How do marijuana legalization policies affect violence rates?

marijuana sold in a dispensary

Social norms around marijuana are becoming more permissive in the United States. The drug has several approved medicinal uses, and survey data show that American perceptions of how harmful marijuana is are declining over time.

Based mostly on these perceptions and a desire to rethink Reagan-era drug policies, vast swaths of the United States have dramatically changed their laws governing use of marijuana. Since 1996, 36 states have legalized medical use and 15 states have legalized recreational use. Now we need research into any number of possible public health effects from these dramatic shifts.

Violence – self-directed and outwardly-directed – is always a major public health concern, although it’s not generally associated with marijuana in the popular mind. Postdoctoral scholar Ellicott Matthay, PhD, researches violence and wanted to see how marijuana policy might change patterns of violence.

“One reason we might expect to see changes,” she explained, “is that alcohol makes people violent, so if cannabis legalization prompts some people to substitute alcohol for cannabis, we might see violence decline.” On the other hand, dispensaries often handle a lot of cash, which could set the stage for violent burglaries.

In a paper published today in JAMA Network Open, Matthay has homed in on this topic. The study examined whether state marijuana legalization has led to changes in rates of violent injuries. The researchers found that recreational marijuana legalization was associated with about a 20% increase in self-harm injuries among males under the age of 40. The increase was greater when cannabis products were available for commercial purchase through dispensaries. Marijuana legalization did not appear to affect violent injuries more broadly – for other age or gender groups, for assault injuries, or where only medical use was legalized.

These findings are, generally speaking, good news for the massive public policy experiment underway. “These research findings are largely a story of no problem, but suggest a potential concern for one population subgroup,” Matthay said.

Why might marijuana legalization increase self-harm injuries? “One thing to consider is that people have this preconceived notion that smoking marijuana makes you mellow, but the products being sold now are not the same as the plant that people were smoking in the 70s. Potency has increased dramatically, and higher potency products may have different effects.”

Other research suggests that heavy marijuana use is associated with poor mental health outcomes, potentially driving suicide attempts. It’s also possible that parents using marijuana become more neglectful, leaving poorly supervised children to injure themselves. Acute intoxication can also hinder judgment and lead to impulsive decisions such as self-harm.

“Substance use around the world is largely an activity of young men. So if marijuana policies do matter, this is the group for whom we might expect to see effects,” said Matthay.

This is the first study to evaluate the impacts of marijuana legalization on self-harm and assault injuries. Matthay and colleagues used a large, nationwide health insurance claims database to compare rates of nonfatal self-harm and assault in states where marijuana was legalized for medical or recreational use to states without legalization from 2003 to 2017.

“This study is the very first pass at the question of whether marijuana legalization and violent injuries might be related. It suggests this is something that we need to monitor and look into more closely,” Matthay said. 

The study’s senior author was Keith Humphreys, PhD, a substance abuse expert at Stanford University. UCSF’s Laura Schmidt, PhD, also worked on the project.