A new study by Montreal researchers shows regular cannabis use is associated with an increase in violent behaviour among young adults after discharge from a psychiatric hospital.
The study followed 1,136 patients between the ages of 18 and 40 for a year once they were released. More than 60 per cent were men.
Conducted by the Institut en Santé Mentale de Montréal (Montreal Institute of Mental Health), the study found that pot use in people with severe mental illness — schizophrenia, bipolar, psychotic and depressive disorders — can lead to crime and violence.
To be clear, said lead researcher Dr. Alexandre Dumais, a psychiatrist at the Philippe-Pinel Institute and associate professor at the Université de Montréal, they’re talking about serious aggressive behaviour from assault with a knife to rape.
While previous research established a link between marijuana use and violent behaviour, the nature of the relationship wasn’t clear. This study shows that with chronic use, the longer individuals report using weed, the greater the risk.
“We saw 144-per-cent greater risk of violent behaviour, or nearly two and half times more the risk than patients who did not use pot,” Dumais said.
The study raises questions as Canada expects to legalize and regulate marijuana from production to consumption by next July.
Published in the journal Frontiers of Psychiatry, Dumais’s results add to the understanding of the negative consequences of chronic cannabis use in psychiatric patients.
It is also the first to demonstrate that this link between violence and marijuana is not two-way, but one-way. Is it the violent, anti-social types who turn to drugs, or do the drugs themselves promote violence? That’s a controversial hypothesis in the scientific literature, Dumais explained.
“We wanted to verify whether there is a temporal link between using and eventual violent behaviour. And that’s what we saw. When people used (pot), there was violent behaviour,” he said. “When there was violent behaviour, there was no increased use of drugs.”
Dumais said his team is conscious of the negative stereotype that the mentally ill are violent. That’s a false stereotype, and not the goal of the study, Dumais said.
“But is there a higher risk among these people? The answer is yes. What we try to do in our lab is find out why.”
According to the scientific literature, drug and alcohol use is quite common in this population, he said, “which likely increases their risks of becoming violent, beyond the fact of having mental illness.”
One unexpected finding is the data showed the association between pot and violent behaviour is stronger than the link to alcohol or cocaine.
“Studies clearly show a link between alcohol, cocaine and violence — there’s no controversy there,” he said. “That (our study) shows a greater link than two other substances that are already well documented, that surprised us hugely as a team.”
To those who say that smoking pot relaxes them, Dumais said that might be true for the occasional user, but the theory is that chronic use changes brain function to lower inhibition and increase irritability.
One of the study limitations is that the violence was self-reported. However, Dumais noted that cases corroborated by the judicial system also underestimate the prevalence of violence.
The study did not take into account the amount of drugs used. The next step is to set up a study to measure dose-related risk of violence, Dumais said, in part to establish a safe amount of pot consumption, similar to current safe-alcohol recommendations.
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