A large systematic review has found an association between cannabis use and violence, both as perpetrator and victim, but the researchers are clear that the data do not establish cannabis as a direct cause. The review, led by researchers at King’s College London, pooled data from 63 studies covering more than 265,000 people and has now been published in Cambridge University Press‘s Psychological Medicine as volume 56, article e296.
What the review examined and how
The researchers at the South London and Maudsley NHS Foundation Trust and King’s College London’s Institute of Psychiatry, Psychology and Neuroscience (IoPPN) used the World Health Organization’s broad definition of violence, covering physical, sexual and psychological violence toward another person, both within and outside intimate relationships. They also separated self-reported violence from cases that resulted in criminal convictions.
Where an individual study measured several forms of violence, the researchers focused on the most serious type reported. They analysed the general population separately from people receiving psychiatric care, which allows the findings to be read in two distinct contexts rather than collapsed into a single, potentially misleading headline figure.
The association between cannabis use and violence is not a new hypothesis, but a review of this size gives it considerably more weight. Dr Marta Di Forti, co-author of the study from the IoPPN at King’s College London, put it plainly: ‘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 violence: the numbers, with their caveats
In the general population, cannabis users were around twice as likely to have committed a violent act compared with non-users. Among people in psychiatric care, the figure was higher: cannabis users were around two and a half times as likely to be violent as other psychiatric patients who did not use cannabis. For violence that led to a criminal conviction, cannabis users were three to four times as likely to have been convicted, though the researchers note that the conviction findings were based on a relatively limited number of studies.
Professor Sir Robin Murray, senior author of the study from the IoPPN, observed that 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. That difference matters: longitudinal designs are generally better positioned to track sequences of events, so a smaller effect in those designs is worth noting.
The review also found that people who used cannabis were about one and a half times as likely to become victims of violence. That association was stronger among women than among men, though the review does not explain why.
Professor Murray added: ‘As cannabis becomes more commercialised and legalised 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.’
What the association does not tell us
The researchers are explicit that this is an association, not proof of causation. Cannabis use frequently co-occurs with alcohol use, other drug use, and difficult social circumstances, all of which can independently increase the risk of violent behaviour or victimisation. Unpicking cannabis’s specific contribution from that mix of factors is something this review cannot do.
There are further limits worth naming. Only a small number of the studies measured how much cannabis people used or how often. Most of the research was conducted before high-potency cannabis became common. The researchers suggest that daily use of today’s more potent products could potentially be associated with stronger effects, but that remains speculative on current evidence.
On the practical side, the researchers say mental health services should routinely ask about cannabis use when evaluating a person’s risk of violence, and that public health information on cannabis should include the possible connection with violence. Whether services have the capacity or the protocols to act on that recommendation is a separate question the review does not address.
The study was supported by the National Institute for Health and Care Research Biomedical Research Centre: Maudsley and the Medical Research Council (grant MR/T007818/1). The full paper appears in Psychological Medicine, volume 56, article e296.
