Finasteride is used to treat benign prostatic hypertrophy (BPH). BPH commonly occurs in men over 55 years of age and causes the prostate gland to enlarge. Enlarged prostate glands can cause urinary problems such as frequent and difficult urination, especially at night. Finasteride shrinks the prostate and relieves these symptoms. This medication is sometimes prescribed for other uses; ask your doctor or pharmacist for more information.

Finasteride, a synthetic 4-azasteroid compound, is a specific inhibitor of steroid Type II 5a-reductase, an intracellular enzyme that converts the androgen testosterone into 5a-dihydrotestosterone (DHT). Finasteride is 4-azaandrost-1-ene-17-carboxamide, N-(1,1-dimethylethyl)-3-oxo-, (5a,17b)-. The chemical formula of finasteride is C23H36N2O2 and its molecular weight is 372.55. Finasteride is a white crystalline powder with a melting point near 250°C. It is freely soluble in chloroform and in lower alcohol solvents but is practically insoluble in water.

Finasteride is a drug that inhibits testosterone metabolism and therefore is used to reduce prostate size, as growth of the prostate is normally testosterone dependant. As a result there is often an improvement in urinary flow and other urinary symptoms. Finasteride inhibits expression of the enzyme, 5-alpha reductase, which regulates production of dihydrotestosterone (DHT). By lowering DHT levels in the scalp, it reduces DHT's harmful effect on hair follicles. Finasteride decreases DHT concentrations in the serum and the scalp by up to 70 and 60%, respectively.

Testosterone is the most potent of the androgenic hormones. It is one of the most important in its influence on sebaceous glands, hair follicles and prostate tissue. Testosterone does not act directly on skin, hair follicles and prostate tissue, however. Within cells, testosterone is converted into dihydrotestosterone (DHT) by 5 alpha-reductase; it is testosterone in its DHT form that influences skin structures and the prostate gland. Androgenic hormones have multiple actions in the body, including actions in the skin, hair follicles and the prostate gland among other internal organs. Hair follicles and sebaceous (oil-producing) glands in the skin are particularly responsive to androgenic hormones: cellular growth and oil production of sebaceous glands are stimulated, and hair growth is significantly influenced by androgens. Male pattern hair loss, or androgenetic alopecia, is an androgen-dependent disorder. In genetically susceptible men, dihydrotestosterone (DHT), a potent metabolite of the male androgen testosterone, contributes to male pattern hair loss. The conversion of testosterone to DHT is regulated by the enzyme 5-alpha reductase.