OBJECTIVE: Medication Regimen Complexity Index (MRCI) is a tool to quantify the complexity of prescriptions through weighting the scores of dosage form, dosing frequency and additional directions. A meta-analysis showed that high MRCI score was associated with medication non-adherence and readmission. However, the factors associated with higher MCRI score are still unclear, especially for elderly. This research aims to explore the factors associated with MRCI score in elderly in Taiwan. METHOD: Patients’ medication and relevant data were evaluated based upon the previous prospective observational study conducted in an academic medical center in Taiwan. Their responses of survey questions in the first visits (e.g., insomnia, mental/functional status, quality of life) were examined to explore the associations with MRCI scores using appropriate statistical approaches, including independent t test and linear regression. RESULT: Of all enrolled 219 patients, elderly who were prescribed with anxiety and hypnotic medications, diagnosed with Chronic Pulmonary Disease, Renal disease, diabetes with chronic complication, or having poor functional status tended to have higher MRCI scores as compared to the counterparts (all ps <0.05). Controlling for the other factors using multivariate linear regression analysis, patients with the following factors were statistically, significant increasing on the MRCI scores than that of counterparts: who had chronic pulmonary disease, diabetes with chronic complication, were using anxiety and hypnotic medications, and had poor functional status. CONCLUSION: Elderly patients with some specific diseases and were using anxiety and hypnotic medications, and poor functional status currently tended to have higher MRCI score. The complicated medication dosage forms or additional directions for these patients might contribute to such higher. Further researchers to explore the application of MRCI in real world or in electronic medication charts are necessary to enhance better quality of care for those elderly patients tended to have higher MRCI scores in medical settings.