BACKGROUND: The barriers and facilitators to engagement with Opiate Substitute Treatment and Needle and Syringe Provision need to be better understood to develop interventions to attract people into services.

OBJECTIVE(S): We conducted a multi-method realist evaluation to generate an in-depth theory of the factors that influence the optimal provision of services in terms of access, engagement, retention and successful exit.

DESIGN: Multi-method involving an online survey of United Kingdom drug and alcohol service commissioning leads and a meta-synthesis of qualitative literature (phase 1) to facilitate the development of an initial theory of optimal provision; and, in phase 2, a realist evaluation to test and refine the initial theory, involving in-depth interviews across three United Kingdom sites, with service commissioners, managers, staff and service users. We used routine treatment and staffing data to estimate the additional staff costs of implementing optimal service provision within one context of our refined theory.

RESULTS: Findings indicated that optimal provision could be understood with reference to five main contexts [(1) agency and empowerment, (2) self-esteem and respect, (3) knowledge and communication, (4) goals, needs and preferences and (5) resources and demands]. The analysis suggested that optimal provision could be facilitated via specific mechanisms of action, operating at the 'Systems level' (policy, legislation and funding) and 'Service level' (delivery of services, service pathways, staff roles and responsibilities and organisational culture). Our analysis also identified the potential importance of independent 'mediating mechanisms' (e.g. confidence, trust and self-efficacy) which can operate to increase the likelihood of successful service outcomes. A costing analysis estimated the costs associated with a smaller caseload for shared care workers and the implementation of a salary increment scheme at one of the study sites.

LIMITATIONS: The survey sample size limited generalisability. There was a limited number of United Kingdom studies within the meta-synthesis. The findings of the realist evaluation highlight that provision was not optimised in the observed sites, limiting examples of best practice. However, the conceptual theory of optimal provision can direct future research to facilitate the development and implementation of optimal policy and practice.

CONCLUSIONS: We have provided a rich understanding of the contexts, mechanisms and actions by which optimal delivery of Opiate Substitute Treatment and Needle and Syringe Provision services may increase the likelihood of successful service provision. A multiagency approach applying various mechanisms within the various contexts of optimal provision may need to be implemented to optimise the provision.

FUTURE WORK: Future research should focus on the development of a multiple-system toolkit or intervention to help drug services to implement optimal provision. More research is also needed to assess, the barriers and facilitators, that vulnerable underserved population and people from minority ethnic groups may face, and the mechanisms to improve provision for these populations.