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: = 0.45) identified leadership (β = 0.42), infrastructure (β = -0.35), and stigma (β = -0.30) as significant predictors of sustainability. Mixed-methods integration revealed these adaptations were the primary mechanism through which effective leadership operated - a dynamic invisible to quantitative measures alone. 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.