Cannabis legalization may lead to a decline in daily opioid use

Legalizing cannabis for both medical and recreational use may lead to a decline in daily opioid use among people who inject drugs in the United States, according to a new study led by a Boston University School of Public Health researcher (BUSPH).

Published in the journal Drug and Alcohol Dependence, the study found that US states that legalized marijuana for medical and adult recreational use saw a 9-to-11-percentage-point decline in daily opioid use among this population, compared to states that legalized marijuana for medical use only.

While the harms and benefits of cannabis use and cannabis reform continue to be debated on the national stage, these findings highlight one major potential advantage of widespread access to marijuana: this increased access may enable people to substitute their use of the unstable and toxic opioid supply with comparatively safer cannabis and, thus, lower their chances of experiencing opioid-related harms or dying from an overdose. In the US, opioids contribute to more than 75 percent of fatal drug overdoses.

The study was published on the heels of a significant shift in US drug policy that will indeed lower restrictions on cannabis. Last December, President Donald Trump signed an executive order to downgrade cannabis from a Schedule 1 classification (assigned to drugs such as heroin and ecstasy) to a Schedule 3 classification, which refers to drugs that pose minimal to moderate risk of physical or psychological dependence. Nearly all US states and Washington, DC have legalized cannabis for medical use, while 48 percent of states allow cannabis for adult recreational use.

People who inject drugs are part of a population that is at the epicenter of the opioid crisis in America, and they stand to benefit the most from policies that increase access to cannabis. By focusing on this group, the study builds upon past research on cannabis use and opioid mortality that has primarily examined the general population-which has a lower risk of experiencing opioid-related harms-with mixed results.

The magnitude of decrease in opioid use that we observed among a population that is experienced with opioid use and likely to experience unpleasant withdrawal symptoms after reducing this use is very profound and important."

Dr. Danielle Haley, study lead and corresponding author, assistant professor of community health sciences at BUSPH

The takeaway, she says, is that creating a safe and regulated supply of a substance is a valuable overdose prevention tactic because it can reduce use of non-regulated and more dangerous substances. "Legalized cannabis tends to be higher quality and more potent. As these products become more available and cheaper, people might be able to reduce their opioid use even without increasing how often they use cannabis." 

For the study, Dr. Haley and colleagues utilized data from the Centers for Disease Control and Prevention's National HIV Behavioral Surveillance, including self-reported use of cannabis and non-medical opioid use among within the last 12 months among nearly 29,000 people who inject drugs, comparing data from states that did not legalize cannabis, legalized it for medical use only, or legalized it for both medical and adult recreational use. The data spanned 13 states in four waves: 2012, 2015, 2018, and 2022.

The decline in opioid use was equivalent across all racial and ethnic groups, as well as among males and females. 

"This study adds to a growing body of evidence that sensible changes to our outdated drug policies can have a positive health impact, especially among some of our most vulnerable neighbors," says study coauthor Dr. Leo Beletsky, professor of law and health sciences at Northeastern University.

The team did not observe overall links between cannabis legalization and daily cannabis use, but cannabis use did increase by five percentage points among White participants living in states that transitioned from no legalization to legalizing cannabis for medical use only. This increase among White participants could reflect long-standing racial inequities in healthcare that make it easier for White people to navigate health systems and services than people of other races, the researchers say.

Understanding how policies related to substance use benefit the health of people who use drugs is essential for effective cannabis reform. 

"What this study shows is the potential impact of decriminalization paired with access to a regulated supply," says Stephen Murray, adjunct clinical assistant professor of community health sciences at BUSPH, who is also an overdose survivor and former paramedic with expertise in overdose prevention. Murray was not involved in the study. "When legal barriers are removed and people have safer alternatives available, we see meaningful reductions in daily opioid use-even among people with long histories of injection drug use. That's a powerful signal."

But the findings also serve as a reminder that the design and implementation of these policies matter, he says. "Commercialized access to cannabis does not benefit all communities equally, and without intentional equity-focused policy, longstanding racial disparities in healthcare access and criminalization can persist even under legalization."

The researchers say future research should further investigate links between legal medical and recreational cannabis and reduced opioid use, as well consider benefits in other areas, such as a reduction in cases of blood-borne infections through injection.

The study's senior author is Dr. Hannah Cooper, Rollins Chair of Substance Use Disorders Research and professor of behavioral, social, and health education sciences at Emory University's Rollins School of Public Health.