Finasteride (marketed as Proscar, Propecia, Fincar, Finpecia, Finax, Finast, Finara, Finalo, Prosteride, Gefina, Finasterid IVAX) 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