Finasteride is a potent competitive 5α-reductase inhibitor, active at a dose as low as 1 mg/day. After a single dose, the effects on 5α-reductase last as long as 7 days. Both hepatic and target tissue 5α-reductase are inhibited. Plasma testosterone and estradiol are unaffected and luteinizing hormone levels do not change. During chronic treatment with finasteride 5 mg/day, the effects on 5α-reductase are maintained. Since the only significant effect of chronic finasteride therapy appears to be 5α-reductase inhibition, and testosterone or estradiol levels are not affected, neither libido nor potency is lost. Testosterone is the active androgen at the muscular level; therefore, muscular catabolism is not expected.