A new study has found that states which legalised cannabis for both medical and recreational use experienced a 9–11% reduction in daily opioid use among people who inject drugs, according to a new study published in the journal Drug and Alcohol Dependence.
Researchers from the Boston University School of Public Health (BUSPH) analysed data from 28,069 people who inject drugs (PWID) across 13 US states between 2012 and 2022. They found that cannabis access may enable people to substitute the unstable and toxic opioid supply with comparatively safer cannabis.
“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,” said lead author Dr Danielle Haley.
The study used data from the Centers for Disease Control and Prevention’s National HIV Behavioral Surveillance, examining self-reported use of cannabis and non-medical opioid use within the last 12 months across four years: 2012, 2015, 2018 and 2022.
Researchers found that states transitioning from medical-only cannabis laws to allowing both medical and adult recreational use saw significant reductions in daily opioid misuse. The decline was particularly pronounced when examining injected opioids specifically, where the probability dropped between 2–19% depending on the model used.
Previous research has found that people who use opioids often report turning to cannabis to manage withdrawal symptoms, reduce cravings and support recovery efforts. Among PWID, more frequent cannabis use has been associated with injecting opioids less often and improved retention in opioid treatment programmes – both of which reduce overdose risk.
“What this study shows is the potential impact of decriminalisation paired with access to a regulated supply,” said Stephen Murray, an adjunct clinical assistant professor of community health sciences at BUSPH and an overdose survivor. “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.”
The researchers caution that while the study is observational and cannot prove causation, the findings highlight one major potential advantage of widespread cannabis access. Haley emphasised 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. “Legalised 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.”
Future research should investigate links between legal, medical and recreational cannabis and reduced opioid use, as well as consider benefits in other areas such as reduction in blood-borne infections through injection, the study authors said.
