Finasteride is a synthetic 4-azasteroid that is a specific competitive inhibitor of 5 alpha-reductase, an intracellular enzyme that converts testosterone to dihydrotestosterone (DHT). It has no binding affinity for androgen receptor sites and itself possesses no androgenic, antiandrogenic, or other steroid hormone-related properties. It is well absorbed after oral administration, with absolute bioavailability in humans of 63% (range 34-108%). The mean time to maximum concentration is 1-2 hours, and it is approximately 90% plasma protein bound. The elimination half-life averages 6-8 hours. The agent is metabolized to a series of five metabolites, of which two are active and possess less than 20% of the 5 alpha-reductase activity of finasteride. Little is known about potential drug interactions, although they appear to be minimal and not clinically relevant. The drug is indicated for the treatment of symptomatic benign prostatic hyperplasia. Its efficacy in regression of prostate gland enlargement is rapid and predictable, although correlation with subsequent improvement in urinary flow and symptoms is highly variable. Dosages of 0.5-100 mg/day regress prostate enlargement; the recommended dosage is 5 mg once/day. Finasteride may hold promise for other DHT-mediated disorders such as acne, facial hirsutism, frontal lobe alopecia, and prostate cancer, but its use in these conditions remains investigational. The frequency of adverse drug events is low, with the most common side effects being impotence, decreased libido, and decreased volume of ejaculate. No reports of intentional overdose have been reported, and dosages of up to 80 mg/day for 3 months have been taken without adverse effect.