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

5-alpha 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. 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 alpha-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. 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 alpha-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). 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