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Cannabis and mental illness link to violence


Canadian research finds that marijuana use by people with mental illness is a strong predictor of violent behavior. Jeff Glorfeld reports.


New findings add to our understanding of the negative consequences of chronic cannabis use amid psychiatric patients.
New findings add to our understanding of the negative consequences of chronic cannabis use amid psychiatric patients.
AJ PHOTO/BSIP

Habitual cannabis use among the mentally ill is being linked to an increase in violent behaviour, according to a study by researchers at the University of Montreal.

The research from a team led by Alexandre Dumais of the university’s department of psychiatry studied 1136 patients aged from 18 to 40. All had recurring mental illnesses and had been seen by doctors at least five times during the preceding year, following discharge from psychiatric care.

Previous research, such as a 2014 study led by Thor Norstrom of Stockholm University, has linked cannabis dependence to violence.

According to the latest study, published in Frontiers in Psychiatry, patients who reported at each follow-up visit that they continued to smoke cannabis were more than twice as likely as non-users to engage in violent behaviour.


The researchers say their findings confirm the detrimental role of chronic cannabis use in patients with mental illness.

“The association between persistent cannabis use and violence is stronger than that associated with alcohol or cocaine," Dumais says.

The study concluded that persistent cannabis use among patients who leave a psychiatric hospital and enter an outpatient clinic should be considered as an indicator of future violent behaviour.

Dumais notes that the behaviour tends to fade with time, which could be explained by patients becoming more involved in their treatment, and by better support from friends and family.

“Even though we observed that violent behaviour tended to decrease during follow-up periods, the association remained statistically significant," he says.

The study terms the finding “particularly relevant because they “add to our understanding of the negative consequences of chronic cannabis use amid psychiatric patients”.

Read about the science behind medical cannabis in the Spring issue of Cosmos magazine. In stores now!

Jeff Glorfeld is a former senior editor of The Age, and is now a freelance journalist based in regional Victoria.
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