Evaluation of methods to estimate missing days’ supply within pharmacy data of the Clinical Practice Research Datalink (CPRD) and The Health Improvement Network (THIN)

Purpose The extent to which days’ supply data are missing in pharmacoepidemiologic databases and effective methods for estimation is unknown. We determined the percentage of missing days’ supply on prescription and patient levels for oral anti-diabetic drugs (OADs) and evaluated three methods for estimating days’ supply within the Clinical Practice Research Datalink (CPRD) and The Health Improvement Network (THIN).