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Uptake and retention for opioid agonist treatment (OAT) remains low. Novel extended-release formulations may improve OAT accessibility by reducing the frequency of healthcare visits. Our aim was to examine uptake, characteristics, treatment patterns and retention of individuals initiating extended-release subcutaneous buprenorphine (BUP-ER), a monthly injectable OAT. We conducted a population-based cohort study among adults aged 18+ initiated on BUP-ER between February 3, 2020 and March 31, 2022 in Ontario, Canada. Using administrative health data, we defined continuous BUP-ER use based on repeat injections within a 56-day period and used Kaplan-Meier curves to estimate time on treatment. Among new BUP-ER recipients, we described individual and prescriber characteristics, healthcare utilization and treatment patterns. 2366 individuals initiated BUP-ER. The median time to BUP-ER discontinuation was 183 days (interquartile range: 66–428 days) and 52.0% of individuals were co-prescribed buprenorphine/naloxone at least once throughout the period of BUP-ER receipt. Among individuals who initiated on a dose of 300 mg BUP-ER and had three or more injections, 18.8% continued to receive only 300 mg doses (N=276 of 1470). Furthermore, 28.6% of those whose dose was reduced to 100 mg (N=341 of 1194) had a subsequent dose increase to 300 mg. On average, people initiating BUP-ER discontinue within the first 6 months of treatment. While BUP-ER is likely providing an important OAT option, the high occurrence of discontinuation, supplementation with buprenorphine/naloxone, and frequent dose increases suggest inadequacy of current dosing recommendations among a proportion of individuals. • On average, time to extended-release subcutaneous buprenorphine discontinuation was 183 days. • Half of new recipients were co-prescribed buprenorphine/naloxone while on treatment. • Almost 1 in 5 people were sustained on a high dose of subcutaneous buprenorphine. • Return to a high dose of subcutaneous buprenorphine was also common (28.6%). [ABSTRACT FROM AUTHOR]