Background: Denosumab (Prolia®) is administered every 6 months for osteoporosis treatment. Co-administration of calcium and vitamin D is required to minimize hypocalcemia risk. We evaluated clinical outcomes based on the administration interval of denosumab and co-prescription with calcium-vitamin D combination products. Methods: A retrospective study was conducted using electronic medical records from 668 patients who started denosumab therapy between January 1 and December 31, 2018, at Seoul National University Bundang Hospital. Clinical outcomes, as measured by changes in T-score, were evaluated by the intervals and concurrent prescriptions with calcium–vitamin D combination products. Results: Of the 668 patients, 333 patients met the eligibility criteria. These patients were divided into two groups based on appropriateness of the administration interval: “Appropriate” (304 patients, 91.3%) and “Inappropriate” (29 patients, 8.3%). T-score changes were significantly higher in the “Appropriate” than in the “Inappropriate” group (0.30±0.44 vs. 0.13±0.37, p=0.048). At the beginning of the treatment, 221 patients (66.4%) were prescribed calcium-vitamin D combination products, but the changes in T-scores were not significantly different by the prescription status of the product (0.29±0.46 vs. 0.28±0.38, p=0.919). Conclusion T-scores were significantly improved in patients with appropriate administration intervals. No significant changes in T-scores were observed by the prescription status with calcium-vitamin D combination products. For optimal treatment outcomes, prescribers should encourage adherence to the approved prescription information on dosage and administration, and pharmacists should provide medication counseling for patients.