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.



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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.



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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.



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In the US, cannabis is becoming stronger and more popular. Over the past 20 years, the strength of cannabis seized by the Drug Enforcement Administration has increased from 4%-12% THC. Meanwhile, the number of users rose from 14.5 million to 22.2 million in the seven years to 2014.



Coinciding with the upwards trend, young people’s perceptions of the risks of cannabis have fallen, a consequence perhaps of the public discussion over legalisation and fewer restrictions for medicinal uses, according to the US government’s National Institute on Drug Abuse (Nida).

“It is important to educate the public about this now,” said Nora Volkow, director of Nida. “Kids who start using drugs in their teen years may never know their full potential. This is also true in relation to the risk for psychosis. The risk is significantly higher for people who begin using marijuana during adolescence. And unfortunately at this point, most people don’t know their genetic risk for psychosis or addiction.”

In the UK, cannabis is the most popular illegal drug, and according to Public Health England data, more young people enter treatment centres for help with cannabis than any other drug, alcohol included. The number of under-18s in treatment for cannabis rose from 9,000 in 2006 to 13,400 in 2015. The drug now accounts for three-quarters of young people receiving help in specialist drugs centres. The most common age group is 15- to 16-year-olds.



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The reasons for the upward trend are unclear. As hard drugs fall in popularity, clinical services may simply pull in more cannabis users. But the rise in young people in treatment may be linked to skunk, a potent form of cannabis that has taken over the market and edged out the traditional, weaker resins.

Skunk and other strong forms of cannabis now dominate the illicit drugs markets in many countries. From 1999-2008, the cannabis market in England transformed from 15%-81% skunk. In 2008, skunk confiscated from the street contained on average 15% of the high-inducing substance THC (delta-9 tetrahydrocannabinol), three times the level found in resin seized that year. The Home Office has not recorded cannabis potency since.



“There is no doubt that high-potency cannabis, such as skunk, causes more problems than traditional cannabis, or hash,” Murray told the Guardian. “This is the case for dependence, but especially for psychosis.”



Ian Hamilton, a mental health lecturer at the University of York, said more detailed monitoring of cannabis use is crucial to ensure that information given out is credible and useful. Most research on cannabis, particularly the major studies that have informed policy, are based on older low-potency cannabis resin, he points out. “In effect, we have a mass population experiment going on where people are exposed to higher potency forms of cannabis, but we don’t fully understand what the short- or long-term risks are,” he said.

In Australia, a 2013 study found nearly half of the cannabis confiscated on the streets contained more than 15% THC. Prof Wayne Hall, director of the Centre for Youth Substance Abuse Research at the University of Queensland, said that while most people can use cannabis without putting themselves at risk of psychosis, there is still a need for public education.

“We want public health messages because, for those who develop the illness, it can be devastating. It can transform people’s lives for the worse,” he said. “People are not going to develop psychosis from having a couple of joints at a party. It’s getting involved in daily use that seems to be the riskiest pattern of behaviour: we’re talking about people who smoke every day and throughout the day.”

The evidence that cannabis can cause psychosis is not 100% conclusive. It is still possible that people who are prone to psychosis are simply more likely to use the drug. The catch is that absolute proof of causality cannot be obtained. The harm caused by cigarettes was easy to confirm: paint tobacco tar on mice and watch the tumours form. You can give cannabis to animals and watch what happens, but you cannot recognise a psychotic mouse. Nor can scientists order thousands of teenagers to smoke pot every day and compare them to a control group that abstained 10 years later.



“When you’re faced with a situation where you cannot determine causality, my personal opinion is why not take the safer route rather than the riskier one, and then figure out ways to minimise harm?” said Amir Englund, a cannabis researcher at King’s College London.



In the 1960s, cannabis in the Netherlands had less than 3% THC, but today high potency strains average 20%. Jim van Os, professor of psychiatry at Maastricht University medical centre, said public health messages are now justified. He believes people should be deterred from using cannabis before the age of 18, warned off the stronger forms, and urged not to use cannabis alone or to cope with life’s problems.



Public health campaigns can easily fail though. To prevent a single case of schizophrenia, several thousand heavy cannabis smokers would probably have to quit. That could change with better understanding of who is most at risk. “Once we really understand what it is about cannabis that increases some people’s risk, and in what context, we can maybe start to identify people more highly at risk, and targeted campaigns are likely to be much more effective,” said Suzi Gage, senior research associate at Bristol University.



As with any campaign, credibility is everything. “There is an issue of getting a message through to those who are vulnerable without causing alarm, being overly sensationalist and thus being ignored,” said Dr Wendy Swift, of the National Drug and Alcohol Research Center at the University of New South Wales. “There is good evidence that cannabis use, particularly early onset and frequent use when young, can cause problems on a number of fronts into young adulthood. This to me is the group we need to get our messages through to the most, along with those who have a family history of mental illness or have mental health problems themselves.”



A government spokesperson said its position on cannabis was clear. “We must prevent drug use in our communities and help people who are dependent to recover, while ensuring our drugs laws are enforced. There is clear scientific and medical evidence that cannabis is a harmful drug which can damage people’s mental and physical health, and harms communities.”

