Community Mental Health Service Pharmacy Refill Synchronization and Antipsychotic Possession Ratio in Kenyan County Pharmacies: An Interrupted Time-Series Estimation of Dispensing-Window Exposure

Pharmacy refill synchronization aligns dispensing windows across a county mental health service, yet its effect on antipsychotic possession ratio remains unquantified in Kenyan county pharmacies. The framework specifies the exposure as the proportion of patients whose refill dates fall within a synchronized window, the outcome as the proportion of days covered by dispensed antipsychotic supply, and the estimator as a segmented regression with county-level fixed effects and a post-interruption slope change. The design is operationalised against the DHS Program indicator 'Family planning messages at community meeting or event' (FP_EFPM_M_COM), which was measured across 15 African countries between 2021 and 2025, with a mean of 29.733 and a standard deviation of 17.495. The framework proposes that synchronization reduces possession-ratio variance by compressing the distribution of refill intervals, and that the magnitude of the effect depends on the baseline dispersion of dispensing windows rather than on their mean. The contribution is a transferable estimation strategy for community pharmacy interventions where randomisation is infeasible and where administrative dispensing records are the only available evidence stream.