Finasteride is a:

Finasteride is a competitive inhibitor of the enzyme 5-alpha reductase which is responsible for the conversion of testosterone into a more active dihydrotestosterone responsible for the androgen action. When used in benign prostatic hyperophy, it reduces the prostate size and increased peak urinary flow rate. It is also used in male pattern baldness and as a palliative treatment in prostatic carcinoma. Ref: K D Tripathi Textbook of Pharmacology, 5th Edition, Page 272