Cannabis use does not reduce long-term heroin use, study finds

People with heroin dependency don't use less of that drug if they start also using cannabis, according to a new study.

The findings cast some doubt on the idea that cannabis might help people reduce their dependence on opioids, experts say.

"Cannabis is becoming increasingly recognized as a therapeutic product," says study author Dr. Jack Wilson, a researcher at The Matilda Centre for Research in Mental Health and Substance Use at the University of Sydney.

But it may not be therapeutic in this specific case. "Despite suggestions that cannabis may be used as a method for reducing opioid use, we found no evidence to suggest a relationship between the use of these [substances]," said Wilson.

The opioid epidemic in the United States is worsening, with the Centers for Disease Control and Prevention reporting over 80,000 deaths in 2021. In response to this crisis, states like New York and Illinois have amended their medical marijuana laws in recent years, now permitting the use of cannabis as an alternative to prescription opioids.

This legislative shift is the result of growing discussions about the potential role of cannabis in reducing opioid dependency, but research over the years has yielded mixed results.

"Increasing the availability of cannabis is unlikely to have an impact one way or the other on the opioid crisis and overdose death rate," said Dr. Andrew Saxon, a professor of psychiatry and behavioral sciences at the University of Washington School of Medicine, and a member of the American Psychiatric Association's council on addiction psychiatry.

In this new study, researchers studied over 600 patients with heroin dependency, including those who both were and were not in treatment. Further, they inquired about the subjects' use of heroin and other drugs, including prescription opioids.

Unlike earlier studies that only looked at the short-term effects of both cannabis and opioid use, this study interviewed participants over a 20-year period.

The study found that cannabis use was very common among those who were dependent on heroin. However, there wasn't a consistent relationship between the patterns of use of the two drugs, and no evidence to suggest that cannabis use reduced long-term opioid use.

Doctors are still trying to understand how cannabis use impacts opioid use, and vice versa, says Dr. Stephanie Widmer, a medical toxicologist and emergency medicine physician practicing in New York. "Much more research needs to be done," she says.

The intersection of cannabis and opioid use disorders presents a complex challenge for policymakers and healthcare providers, experts say.

"Opioid use disorders are such a persistent and complex condition," says study author Wilson, in part because everyone has a unique set of psychical and psychological needs that affect their treatment plan.

"Rather than implementing policies that allow people to substitute their opioids for cannabis, it may be more impactful to design a policy that ensures that all people with opioid use disorder are accessing effective treatments early and often," Wilson says.

Khushali Jhaveri, MD, a board-certified internal medicine physician, is a hematology/oncology fellow at Moffitt Cancer Center and a member of the ABC News Medical Unit.