Marijuana and violence: How much more evidence do we need of the connection?

For decades, marijuana advocates have promoted the image of a harmless drug producing relaxation, laughter, and peaceful behavior. That image has helped normalize marijuana use and fuel a multibillion-dollar commercial industry.

But scientific research increasingly challenges this reassuring narrative.

A major systematic review and meta-analysis published last month in the journal Psychological Medicine examined 63 studies involving more than 265,000 participants. Researchers found significant associations between marijuana use and violence across both general-population and psychiatric samples.

Among the general population, marijuana users had approximately twice the odds of perpetrating violence. Among psychiatric patients, the odds were nearly two and a half times as high.

The association with marijuana use was even stronger — 3.75 times as high in the general population — in studies examining violence resulting in criminal convictions. And among psychiatric patients, the association between marijuana and criminal violence was 4.21 times as high.

These findings deserve national attention, but this is hardly the first warning. A 2020 meta-analysis published in the American Journal of Psychiatry examined 30 studies involving nearly 300,000 adolescents and young adults. Researchers found that the odds a subject committed physical violence more than doubled if he was a marijuana user. Other studies have identified associations between marijuana use and intimate-partner violence .

Research involving patients with schizophrenia has raised additional concerns. The connection between marijuana, serious psychiatric illness, and violence is particularly troubling. Nearly four decades ago, a landmark 1987 study published in The Lancet followed 45,570 Swedish military conscripts over 15 years. Researchers found that those who had used marijuana more than 50 times had approximately six times the risk of developing schizophrenia compared with non-users.

Subsequent research has repeatedly associated marijuana consumption — particularly frequent use of high-potency products — with increased risks of psychosis.

Why does this matter when discussing violence? Schizophrenia and other psychotic disorders can involve paranoia, delusions, impaired judgment, and distorted perceptions of threats.

A 2009 meta-analysis published in PLOS Medicine found that schizophrenia and other psychoses were associated with increased violence risk, particularly when accompanied by substance abuse. The odds of violence were approximately twice as high without substance abuse but nearly nine times as high when substance misuse was also present. Although that analysis examined substance abuse broadly rather than marijuana alone, it highlights a potentially dangerous intersection of psychiatric illness and drug consumption.

Importantly, most people with schizophrenia are not violent . Nevertheless, the combined research raises questions about marijuana’s contribution to psychiatric vulnerability and behavioral risk. The scientific evidence does not establish that marijuana causes every violent incident involving its users.

Alcohol, other drugs, psychiatric conditions, and social circumstances may contribute as well. But uncertainty about causation is no justification for dismissing repeatedly observed risks.

Public-health authorities routinely warn consumers about potential dangers before every biological mechanism has been conclusively established. Why should marijuana receive different treatment?

There is another troubling dimension receiving far too little attention: The consequences of violence are not confined to marijuana users. They often fall upon spouses, intimate partners, children, neighbors, strangers, police officers, and emergency responders. Yet marijuana commercialization is frequently discussed as though its consequences were simply a matter of personal preference.

We hear about consumer freedom, business opportunities, employment, and government tax receipts. We hear almost nothing about the potential costs associated with violence, psychiatric emergencies, impaired driving , addiction , and family disruption. Those costs do not disappear just because they are difficult to measure. They are absorbed by victims, families, hospitals, employers, insurers, law enforcement, and taxpayers.

While marijuana businesses collect revenue from increasingly potent products, the public bears costs never appearing on the industry’s balance sheet. The new research does not establish how much violent crime marijuana directly causes or whether legalization increases violent crime. Those questions require additional investigation. But the findings reinforce a serious public-health concern that can no longer be dismissed as an isolated observation.

Public-health agencies should inform consumers about these associations. Medical professionals should assess marijuana use when evaluating psychiatric and behavioral problems. Policymakers should examine potential social costs rather than focusing exclusively on tax revenue.

A drug does not become harmless because it is legal, profitable, or socially fashionable. For years, Americans have been assured that marijuana’s consequences are largely limited to individuals choosing to consume it. The scientific research presents a considerably more troubling picture. The warning signs have accumulated for decades. How much more evidence will it take before public health officials, lawmakers, and the marijuana industry acknowledge the warning signs?

Scott Chipman is vice president of Americans Against Legalizing Marijuana .