BACKGROUND: Understanding the factors that hinder or support engagement with Opioid Agonist Therapy (OAT) and Needle and Syringe Provision (NSP) is essential for increasing retention and reducing drug-related harm. This study comprised a multimethod realist evaluation to develop a theory of the factors influencing service provision, focusing on access, engagement, retention, and successful exit (clients outcome met) from services.

DESIGN: Phase 1 involved an online survey of UK drug and alcohol service commissioners, focus groups with interest holders, and a qualitative systematic review to build an initial programme theory of optimal service provision. Phase 2 tested and refined this theory through 86 in-depth interviews across three UK sites with commissioners, managers, staff, and people who use services (both regular attendees and those with limited contact).

RESULTS: The Realist Evaluation identified five interrelated contexts (Agency and Empowerment; Self-esteem and Respect; Knowledge and Communication; Goals, Needs and Preferences; and Resources and Demands), within which mechanisms can shape optimal service provision. Person-centred approaches, low-threshold access, and timely provider contact foster agency. Confidentiality, non-stigmatising care, and strengths-based support build self-esteem, while skilled staff, training, and peer networks enhance knowledge and communication. Flexible appointments, integrated services, and shared stakeholder responsibility align care with individual goals and needs. Adequate staffing, training, supervision, and sustainable funding enable responsive, resilient services.

CONCLUSIONS: These findings show how interacting relational and structural mechanisms generate optimal outcomes, guiding policy and service design. Implementing a multi-agency approach, integrating different mechanisms across the identified contexts, may be necessary to achieve optimal service delivery.