<p>Finasteride, a competitive inhibitor of steroid Type II 5&alpha; reductase, an intracellular enzyme that converts the androgen testosterone into 5&alpha; dihydrotestosterone (DHT). Finasteride is a competitive and specific inhibitor of Type II 5&alpha; reductase, an intracellular enzyme that converts the androgen testosterone into DHT. Finasteride has no affinity for the androgen receptor and has no androgenic or antiandrogenic effects. Inhibition of Type II 5&alpha; reductase blocks the conversion of testosterone to DHT, resulting in significant decreases in serum and tissue DHT concentrations. In men with male pattern hair loss, the balding scalp contains miniaturised hair follicles and increased amounts of DHT compared with hairy scalp. Administration of Finasteride decreases scalp and serum DHT concentrations in these men.</p>