Introduction. Opioid agonist therapy (OAT) is the critical treatment for people with opioid dependence. OAT improves survival and reduces multiple drug related harms such as blood borne viral and bacterial infections, drug related criminal activity and improves social functioning. Previous research with samples recruited in primary care and treatment settings has shown that OAT reduces injection drug use frequency We sought to model transitions between states of drug injecting and non-injecting over time and the impact of OAT on these transitions in a community-based cohort of people who inject drugs.
Methods. Data come from SuperMIX, an ongoing prospective cohort study of people who inject drugs in Melbourne, Victoria, established in 2008. Annual face to face interviews with participants measure the frequency of injecting and other drug use in the period between interviews, along with OAT engagement and a range of related health and social factors. Directed Acyclic Graphs established a data-generating process to test for a causal effect between OAT engagement and cessation of, and relapse to, any injecting drug use between annual interviews, and this informed statistical model composition. We then used multi-state (bivariate random effect) discrete-time survival modelling with interval-censored annually collected person-period data to model transitions between states of injecting and non-injecting to test for causal effects of OAT engagement on these transitions (with adjustments for a range of health and social factors related to injecting).
Results. From 1496 participants with 4875 person-period follow-up (FU) observations, we observed 281 injecting cessation and 139 relapse to injecting events. Preliminary analyses provide evidence of a time-dependent positive association between any OAT engagement and injecting cessation (adjusted Hazard Ratio [aHR]=2.29, 95% confidence interval [95%CI] = 1.08, 4.84 at 7-years follow up). Similarly, sustained engagement in OAT increased likelihood of cessation (aHR=1.12, 95%CI=0.99, 1.26). Any OAT was found to increase injecting cessation among those who identified as Aboriginal or Torres Strait Islander (aHR=5.99, 95%CI=1.07, 33.6). There was a small to moderate correlation (r=.28) between each of the state-specific random effects (injecting cessation and injecting resumption) indicating participants were most likely to belong to one of two groups: frequent movers (high probability for both states – short periods of cessation and resumption) or non-movers (low probability for both states – longer periods of cessation and relapse).
Conclusion. We found evidence of a causal relationship between OAT and cessation of injecting drug use, and weaker evidence of a similar relationship for sustained OAT engagement, in a community-recruited cohort of people who inject drugs. These relationships were evident after adjusting for key confounders. Our findings strengthen the evidence of the cumulative effect of OAT engagement on long term cessation of injecting drug use which in turn reduces connected harms such as opioid overdose and blood borne viral infections.