Male breast cancer and 5α-reductase inhibitors finasteride and dutasteride.

We examined the association between 5α-reductase inhibitors and male breast cancer. Study participants were men 40 to 85 years old, with prescription and medical coverage, enrolled in the United States IMS LifeLink™ Health Plan claims database between 2001 and 2009. Cases required a primary breast cancer diagnosis (ICD-9-CM 175.x) on 2 different dates and a procedural code for mastectomy or lumpectomy/partial mastectomy with evidence of continuous care (radiation/chemotherapy or diagnoses in 2 or more months). Eligible controls were within 5 years in age and had duration of prior health care enrollment within 6 weeks. Risk set sampling selected 20 controls per case. We assessed the rate ratio for male breast cancer with 5α-reductase inhibitor exposure using conditional logistic regression. Analyses were stratified by duration of health care enrollment before diagnosis (1 year or more, 2 years or more and 3 years or more), each incremental 180 and 365 days of cumulative 5α-reductase inhibitor exposure, and period specific time frames before diagnosis (years 1, 2 and 3). We identified 339 breast cancer cases matched to 6,780 controls. No statistically significant associations were observed between 5α-reductase inhibitors and breast cancer regardless of exposure assessment before the index date (1 year or more-RR 0.70, 95% CI 0.34-1.45; 2 years or more-RR 0.59, 95% CI 0.24-1.48; or 3 years or more-RR 0.75, 95% CI 0.27-2.10). Each subsequent 180 days (RR 1.02, 95% CI 0.67-1.53) and 365 days (RR 1.03, 95% CI 0.45-2.37) of cumulative 5α-reductase inhibitor therapy and period specific rate ratios also showed null associations. The lack of an association in our study suggests that the development of breast cancer should not influence the prescribing of 5α-reductase inhibitor therapy.