Is there a long-term relationship between cannabis and heroin use? An 18-20-year follow-up of the Australian Treatment Outcome Study (ATOS)

Cannabis and opioid use disorders (OUD) make up over three quarters of all illicit substance use disorders worldwide, and they frequently co-occur. Considering that opioid use is associated with a considerably greater health burden, recent studies argue that cannabis may be used as a substitute for opioids by means of an alternative treatment for pain, or as a method of reducing opioid withdrawal symptoms. Using data collected as part of the Australian Treatment Outcome Study (ATOS), the current paper aims to examine the long-term relationship between cannabis and heroin use over 20-years.

615 people with heroin dependence were recruited to ATOS in 2001-2002 and reinterviewed at 3-, 12-, 24-, 36-months, 11 and 20-years post-baseline (follow-up rates of 89%, 81%, 76%, 70%, 70% and 65% respectively). Heroin and cannabis use were assessed at each time point using the Opiate Treatment Index (OTI). A random intercept cross-lagged panel model (RI-CLPM) was conducted to identify within-person relationships between cannabis use and heroin use at subsequent follow-ups.

After accounting for a range of demographic, other substance use, mental and physical health measures, an increase in heroin use at 3-months and 24-months post-baseline was associated with a decrease in cannabis use at 12-months (γ3 = -0.27, SE = 0.09, p = <0.01) and 36-months post-baseline (γ5 = -0.22, SE = 0.08, p = <0.01). Meanwhile, an increase in cannabis use at 24-months was associated with an increase in heroin use at 36-months (β5 = 0.21, SE = 0.10, p = 0.03).

Although there was some evidence to suggest an ‘escalation effect’ where cannabis use may have promoted greater heroin use, and in turn suppressed subsequent cannabis use, this was not the case for most of the cross-lag time intervals. Nevertheless, caution should be taken when considering the role of cannabis as a substitute for opioids among those with opioid use disorder.