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A review of 63 studies found an association between cannabis use and violence, but the evidence does not establish that cannabis causes violence. The strongest associations came from studies that measured both at one point in time, while the review rated overall evidence certainty as very low.
A King’s College London systematic review found an association between cannabis use and violence across 63 studies involving more than 265,000 people, but it does not establish that cannabis causes violence. A critique published by The Conversation says a Daily Telegraph headline that cannabis users are “twice as likely to be violent” overstates uncertain evidence, particularly because many studies could not establish which came first.
The review did identify a statistical link, according to The Conversation’s account. But the size of that link differed depending on study design. Of the 63 studies, 46 were cross-sectional: they measured cannabis use and violence at a single point in time. Those studies found the strongest association, with more than twice the odds of violence among cannabis users. Because they do not establish timing, they cannot show whether cannabis use preceded violence, followed it, or occurred alongside other factors.
Studies that followed people over time found a weaker association. The source reports that cannabis users had 17% higher odds of violence than non-users in the longitudinal studies. Odds are not the same as probability, so describing a result in odds as being “twice as likely” can also mislead readers. How much the two measures differ depends on how common the outcome is.
The critique points to factors that could contribute to the observed association, including poverty, alcohol use, other drugs and difficult childhood experiences. The review attempted to account for other factors, but the critique says its authors generally relied on study versions that adjusted for fewer of them. That leaves room for alternative explanations; it does not show that the association is false.
How Study Design Changes the Claim
The distinction matters because a headline suggesting cannabis makes people violent can influence public understanding and policy, while the studies described here show an association, not proof of cause. A single-time-point study cannot tell whether cannabis use came before violence or whether other circumstances helped produce both. The weaker result from studies that tracked people over time gives readers a different picture from the most prominent headline figure.
The size of the review also needs careful interpretation. Combining more than 265,000 people can reduce the chance that a result is due to an unusual sample, but it cannot correct systematic problems in the underlying studies. If researchers define cannabis use or violence differently, or measure either poorly, a large pooled total does not remove those limitations.
For readers, the practical takeaway is neither that cannabis has no connection to violence nor that it makes users violent. The review’s reported association may be real, but its size and meaning remain uncertain. That distinction is central to interpreting claims about substance use, risk and public safety.
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What the 63 Studies Measured
The systematic review brought together research with different methods and definitions. Studies varied in what counted as cannabis use, from any past use to more frequent use, and in what counted as violence, from physical incidents to criminal convictions. The review also grouped outcomes such as domestic incidents, sexual assault, fights and robbery, which may have different causes.
Results varied substantially. The Conversation says three analyses using the same US national drug survey, all included in the review, reported odds ratios ranging from about 1.3 to nearly 10. The variation may reflect differences in definitions, measurement and surveyed populations. The critique also notes possible publication bias: studies finding little or no association may be less likely to appear in published research. The review found signs of that issue, and its methods could not rule it out.
“Cannabis users are “twice as likely to be violent”.”
— Daily Telegraph, as quoted in The Conversation
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Limits on the Violence Estimate
The review’s supplementary material rated the certainty of the evidence as very low, according to The Conversation. That means the authors had little confidence in the size of the estimate, although they considered the association itself likely to be real. The source says this qualification was not included in the Telegraph report.
It remains unclear how much of the association is attributable to cannabis use itself, how much is explained by other factors, and whether the pooled figures apply similarly across different forms of cannabis use and violence. The studies’ varying measures and the possibility of unpublished null findings add further uncertainty. The available material does not establish a causal effect or settle the size of any such effect.
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What Further Research Must Resolve
The reporting discussed here does not announce a new study or a follow-up timetable. The unresolved issues point to what further research would need to clarify: whether cannabis use precedes violence, how results change after careful adjustment for other risks, and whether findings hold when both cannabis use and violence are measured consistently.
Until stronger evidence addresses those questions, the review should be described as finding an association with very low certainty about its size, not as showing that cannabis causes violence or that users are definitively twice as likely to be violent. Researchers and news outlets can also make the evidence easier to interpret by distinguishing odds from probability and by explaining study limitations alongside headline estimates.
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Key Questions
Did the review find a link between cannabis use and violence?
Yes. The review reported an association across 63 studies, but an association does not by itself establish that cannabis use causes violence.
Does the evidence show cannabis users are twice as likely to be violent?
Not as a settled probability claim. The strongest result came from cross-sectional studies and concerned more than twice the odds of violence. The critique says that wording overstates what the evidence can show.
Why do longitudinal studies matter?
They follow people over time, which can help establish whether cannabis use came before violence. The source reports a weaker association in these studies: 17% higher odds among cannabis users than non-users.
How certain is the evidence?
The review’s supplementary material rated the evidence as very low certainty, according to The Conversation. The critique says the association may be real, but the estimate’s size is uncertain.
What other factors could help explain the association?
The critique identifies poverty, alcohol, other drugs and difficult upbringing as factors linked to violence that may also be more common among cannabis users. Their contribution is not settled by the reported findings.
Source: fediverse
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