Finasteride is an antiandrogen which acts by inhibiting type II 5-alpha reductase, the enzyme that converts testosterone to dihydrotestosterone (DHT). It is used as a treatment in benign prostatic hyperplasia (BPH) in low doses, and prostate cancer in higher doses. It is also indicated for use in combination with doxazosin therapy to reduce the risk for symptomatic progression of BPH. Additionally, it is registered in many countries for androgenetic alopecia (male-pattern baldness).

Finasteride was approved initially in 1992 as Proscar, a treatment for prostate enlargement, but the sponsor had studied 1 mg of finasteride and demonstrated hair growth in male pattern hair loss. On December 22, 1997, the FDA approved finasteride to treat male pattern hair loss.

The Prostate Cancer Prevention Trial (PCPT) showed at a dosage of 5mg per day, as is commonly prescribed for BPH, though much higher than the 1mg generally prescribed for hair loss, participants taking finasteride were 25% less likely to have developed prostate cancer at the end of the trial compared to those taking a placebo. Further, finasteride increased the specificity and selectivity of prostate cancer detection, thus, a seemly increased rate of high Gleason grade tumor.

Recognized side effects, experienced by around >1% of users, include erectile dysfunction, and less often gynecomastia (breast gland enlargement).As expected from its short 6-8 hour half-life, in trial studies, side effects ceased after dosage was discontinued.

Side effects
Finasteride is not indicated for use by women. Finasteride is in the FDA pregnancy category X. This means that it is known to cause birth defects in an unborn baby. Women who are or who may become pregnant must not handle crushed or broken finasteride tablets, because the medication could be absorbed through the skin. Finasteride is known to cause birth defects in a developing male baby. Exposure to whole tablets should be avoided whenever possible, however exposure to whole tablets is not expected to be harmful as long as the tablets are not swallowed. It is not known whether finasteride passes into breast milk, and thus should not be taken by breastfeeding women. Finasteride may pass into the semen of men, but Merck states that a pregnant woman's contact with the semen of a man taking finasteride is not an issue for concern.

Finasteride has been linked with depression.The drug also caused reductions in allopregnanolone, a potent, endogenous positive modulator of the GABA-A receptor, in very large doses in rodent studies.

Finasteride can also be used to mask steroid abuse, and many professional sports have banned finasteride use for this reason.

Finasteride is under investigation by the Swedish Medical Products Agency for possibly causing irreversible sexual side effects.

Use as a treatment for hair loss
In a 5-year study of men with mild to moderate hair loss, 48% of those treated with Propecia (finasteride 1mg) experienced some regrowth of hair, and 42% had no further loss. Average hair count in the treatment group remained above baseline, and showed an increasing difference from hair count in the placebo group, for all five years of the study. Propecia is effective only for as long as it is taken; the hair gained or maintained is lost within 6-12 months of ceasing therapy.[9] In clinical studies, Propecia, like minoxidil, was shown to work on both the crown area and the hairline,but is most successful in the crown area.

Some users, in an effort to save money, buy Proscar instead of Propecia, and split the Proscar pills to approximate the Propecia dosage. Doing so is generally considered unadvisable if women of pregnancy age are in the household; this is because finasteride, even in small concentrations, can cause birth defects in a developing male fetus. The birth defects involve the development of male genitalia (no such effects have been noted in developing female fetuses). On most product inserts, it will be mentioned that the dust or crumbs from broken Propecia tablets should be kept away from pregnant women.

Propecia has been shown to be ineffective for treating hair loss in women.[citation needed] However, Propecia's supporters respond that the study was on post-menopausal women whose hair loss was more likely related to the loss of estrogen versus a sensitivity to testosterone. Many doctors prescribe it for women, but not without either careful birth control measures or assurance that the woman cannot become pregnant.