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Opioid and cannabis use disorders are a major concern worldwide, constituting almost 80 per cent of all illicit drug use disorders. Their co-occurrence is of particular concern to clinicians and public health officials, especially with a backdrop of opioid harms and less restrictive cannabis policies globally. While there are claims that cannabis might help reduce opioid use, evidence supporting this hypothesis has been limited and inconsistent. Now a long-term study examines the relationship between cannabis, heroin and other opioid use over a 20-year period.

The Australian Treatment Outcome Study (ATOS) is a longitudinal study tracking individuals entering treatment for heroin dependence in Sydney, Australia. The study began with about 600 participants, including both individuals in treatment and a comparison group not in treatment. Participants were assessed at multiple points over 20 years, providing data on their use of cannabis, heroin, and other opioids, with modelling analysing the relationship between cannabis and heroin use, taking into account various other factors like someone’s criminal history, psychopathology, and general health.

The study makes several key findings. Over long-term follow-up, there was a significant decrease in daily heroin use and a fluctuation in cannabis use frequency. The study found that increases in heroin and cannabis use at one time point predicted further increases in the respective substance use at subsequent follow-ups. An increase in heroin use at certain points was associated with a decrease in cannabis use at later points. And there was no relationship detected between use of cannabis and use of other opioids, including prescribed opioids. These findings suggest a complex, non-linear relationship between cannabis, heroin and other opioid use over a long period.

Despite the prevalence of cannabis use among individuals with long-term opioid use disorder, the study provides little consistent evidence for a strong association between cannabis and heroin use, or cannabis and general opioid use, over the extended period. These results imply that the interaction between these substances is more nuanced than previously thought. The authors say that clinicians and policymakers should be cautious about using cannabis as part of a strategy to manage opioid use considering growing global legalisation.


Further information

The Long-Term Relationship Between Cannabis and Heroin Use: An 18- to 20-year Follow-Up of the Australian Treatment Outcome Study (ATOS): American Journal of Psychiatry

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