Stanford study finds medical marijuana does not reduce opioid deaths

Legalizing medical marijuana does not reduce the rate of fatal opioid overdoses, according to a new study by researchers at the Stanford University School of Medicine, and published in the Proceedings of the National Academy of Sciences.

The researchers evaluated the connection between legalized medical marijuana and fatal opioid overdoses. A previous study in 2014 found lower rates of fatal opioid overdoses in the states that had legalized marijuana for medical purposes than in states where marijuana remained illegal. However, the new study, which revisited the issue after many more states had legalized medical marijuana, found no evidence of a connection between opioid deaths and the availability of medical cannabis.

In 1996, California became the first state to legalize medical marijuana. By 2010, 13 states, most of them in the West, had legalized medical marijuana. Today, 47 states permit some version of medical marijuana.

The new study confirmed the findings from the 2014 study, but when the researchers looked at opioid deaths up to 2017, by which point most states had legalized some form of medical marijuana, if not recreational marijuana, they found that the opposite was true. States with legal medical marijuana had a higher rate of deaths due to opioid overdose, researchers said.

After the 2014 study was released, medical marijuana proponents and some public officials interpreted the results to mean that, given access to legalized pot, people would turn to it rather than opioids for pain relief or recreation. Yet, when the Stanford researchers compared states that have more restrictive medical marijuana laws with those that allow recreational marijuana, they found no correlation between opioid overdose mortality and the level of restriction.

Further, given that only 2.5 percent of the U.S. population uses medical marijuana, it's unlikely that use could affect mortality statistics, according to Chelsea Shover, PhD, postdoctoral scholar and lead author of the study.

The results of the 2014 study may have reflected policies and conditions in states that legalized medical marijuana early, according to Keith Humphreys, PhD, professor of psychiatry and behavioral sciences and senior author. Those states tended to be wealthier and more politically liberal, with greater access to addiction treatment and to naloxone, which reverses the effects of opioids and can prevent overdose fatalities.

The states that legalized marijuana early also incarcerate fewer people for drug use, Humphreys said. When people are released from prison, where they lack access to drugs and lose tolerance to them, they may try to use the same levels as they did before they were incarcerated, leading to overdose.

The researchers said they believe that medical marijuana provides benefits and that research into its effectiveness should continue.

"There are valid reasons to pursue medical cannabis policies, but this doesn't seem to be one of them," Shover said in a statement. "I urge researchers and policymakers to focus on other ways to reduce mortality due to opioid overdoses."