A large review led by researchers at King’s College London has found that people who use cannabis are more likely both to commit violent acts and to experience violence themselves.
Researchers from the Institute of Psychiatry, Psychology & 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 inside and outside intimate relationships.
They also separated self-reported violence from cases that resulted in criminal convictions. When individual studies measured several forms of violence, the 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 & Neuroscience (IoPPN) at King’s College London, and South London and Maudsley NHS Foundation Trust, said:
“As a clinician, I’ve seen how heavy cannabis use can be a factor in violence. We’ve been cautious for a long time about how far we could go in describing this link, but a review of this size lets us speak with more confidence 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 as likely to become victims of violence. This association was stronger among women than among men.
Professor Sir Robin Murray, senior author of the study from the Institute of Psychiatry, Psychology & Neuroscience at King’s College London, said:
“In the general population, people who used cannabis were around twice as likely to have committed a violent act as those who didn’t; for those in psychiatric care, cannabis users were around two and a half times as likely to be violent as other psychiatric patients who didn’t use cannabis. The link held in studies that followed people over time, though it was smaller than in studies that looked at a single point in time. For violence that led to a criminal conviction, people who used cannabis were three to four times as likely to have been convicted.
“As cannabis becomes more commercialized and legalized around the world, the conversation about its risks has focused mostly on the person using it. This review shows that violence, both committing it and experiencing it, needs to be part of that conversation too.”
Implications for Mental Health and Public Health
Based on the findings, the researchers say mental health services should routinely ask about cannabis use when evaluating a person’s risk of violence. They also argue that public health information about cannabis should include the possible connection with 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 can also increase the risk of violent behavior or victimization, including alcohol use, other drug use and difficult social circumstances.
Important Limitations
Only a small number of the studies measured how much cannabis people used or how often they used it. The findings involving criminal convictions were also based on a relatively limited number of studies.
Most of the research was conducted before high potency cannabis became common. The researchers suggest that daily use of today’s more potent cannabis products could potentially 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.
