No evidence that cannabis use lowers opioid dependence

A 20-year-long study has found insufficient evidence to link opioid use with cannabis use. This means, the study suggests, that cannabis may not be effective harm reduction for opioid use disorders.

The Australian study, published in the American Journal of Psychiatry, tracked 615 people with heroin dependence from 2001 to 2022.

“There are claims that cannabis may help decrease opioid use or help people with opioid use disorders keep up with treatment,” says lead author Dr Jack Wilson, a researcher at the University of Sydney.

But Wilson says these claims aren’t based on thorough evidence. “It’s crucial to note those studies examine short-term impact, and focus on treatment of chronic pain and pain management, rather than levels of opioid use in other contexts.”

The University of Sydney study, conversely, draws on longer-term information. The researchers used data from the Australian Treatment Outcome Study, a longitudinal study which tracks people who entered treatment for heroin dependence in Sydney. Participants were recruited and interviewed once in 2001-2002, then followed up for interviews 6 times over the next 18-20 years.

The researchers used several statistical techniques to adjust for age and other factors, allowing them to look at the link between cannabis and opioid use.

“Our investigation shows that cannabis use remains common among this population, but it may not be an effective long-term strategy for reducing opioid use,” says Wilson.

There was some evidence of a link between the two substances at earlier follow-up interviews, but the researchers say this evidence was inconsistent.

“Opioid use disorders are complex and unlikely to be resolved by a single treatment,” says Wilson.

“The best way to support them is evidence-based holistic approaches that look at the bigger picture, and include physical, psychological, and pharmacotherapy therapies.”

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