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Does cannabis make people violent?

by Stephen Armstrong The Sunday Times, posted 28 06 2020

The drug, which may have been a factor in the Reading stabbings, is under the spotlight over fears it triggers psychosis and violence

Could cannabis make somebody kill? In the past week, a picture has been forming of Khairi Saadallah, 25, arrested after last weekend’s Reading knife attack in which David Wails, Joe Ritchie-Bennett and James Furlong died.

Saadallah had suffered mental health problems, receiving treatment for post-traumatic stress disorder and other difficulties. He also reportedly smoked cannabis — a fact to which some commentators drew attention. But scientists, too, are becoming increasingly interested in such details.

Sir Robin Murray, professor of psychiatric research at King’s College London, is reviewing research into the drug’s impact on crime and believes there is a relationship between heavy consumption and violence.

“I’m a psychiatrist: I see people who have become psychotic, smoke excessively and become paranoid,” he says. “[When you give] cannabis to animals, you see it stokes up the chemicals associated with psychosis. If you think the world is against you and people will harm you, it’s possible you will fight back — so cannabis violence is often senseless and bizarre.”

The issue is attracting millions of pounds in research grants after countries such as Portugal and some American states decriminalised cannabis.

A 2019 study from King’s College London and a 2018 study from the University of Oulu in Finland found that smoking high-potency cannabis every day could increase the risk of developing psychosis by nearly five times.

“There is no question that cannabis causes transient psychosis,” says the American journalist Alex Berenson, author of Tell Your Children: The Truth About Marijuana, Mental Illness and Violence. “It can increase the risk of conditions such as schizophrenia. Cannabis violence has a particular flavour — it’s often knife crime, often random, often overly violent, sometimes from people with no history of violence. The idea that cannabis doesn’t cause violence is the result of generations of mythmaking.”

Others disagree, including a study by the US National Epidemiological Survey on Alcohol and Related Conditions. “There is no empirical evidence to support this claim,” argues Carl Hart, Dirk Ziff professor of psychology at Columbia University. “In the US before Covid there were researchers like my team who give thousands of doses of marijuana, as much as eight cigarettes per day for weeks at a time — and if that caused violence, our government would not have given us research funding.”

In Wolverhampton last year, cannabis took some of the blame for the rape and murder of 14-year-old Viktorija Sokolova, who was hit with a hammer by Ayman Aziz, 17. It was also implicated in Aaron Campbell’s rape and murder of Alesha MacPhail, 6, on the Isle of Bute in 2018; the 2005 murders of the teenagers Steven Bayliss and Nuttawut Nadauld in Berkshire by their friend Tom Palmer; and Robbie McIntosh’s stabbing to death of Anne Nicoll, a civil servant, in Dundee in 2002.

Then there is the terror link. Sudesh Amman, shot dead by police in Streatham, south London, in January after stabbing three people, had served time for cannabis possession. Salman Abedi, the Manchester Arena bomber, was a cannabis-smoking university dropout. PC Keith Palmer’s killer, Khalid Masood, was a heavy drug user who stabbed a man in the face while high. Muhiddin Mire, who attacked Lyle Zimmerman with a knife at Leytonstone Tube station in east London in 2015, had mental health problems after smoking cannabis and had been treated for paranoia.

According to the Office for National Statistics, for the year ending March 2019, 18% of homicide suspects in England and Wales were under the influence of alcohol — with 5% taking an illicit drug and 9% under the influence of both.

More than one factor is usually involved. Hart points to an Ontario court blaming cannabis for the 2018 murder of John Kehl, 67, by his son Adam, 30. But Adam was also taking escitalopram, an antidepressant associated with hallucinations and vivid dreams. A 2015 study of Swedish data found that young people taking antidepressants of the same type were 43% more likely to be convicted of assault.

“There is work linking [cannabis] with psychosis, but no one has made anything really stand up. Cannabis by itself does not cause violence,” says Richard Hammersley, professor of health psychology at the University of Hull. “If you mix cannabis with alcohol or synthetic cannabinoids like spice, that’s a different story, especially if you have psychotic problems and take a mixture of God knows what. But cannabis has been tested for centuries by millions of recreational users and there’s no causal link.”

If tetrahydrocannabinol (THC), the principal psychoactive constituent in the drug, does trigger psychosis, the key chemical likely to be involved is dopamine — the pleasure/reward neurotransmitter that, according to recent research, appears to be overactive in some parts of a schizophrenic brain and underactive in others.

In interfering in the dopamine pathway, THC might cause an imbalance and trigger a psychotic episode.

“It’s not easy to isolate as an independent variable. It’s mixed with other drugs, and used to self-medicate by people with mental health issues,” says Dr Abigael San, a chartered clinical psychologist. “If people have aggression issues, the dopamine reward system can be triggered by violence. If a heavy cannabis smoker from a violent background is hallucinating and the fight-or-flight system switches on, they can become aggressive and be prone to violence.”

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