Community Mental Health Service Essential Medicines Stockout Duration and Antipsychotic Switching in Malawian District Clinics: A Difference-in-Differences Estimation of Supply-Chain Exposure

Essential medicines stockouts in community mental health services are widely reported, but their behavioural consequences for prescribing remain poorly quantified. The framework treats stockout duration as a supply-chain exposure with a dose-response structure, distinguishes switching driven by clinical judgement from switching forced by unavailability, and specifies the parallel-trends and no-anticipation assumptions required for identification. Because no clinic-level dispensing or stockout data were available for this analysis, the paper is methodological rather than empirical: it formalises the exposure, defines the estimand, derives the difference-in-differences specification, and sets out the threats to validity that a field implementation would face. To anchor the design in measurable quantities, the paper draws on a real DHS Program indicator of family planning messages delivered at community meetings across fifteen African countries between 2021 and 2025, with a mean of 29.733 and a standard deviation of 17.495, to illustrate the scale of between-country heterogeneity that a multi-site supply-chain study must accommodate. The contribution is a pre-specified, falsifiable estimation strategy for linking stockout duration to antipsychotic switching, together with an explicit account of the conditions under which the resulting estimates would and would not support causal interpretation.