RESUMEN
Medication waste may be caused by medication oversupply; however, the degree of medication oversupply in Japan is unclear. This study aimed to quantify the degree of oversupply of chronic disease medications per patient, the proportion of oversupplied patients, and the excess days and costs of the oversupplied medications in Japan. This retrospective nationwide cohort study using a large insurance claims database from Japan was conducted in patients aged ≥55 years who received one or a combination of the following five classes of medications dispensed in FY 2019: third-generation calcium antagonists, angiotensin 2 receptor blockers, statins, dipeptidyl peptidase-4 inhibitors, and biguanides. Medications with the same ingredient having the same specification were treated as the same medication. Medication oversupply was defined as a medication possession ratio (MPR) during persistence >1.0. The proportions of oversupplied patients and excessively oversupplied patients with ≥30 excess days/year were approximately 16 and 1-2% for all drug classes, respectively. Three-quarters of the oversupplied patients had fewer excess day (≤14/year), and the median oversupplied medication cost was less than 1000 yen/year for all classes. However, there was a patient with oversupplied medication estimated as 983 excess days per year and a patient with oversupplied medication costs of nearly 90000 yen per year. Using the MPR and excess days as indicators, it is necessary to accelerate estimation of the oversupply per patient, as well as the development of patient intervention strategies and a national system to reduce medication oversupply.