Background: Five-alpha reductase inhibitors (5-ARIs), specifically finasteride and dutasteride, have been shown to significantly reduce prostate cancer incidence. However, these agents were also associated with a significant increase in the detection of high-grade prostate cancer leading to an FDA black box warning in 2011. Little is known about the effect of this warning on the subsequent use of these 5-ARIs. The purpose of this analysis was to assess use patterns of finasteride and dutasteride before and after the black box warning. Methods: This cohort study evaluated men enrolled in the Prostate, Lung, Colorectal and Ovarian (PLCO) screening trial who had \ge12 months of Medicare Part D coverage from 2008 to 2015, and had not been diagnosed with prostate cancer through 2007. Socio-demographic factors and benign prostatic hyperplasia (BPH) status were ascertained from follow-up questionnaires, while medication use was ascertained from linkage to Medicare Part D claims data. Results: Of 14,833 eligible men, 88.7% identified as non-Hispanic white, 1.7% as African-American, 5.2% as Asian/Pacific Islander and 1.7% as Hispanic. The median age was 72 years; 41.8% reported a BPH diagnosis. Only 13.6% and 4% of the population took finasteride or dutasteride, respectively, at any time from 2008 to 2015. During this period, finasteride use significantly increased from 3.6% to 9.7% and was highest among men with BPH; dutasteride use remained low and decreased from 2.8% to 1.9%. Conclusions: Finasteride use significantly increased after the FDA’s 2011 black box warning, while dutasteride use remained low and steady throughout the study period.