Researchers at the Institute of Psychiatry, Psychology and Neuroscience (IoPPN) combined data from 63 studies involving more than 265,000 people. They analyzed findings from both the general population and people receiving psychiatric treatment.

What the cannabis and violence study found

The researchers used the World Health Organization’s broad definition of violence. This includes physical, sexual and psychological violence against other people, both within and outside intimate relationships.

The review also distinguished between self-reported violence and incidents that resulted in a criminal conviction. When individual studies measured several forms of violence, the researchers focused on the most severe types reported.

Dr Marta Di Forti, co-author of the study from King’s College London’s Institute of Psychiatry, Psychology and Neuroscience (IoPPN) and South London and Maudsley NHS Foundation Trust, said:

“As a clinician, I have seen how heavy cannabis use can contribute to violence. We have long been cautious about how far we can go in explaining this association, but a review of this scale allows us to talk about the risks with more confidence, while being careful not to prejudice people who use cannabis more broadly.”

Cannabis use linked to violence and victimization

The study found that people who used cannabis were about 1.5 times more likely to be victims of violence. This association was stronger among women than among men.

In the general population, people who used cannabis were about twice as likely to commit a violent act as those who did not use cannabis. Among people receiving psychiatric treatment, cannabis users were also about twice as likely to commit a violent act as psychiatric patients who did not use cannabis.

Professor Sir Robin Murray, lead author of the study from King’s College London’s Institute of Psychiatry, Psychology and Neuroscience, said:

“In the general population, people who used cannabis were about twice as likely to commit a violent act compared to those who did not use it. Among people receiving psychiatric treatment, people who used cannabis were about twice as likely to commit a violent act compared to other psychiatric patients who did not use cannabis. This association held up in studies that followed people over time, but was smaller than in studies that looked at a single point in time. It was 3 to 4 times more.”

“As cannabis becomes increasingly commercialized and legalized around the world, discussions about its risks have focused primarily on those who use it. This review shows that both committing and experiencing violence needs to be part of that conversation.”

Implications for mental health and public health

Based on the findings, the researchers say mental health services should routinely ask about cannabis use when assessing a person’s risk of violence. They also argue that public health information about cannabis should include its possible association with violence.

However, the researchers stress that the study identified an association and does not prove that cannabis directly causes violence. Cannabis use often occurs alongside other factors that may increase the risk of violent behavior and victimization, including alcohol use, other drug use and difficult social situations.

Study limitations

Only a small number of studies measured how much cannabis people used or how often they used it. Findings involving criminal convictions were also based on a relatively limited number of studies.

Most of the research was conducted before the widespread use of highly potent cannabis. The researchers suggest that daily use of today’s more potent cannabis products may produce more powerful effects.

This research was supported by the National Institute for Health and Care Excellence (NIHR) Biomedical Research Center: Maudsley and the Medical Research Council (MR/T007818/1). The views expressed are those of the authors and not necessarily those of the NIHR or the Department of Health and Social Care.