2.1. Database and Study Population

This retrospective cohort study used data from the Korean National Health Insurance Service (NHIS) database. The NHIS was launched in 2000 as the single insurer covering the entire population of Korea by integrating 375 insurance associations [15]. The NHIS comprises the eligibility database, national health screening database, basic demographic variables (i.e., age, sex, residential area, type of health insurance, etc.), International Classification of Diseases-10 (ICD-10) disease codes, health care utilization (diagnosis, length of stay, treatment costs, and services received), and prescription records (drug code, days prescribed, and daily dosage) [16].