A new perspective paper in Frontiers in Psychiatry has reignited concern about the cannabis and violence link. It argues that toxicology screens often skip testing for tetrahydrocannabinol, or THC, the one substance most often present at the scene of unexplained violence.

Decades of data, real cases

Researchers Kenneth Finn, Robin Murray, Elizabeth Stuyt and Karen Randall examined 29 real world violence cases. They combined these with decades of epidemiological data. Seventeen cases returned THC positive toxicology results. Meanwhile, twelve involved documented cannabis use where investigators never tested for THC or never reported the result.

Their conclusion leaves little room for ambiguity. The authors state that current evidence supports a causal relationship between cannabis use, psychotic disorders and violence. Notably, that relationship is strongest when use starts in adolescence and involves high potency products.

The cannabis and violence link starts in the brain

The review ties the marijuana and violence connection to what cannabis does to the developing brain. Cannabis induced psychosis converts to schizophrenia in roughly 34 percent of cases. By comparison, alcohol induced psychosis converts in just 10 percent of cases. Meanwhile, among adolescents aged 12 to 19, regular cannabis use carried an 11.2 fold increase in psychotic disorder risk. For cases severe enough to need hospital admission, that figure climbed to 26.7 fold.

Stronger products, higher risk

Potency has changed the picture considerably. Cannabis flower measured just 2 to 4 percent THC through the 1970s and 1980s. Today, it averages closer to 20 percent across parts of the United States, while concentrates can reach 80 to 90 percent. In Canada, 71 percent of dried cannabis sold in 2022 exceeded 20 percent THC. As a result, daily users of products above 10 percent THC face roughly five times the risk of a psychotic disorder. That risk compares with people who have never used cannabis at all.

A dose response pattern in violence

On violence specifically, the authors describe a clear dose response pattern within the cannabis and violence link. Occasional cannabis users showed a 3 percent rate of violent behaviour. By comparison, regular users showed 6 percent, and frequent users showed 11 percent. In addition, persistent use across the lifespan carried a sevenfold increase in the odds of a violent conviction. Among Iraq and Afghanistan era veterans with cannabis use disorder, 27 percent reported difficulty controlling violence in the past month. Only 8 percent of veterans without the condition said the same.

What happened after legalisation

Jurisdictions that have opened retail cannabis markets offer a real world test of the theory. Emergency department visits for cannabis induced psychosis in Canada climbed by roughly 40 percent after legalisation in 2018. The sharpest rise came among adults aged 19 to 24. Similarly, Colorado youth psychosis hospitalisation rates rose from 21.9 to 32.3 per 100,000 over a similar period. Hospitalisations tied to cannabis use disorder increased from 2.0 to 7.9 per 100,000 in the same window.

Gaps in the testing net

Despite these figures, toxicology practice has not kept pace with the science. Several major jurisdictions do not routinely test for cannabinoids in violent crime investigations. Los Angeles County, for example, does not include cannabinoid testing in its standard screening for violent deaths. Several other states restrict or delay release of toxicology results entirely. Therefore, the authors argue that mandating THC testing in cases of unexplained violence would close a significant gap in the data.

What the evidence does not show

Still, the authors acknowledge that most people who use cannabis never become violent and that observational data alone cannot prove causation. Even so, they argue the marijuana and violence connection holds consistently across adolescents, veterans and psychiatric patients alike. That consistency, they say, makes cannabis a modifiable risk factor rather than a bystander.

Screening over blame

For clinicians, the message concerns who to watch rather than who to blame. The review calls for routine cannabis screening in young patients and those with existing mental health conditions. It also calls for clearer public education on the risks tied to high potency products. Given how sharply the risk climbs during adolescence, the authors suggest something simple. Ultimately, an early conversation about the cannabis and violence link, held before any crisis develops, may do more good than any toxicology report.

Source: dbrecoveryresources