BACKGROUND Amidst a sustained drug poisoning crisis, there is growing recognition in Canada of the need to expand injectable opioid agonist treatment (iOAT). iOAT is an intensive treatment that involves the daily self-administration of hydromorphone or diacetylmorphine under healthcare provider supervision, typically accompanied by other health and social services. While this treatment has demonstrated effectiveness in reducing drug use-related risks, high threshold characteristics may also create barriers to engagement. This study examined patients' experiences of barriers and facilitators to iOAT with attention to how social and structural factors (e.g., housing vulnerability, poverty) shape program engagement.

METHODS This study draws on qualitative interviews and fieldwork observations with people accessing four iOAT programs in Vancouver's Downtown Eastside neighbourhood from May 2018 to November 2019. Data included baseline and follow-up interviews and approximately 50 h of observational fieldwork. Analysis leveraged a structural vulnerability lens to examine how social and structural factors shape people's engagement with iOAT.

RESULTS Participants highlighted how improved access to health and social services, compassionate and relational care, and flexible and individualized approaches to treatment delivery that addresses and accounts for the structural vulnerabilities facilitated engagement in treatment. However, dosing supervision, operational capacity and medication formulation were experienced as barriers to treatment. These barriers were magnified by structural vulnerabilities such as housing instability and mobility challenges.

CONCLUSIONS Study findings highlight how people navigate the barriers and facilitators to iOAT engagement in light of the structural vulnerabilities they experience. Adaptations to and ongoing support for iOAT programs may help to facilitate engagement and should focus on equity-oriented and patient- centered treatment models that includes the integration of social supports, support for relational care and treatment planning that supports patient autonomy.