Warning reflects growing consensus that frequent use of the drug raises the risk of psychotic disorders in vulnerable people
The call for action from scientists in the UK, US, Europe and Australia reflects a growing consensus among experts that frequent cannabis use can increase the risk of psychosis in vulnerable people and lead to a range of other medical and social problems.
The warning 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 drugs 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 those 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.”
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.
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.”
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, which 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.
“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.
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.”
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