Finasteride use and risk of male breast cancer: a case-control study using individual-level registry data from Denmark, Finland, and Sweden.

Background: In case reports, concerns have been raised as to whether finasteride use increases the risk of male breast cancer. Previous epidemiologic evidence on the potential link is conflicting. This study aimed to assess whether an association between finasteride use and male breast cancer exists after accounting for potential confounders. Methods: The source population consisted of all men (similar to 35 years) from Denmark (1995-2014), Finland (1997-2013), and Sweden (2005-2014). Cases with incident male breast cancer were identified in the cancer registries and matched with 50 density-sampled, age, and country-matched male population controls per case. Exposure information on finasteride use was derived from the prescription registries. Potential confounders were identified using the directed acyclic graph methodology and measured by use of information from nation-wide registries. Results: The study population comprised 1,005 male breast cancer cases and 43,058 controls. Confounder-adjusted odds of finasteride exposure were not statistically significantly increased [OR, 1.09; 95% confidence interval (CI), 0.77-1.54] in breast cancer cases relative to controls. There was no evidence of a dose-response relationship, as the group with greatest exposure to finasteride was associated with lowest OR of male breast cancer [OR, 0.72 (95% CI, 0.40-1.30)]. Sensitivity analyses did not reveal marked changes in results with different exposure definitions or for specific subgroups. Conclusions: Results from this study provided no evidence that finasteride use was associated with male breast cancer. Impact: This large confounder-adjusted study supports the view that exposure to finasteride is not associated materially with male breast cancer risk.