Exploring contextual barriers and facilitators to sustaining mental health integration in primary care: a mixed-methods analysis of adaptive mechanisms and multi-level dynamics in Lagos, Nigeria.

Background: Mental health interventions in low- and middle-income countries (LMICs) face significant sustainability challenges, often leading to 'programme drift' (protocol deviation ) and 'voltage drop' (reduced effectiveness). While implementation science frameworks emphasise fidelity, they often fail to explain how frontline providers in resource-constrained settings maintain services. Here, we investigate how adaptive mechanisms function as legitimate sustainability strategies within Lagos, Nigeria's Mental Health in Primary Care programme, which contends with chronic underfunding, high staff turnover, and community stigma. Methods: We conducted a convergent mixed-methods study in six Lagos local government areas. Data were collected from 130 stakeholders (policymakers, managers, health workers, care recipients) through quantitative surveys and from a nested subsample of 70 participants through in-depth interviews and institutional ethnography. We analysed quantitative data using multiple regression and qualitative data using thematic analysis, systematically integrating the findings through triangulation to produce meta-inferences about sustainability dynamics. Results: Systemic constraints, particularly underfunding (<2% of health budget) and high staff turnover (30% annually), drove programme drift and community stigma, deterring 40% of patients and contributing to voltage drop. However, this drift often manifested through constructive adaptive mechanisms, including informal peer mentoring networks and role flexibility, which maintained service continuity. Multiple regression (R Conclusions: Adaptive mechanisms represent legitimate and necessary sustainability strategies in resource-constrained settings, not implementation failures. We propose 'functional fidelity' (maintaining core outcomes through flexible processes) and 'adaptive capacity' as crucial theoretical extensions for implementation science in LMICs. Sustainable mental health integration requires frameworks that recognise and support frontline innovation while ensuring quality safeguards are maintained, offering a more realistic pathway to closing the global mental health treatment gap.