OBJECTIVES: To assess the associations between preoperative treatment with 5-alpha reductase inhibitors and the risks of blood transfusion during transurethral resection of the prostate and blood clot evacuation or emergency department visits for hematuria within 1 month after surgery. METHODS: We used data from the Taiwan National Health Insurance Research Database in this population-based cohort study. A total of 3,126 patients who underwent first-time transurethral resection of the prostate from 2004 to 2013 were identified. Adjusted odds ratios estimated by multiple logistic regression models were used to assess the independent effects of the preoperative use of 5-alpha reductase inhibitors on the risks of perioperative hemorrhagic events after adjustment for potential confounders. RESULTS: Two hundred and ninety-seven (9.4%) patients were treated with 5-alpha reductase inhibitors for <3 months, and 65 (2.1%) patients were treated for ≥3 months prior to undergoing transurethral resection of the prostate. The blood transfusion rates for patients who were not treated with 5-alpha reductase inhibitors (controls), patients who were treated with 5-alpha reductase inhibitors for <3 months, and patients who were treated with 5-alpha reductase inhibitors ≥3 months were 9.5%, 8.8%, and 3.1%, respectively. 5-alpha reductase inhibitors tended to decrease the risk of blood transfusion; however, this association was not statistically significant (adjusted odds ratio=0.14, 95% confidence interval: 0.02-1.01). Age ≥80 years, coagulopathy, and a resected prostate tissue weight >50 g were associated with significantly higher risks of blood transfusion than other parameters. CONCLUSIONS: This nationwide study did not show that significant associations exist between 5-alpha reductase inhibitor use before transurethral resection of the prostate and the risks of blood transfusion and blood clot evacuation or emergency visits for hematuria.