A large review led by researchers at King’s College London has found that people who use cannabis are more likely to commit violent acts and experience violence themselves.
Researchers from the Institute of Psychiatry, Psychology and Neuroscience (IoPPN) combined data from 63 studies involving more than 265,000 people. They analyzed the general population separately from people receiving psychiatric care.
What the review examined
The researchers used the World Health Organization’s broad definition of violence, which includes physical, sexual and psychological violence toward another person, both within and outside of intimate relationships.
They also separated self-reported violence from cases that resulted in criminal convictions. When individual studies measured various forms of violence, researchers focused on the most serious type reported. The findings were published in Psychological Medicine.
Dr Marta Di Forti, co-author of the study from the Institute of Psychiatry, Psychology and Neuroscience (IoPPN) at King’s College London and the South London and Maudsley NHS Foundation Trust, said:
“As a doctor, I have seen how excessive cannabis use can be a driver of violence. We have long been cautious about how far we could go in describing this link, but a review of this magnitude allows us to talk more confidently about the risks, while being careful not to stigmatize people who use cannabis more broadly.”
Cannabis use and victimization
The review also found that people who used cannabis were about one and a half times more likely to become victims of violence. This association was stronger among women than men.
Professor Sir Robin Murray, lead author of the study from the Institute of Psychiatry, Psychology and Neuroscience at King’s College London, said:
“In the general population, people who used cannabis were about twice as likely to have committed a violent act as those who did not; for those in psychiatric care, cannabis users were about two and a half times more likely to be violent than other psychiatric patients who did not use cannabis. The link held up in studies that followed people over time, although it was smaller than in studies that looked at a single point in time. For violence that led to a criminal conviction, the People who used cannabis were three to four times more likely to have been convicted.
“As cannabis becomes more commercialized and legalized around the world, the conversation about its risks has largely focused on the person using it. This review shows that violence, both committed and experienced, must also be part of that conversation.”
Implications for mental health and public health
Based on the findings, 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 the possible connection to violence.
However, the researchers emphasize that the review identifies an association and does not prove that cannabis directly causes violence. Cannabis use often occurs alongside other factors that may also increase the risk of violent behavior or victimization, including alcohol use, other drugs, and difficult social circumstances.
Important limitations
Only a small number of studies measured how much cannabis people used or how often they used it. The conclusions regarding criminal convictions were also based on a relatively limited number of studies.
Most of the research was done before high-potency cannabis became common. Researchers suggest that daily use of today’s most potent cannabis products could be associated with stronger effects.
The study was supported by the National Institute for Health and Care Research (NIHR) Biomedical Research Centre: 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.