Opioid Agonist Treatment Retention Among People Initiating Methadone and Buprenorphine Across Diverse Demographic and Geographic Subgroups in Ontario: A Population-based Retrospective Cohort Study

Objective: The aim of this study was to compare time on treatment among individuals initiating buprenorphine/naloxone and methadone and understand how retention varies according to age, sex, and urban/rural residence.Methods: We conducted a population-based retrospective cohort study among individuals aged 18 years and older, residing in Southern Ontario, Canada, who initiated buprenorphine/naloxone or methadone between October 2016 and December 2018 (N=15,724). We compared time on treatment across demographic and geographic subgroups. Our primary outcome was time to treatment discontinuation, defined as missing at least 14 consecutive days of opioid agonist therapy (OAT).Results: We identified 15,724 eligible individuals, among whom 7209 (45.8%) initiated buprenorphine/naloxone and 8515 (54.2%) initiated methadone. The median time to treatment discontinuation was significantly shorter among those initiating buprenorphine/naloxone rather than methadone (114 d, interquartile range:15-665 d vs. 263 d interquartile range: 32-1015 d, respectively, P<0.0001). Time on treatment increased with age in both OAT groups, with the exception of those aged 65 and older, where retention declined. Among methadone recipients, time on treatment was longer among rural relative to urban residents (372 vs. 255 d; P=0.0073), with no variation by sex. Conversely, females treated with buprenorphine/naloxone had significantly longer treatment retention than men (125 vs. 108 d; P=0.0372), with no variation by urban or rural residence.Conclusions: Treatment retention is lower among individuals treated with buprenorphine/naloxone relative to methadone. Moreover, retention varies between and within OAT groups by demographic variables and place of residence. Further research which aims to explain these differences is needed to optimize OAT treatment.