Opioid use disorder (OUD) is defined in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-V) as a "problematic pattern of opioid use leading to clinically significant impairment or distress." The DSM-V also uses the number of criteria present to categorize the severity of OUD, with mild OUD defined as having 2-3 criteria present, moderate OUD defined as having 4-5 criteria present, and severe OUD defined as having ≥6 criteria present. Opioid use disorder and opioid-related deaths are increasing in Ontario and Canada. Compared to 2016, the age-adjusted opioid-related mortality rates in 2020 have increased by 161% in Ontario to 16.5 per 100 000 population and 114% in Canada to 16.7 per 100 000 population. Like deaths, opioid-related healthcare utilization as measured by hospitalization and emergency department visits have also been increasing over the same time period. Compared to 2016, the age-adjusted opioid-related hospitalization rates in 2020 have increased by 19.8% in Ontario to 16.3 per 100 000 population and 5.9% in Canada to 17.8 per 100 000 population. Emergency room visits have increased by 166% during the same time period in Ontario to 84.5 per 100 000 population. The median length of hospitalization from 2016 to 2020 has remained unchanged at 3 days (Range 1 to 207 days), with an average cost per hospitalization of $9 626 (CDN) (Range $26 to $296 831). Opiate substitution treatment with either full (methadone) or partial (buprenorphine) opioid agonist therapy (OAT) has been shown to reduce self-reported opioid use or opiate positive urine drug tests compared to detoxification or psychological treatments. There are 2 main opioid agonists used to treat OUD, methadone and buprenorphine. Buprenorphine has been approved by Health Canada for use in OUD, and is available in immediate (Suboxone®) and extended-release (Sublocade®) formulations. Monthly subcutaneous buprenorphine (SC-BPN-XR) has been shown to be non-inferior to daily sublingual buprenorphine/naloxone (SL-BPN/NX) for medication adherence and treatment retention. The Canadian pharmaceutical costs of Sublocade® and Suboxone® differ substantially , with monthly costs for Sublocade 8-fold higher than Suboxone. The majority of patients meeting criteria for Sublocade® use must pay for the drug out-of-pocket resulting in significant restriction to access for this population that has a high burden of chronic homelessness and poverty. A recent pharmacoeconomic report by the Canadian Agency for Drugs and Technologies in Health (CADTH) concluded that the current evidence base for supporting cost-effectiveness of Sublocade® over Suboxone is limited by the absence of studies directly comparing the effectiveness of the two treatments on clinically important outcomes such as healthcare utilization. With limited evidence and significant uncertainty, the CADTH analysis suggested that a price reduction of at least 73% would be required for Sublocade® to be a cost-effective alternative to Suboxone. A real-world (pragmatic) randomised study is therefore needed to compare the treatment effectiveness of Sublocade® versus Suboxone® in Canadian patients attending RAAM clinics for OAT for moderate- to severe-OUD. This study is needed to demonstrate the superior benefits of Sublocade® on important clinical outcomes such as reduced opioid and healthcare utilization in order to demonstrate its cost-effectiveness and justify expanded access for this at-risk population.