Short answer: Finasteride has a half-life of about 5 to 6 hours. Levels peak 1 to 2 hours after a dose. People vary, so treat these as estimates.
Finasteride is a 5-alpha-reductase inhibitor: it blocks the type II form of the enzyme that converts testosterone into dihydrotestosterone (DHT). It is sold as Propecia for hair loss, as Proscar for an enlarged prostate, and as generic tablets.
Also known as: Propecia, Proscar.
Propecia comes as 1 mg tablets and Proscar as 5 mg tablets. Generic finasteride is sold in both strengths.
The FDA Propecia label approves finasteride 1 mg once a day for male pattern hair loss in men only; it is not indicated for women or children. The Proscar label approves finasteride 5 mg once a day, alone or with the alpha-blocker doxazosin, for symptoms of benign prostatic hyperplasia (BPH) in men with an enlarged prostate.
Women who are or may be pregnant should not handle crushed or broken tablets, because finasteride can affect the genitals of a male fetus. Finasteride lowers PSA: the Proscar label says by about 50% within six months, so a PSA result after six months should be doubled before comparing it with normal ranges. In a 7-year trial of finasteride 5 mg, high-grade prostate cancer was more common than on placebo (1.8% vs 1.1%). Reports after approval include sexual problems that continued after stopping, depression and suicidal thoughts. See also dutasteride, which blocks both forms of the enzyme.
The FDA Propecia label gives a mean terminal half-life of about 5 to 6 hours in men 18 to 60 and about 8 hours in men over 70, with peak levels 1 to 2 hours after a dose. The label says a 1 mg tablet cut DHT by 65% within 24 hours. Food does not change how much is absorbed.
Numbers shown are taken from the source above. Half-life varies from person to person.
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