to tackle issues around illicit drug use.
While the vast majority of people who smoke cannabis will not develop psychotic disorders, those who do can have their lives ruined. Psychosis is defined by hallucinations, delusions and irrational behaviour, and while most patients recover from the episodes, some go on to develop schizophrenia. The risk is higher among patients who continue with heavy cannabis use.
Public health warnings over cannabis have been extremely limited because the drug is illegal in most countries, and there are uncertainties over whether it really contributes to mental illness. But many researchers now believe the evidence for harm is strong enough to issue clear warnings.
“It’s not sensible to wait for absolute proof that cannabis is a component cause of psychosis,” said Sir Robin Murray, professor of psychiatric research at King’s College London. “There’s already ample evidence to warrant public education around the risks of heavy use of cannabis, particularly the high-potency varieties. For many reasons, we should have public warnings.”
The researchers are keen not to exaggerate the risks. In the language of the business, cannabis alone is neither necessary nor sufficient to cause psychosis. But the drug inflicts a clear burden on the vulnerable. Estimates suggest that deterring heavy use of cannabis could prevent 8-24% of psychosis cases handled by treatment centres, depending on the area. In London alone, where the most common form of cannabis is high-potency skunk, avoiding heavy use could avert many hundreds of cases of psychosis every year.