Polypharmacy, defined as the concurrent use of five or more medications, is a major public health concern and the potential cause of medication-related problems. 1) Drug treatment for elderly patients with chronic diseases has increased, and polypharmacy is common owing to complex morbidity. 2,3) The prevalence of polypharmacy is high, and the use of 5, 9, 10 drugs or more drugs was reported to be 91, 74, and 65%, respectively, depending on the facilities. 4) Polypharmacy is an indicator of medication-related problems, which is associated with admission and hospitalization in elderly patients. 5,6) Increased regimen complexity and the use of inappropriate medications have been studied as medication-related problems regarding polypharmacy. 7,8) First, the increase in the number of medications may lead to regimen complexity. 9) George et al. ( 2004) developed a methodology for quantifying regimen complexity as numbers known as the medication regimen complexity index (MRCI). 10,11) This instrument is an open index with no upper limit and consists of sections A (the dosage form), B (dosing frequency), and C (the instructions) of the regimen, which are expressed as the sum of regimen ABSTRACT Background: Along with the increase in the elderly population, concerns about polypharmacy, which can cause medication-related problems, are increasing. This study aimed to find out the association between drug-related factors and readmission in elderly patients within 30 days after discharge. Methods: Data of patients aged ≥65 years who were discharged from the respiratory medicine ward of a tertiary hospital between January and March 2016 were retrospectively obtained. The medication regimen complexity at discharge was calculated using the medication regimen complexity index (MRCI) score, comorbidity status was assessed using the Charlson comorbidity index (CCI), potentially inappropriate medications (PIMs) were evaluated based on the Beer 2019 criteria, and adverse drug events (ADEs) were examined using the ADE reporting system. Multivariable logistic regression analysis was used to evaluate the effect of medication-related problems on hospital readmission after controlling for other variables. Results: Of the 206 patients included, 84 (40.8%) used PIMs, 31 (15%) had ADEs, and 32 (15.5%) were readmitted. The mean age, total medications, MRCI, CCI, and PIMs in the readmission group were significantly higher than those in the non-readmission group. Age significantly decreased the risk of readmission (odds ratio [OR], 0.89; 95% confidence interval [CI], 0.84-0.96) after adjusting for sex, length of hospital stay, and ADEs. The use of PIMs (OR, 2.38; 95% CI, 1.10-5.16) and increased CCI (OR, 1.50; 95% CI, 1.16-1.93) and MRCI (OR, 1.04; 95% CI, 1.01-1.07) were associated with an increased occurrence of readmission. Conclusion: PIMs were associated with a significantly greater risk for readmission than MRCI.