Science1 publisher3 min readPublished
Cannabis use tracks with committing violence and suffering it across 63 pooled studies
King's College London researchers pooled 63 studies of 265,000-plus people and found general-population cannabis users twice as likely to commit violence. The authors stop at association, so the finding supports flagging cannabis in violence risk checks and leaves causation open.
The Scientist · Science desk
What happened
- Cannabis users were also about one and a half times as likely to become victims of violence, an association that was stronger among women than among men.
- For violence that ended in a criminal conviction, cannabis users were three to four times as likely to have been convicted, according to senior author Robin Murray.
- The team applied the World Health Organization's broad definition of violence and counted self-reported violence separately from violence that led to a conviction.
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Why it matters
- decision Services that add a routine cannabis question to violence risk assessment are adopting a predictor; the pooled data do not back telling patients that quitting will lower their risk.
- constraint With few studies recording quantity or frequency of use, the review cannot tell clinicians or regulators what level of use marks higher risk.
- exposure The stronger victimization link among women puts female cannabis users' own safety into the public health information the authors want expanded.
- constraint Legalization and commercial rules are being set against ratios drawn mostly from before high-potency products were common, so the figures may not describe what is sold now.
The comparison that bears most on causation is between study designs. "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," senior author Robin Murray said [7]. A single-point study cannot tell whether cannabis use came before the violence or after it. A study that follows people can at least put the use first. The release does not say how much smaller the longitudinal estimate was [7].
A smaller effect in the follow-up studies is consistent with part of the snapshot association coming from shared causes. The authors name the likely ones: cannabis use often occurs alongside alcohol use, other drug use and difficult social circumstances, and each can raise the risk of committing violence or suffering it [8]. Co-author Marta Di Forti said "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" [13]. Size narrows the uncertainty around a pooled estimate. It does not remove confounding carried in from the individual studies. The psychiatric-care comparison, made against other patients who did not use cannabis, holds psychiatric illness roughly constant but not alcohol or social circumstances [4][8].
Two parts of the evidence are thin. Only a small number of the included studies measured how much or how often people used cannabis [9], so the review cannot check whether heavier use goes with more violence. The conviction result is the largest ratio reported, 1.5 to 2 times the general-population ratio for committing violence [1]. It also rests on a relatively limited number of studies [10].
Potency is a further gap. Most of the research was done before high-potency cannabis became common, and the researchers suggest daily use of today's stronger products could be associated with stronger effects [11]. Testing that needs the frequency and quantity measures that few of the included studies collected [9].
The authors' recommendations stay within what an association can support. They want mental health services to ask routinely about cannabis use when assessing a person's risk of violence, and public health information on cannabis to mention the possible connection [12]. A question in a risk assessment only has to predict. On these data cannabis use does predict, both in the general population and among psychiatric patients [3][4]. Murray put the public side in terms of legalization. "As cannabis becomes more commercialized and legalized around the world, the conversation about its risks has focused mostly on the person using it," he said [17].
The review appeared in Psychological Medicine, with funding from the NIHR Maudsley Biomedical Research Centre and the Medical Research Council [15][16].
What to watch
- Cohort studies that record daily use of high-potency products and can test the authors' suggestion of stronger effects.
- More conviction-outcome studies to firm up the three-to-four-fold conviction ratio, now drawn from a limited number of studies.
- Whether mental health services write a routine cannabis question into violence risk assessment, as the authors recommend.