Propecia (finasteride) is a competitor and specific inhibitor of steroid Type II5alpha-reductase, the intracellular enzyme that converts testosterone to DHT (5alpha-dihydrotestosterone). It blocks the peripheral conversion, then, of test to DHT.
It has no affinity for the androgen receptor and has no antiandrogenic effects, nor does it have any androgenic, estrogenic, antiestrogenic or progestational effects.