Researchers published a systematic review and meta-analysis in Psychological Medicine last month, analysing 63 studies with more than 265,000 participants. The paper reports significant ties between marijuana consumption and violent behaviour in both general-population and psychiatric groups.
In the general population, users exhibited roughly twice the odds of committing violence. Among psychiatric patients, the odds rose to nearly two and a half times higher.
When the focus shifted to violence that led to criminal convictions, the association intensified to 3.75 times higher odds for the general population and 4.21 times for psychiatric patients.
This is not the first warning. A 2020 meta-analysis in the American Journal of Psychiatry, covering 30 studies and nearly 300,000 adolescents and young adults, found that marijuana users more than doubled the odds of physical violence. Other research has linked marijuana to intimate-partner aggression.
Studies on schizophrenia add further concern. A landmark 1987 Lancet study followed 45,570 Swedish conscripts for 15 years and showed that those who used marijuana over 50 times faced about six times the risk of developing schizophrenia compared with non-users.
Later work repeatedly associates frequent use of high-potency cannabis with heightened psychosis risk.
Violence risk matters because psychotic disorders can involve paranoia, delusions and impaired judgement. A 2009 PLOS Medicine meta-analysis found that schizophrenia and related psychoses doubled the odds of violence without substance abuse, but the odds jumped to nearly nine times when substance misuse was present.
Although most people with schizophrenia are not violent, the combined evidence raises questions about cannabis' role in psychiatric vulnerability and behavioural risk. The data do not prove cannabis causes every violent act, and other factors such as alcohol, other drugs, mental illness and social conditions also contribute.
Public-health agencies often warn about potential harms before mechanisms are fully proven; the same standard should apply to marijuana. Violence consequences extend beyond users to spouses, children, neighbours, police and emergency responders, imposing costs on families, hospitals, insurers and taxpayers.
While cannabis firms profit from potent products, the broader society bears hidden expenses. The new research does not quantify how much violent crime cannabis directly causes or whether legalization raises such crime, but it underscores a public-health issue that can no longer be ignored.
Authorities should inform consumers about these links, medical professionals ought to assess cannabis use when addressing psychiatric or behavioural issues, and policymakers need to weigh social costs alongside tax revenue.
Legal status, profitability or social acceptance do not render a drug harmless. Decades of scientific findings suggest marijuana's impact reaches far beyond individual users.
Scott Chipman is vice president of Americans Against Legalizing Marijuana.