Quantifying regional variation in Ivermectin use during the pandemic using regularized synthetic controls

Abstract We analyse 36 months of weekly US prescription claims spanning the COVID-19 pandemic to investigate overconsumption of the antiparasitic drug Ivermectin (IVM). To quantify the IVM overconsumption following the heightened public attention as a COVID-19 treatment, we adopt a causal framework, comparing IVM prescription trends to those of a large set of control medications. We employ a regularized synthetic control method using continuous spike-and-slab shrinkage priors to estimate state-level deviations in IVM consumption. This approach offers decision-theoretic guarantees on predictive risk, for downstream policy analysis at multiple-time points after the intervention. Its empirical robustness is demonstrated through extensive validation checks. We find a modest increase in IVM prescriptions following early reports of its potential therapeutic use, with no significant surge over the subsequent 8 months, followed by a pronounced increase coinciding with the peak in COVID-19 cases. Strikingly, elevated IVM use persisted even after COVID-19 vaccines became widely available and federal countermeasures were implemented. Our estimation captures the heterogeneity in long-term effectiveness of these countermeasures across states. We find that state-level political affiliation significantly explains variation in overconsumption, even after accounting for COVID-19 incidence, highlighting regional disparities and the need for more targeted and trusted public health messaging.