5-α reductase inhibitors, benign prostatic hyperplasia, and risk of male breast cancer

PURPOSE: 5-α reductase inhibitors (5-ARI) have been suggested to increase the risk of male breast cancer. The aim of this study was to study the risk of breast cancer in men on 5-ARI, in men with benign prostatic hyperplasia (BPH) not on 5-ARI, and in men without BPH. METHODS: We performed a population-based cohort study in Sweden with data from The Prescribed Drug Register, The Patient Register, and The Cancer Register. Men on 5-ARI, men on α-blockers, or men who had undergone a transurethral resection of the prostate (TUR-P) prior to or during 2006–2008 were included as exposed to BPH and a specific treatment thereof. For each exposed man, five unexposed men were selected. Risk of breast cancer was calculated in Cox proportional hazard models. RESULTS: There were 124,183 exposed men and 545,293 unexposed men, and during follow-up (median 6 years), 99 men with breast cancer were diagnosed. Compared to unexposed men, men on 5-ARI had a hazard ratio (HR) of breast cancer of 0.74 (95 % confidence interval (CI) 0.27–2.03), men on α-blockers had HR 1.47 (95 % CI 0.73–2.95), and men with a TUR-P had HR 1.99 (95 % CI 1.05–3.75). CONCLUSION: No increased risk of breast cancer was observed for men on 5-ARI. However, the increased risk of breast cancer among men who had undergone a TUR-P, a strong indicator of BPH, suggests that the endocrine milieu conducive to BPH is associated with male breast cancer.