Important: this page is educational. Finasteride is a prescription drug and requires a clinician evaluation.
What is finasteride?
Finasteride is a 5-alpha reductase inhibitor that reduces the conversion of testosterone to dihydrotestosterone (DHT). It is sold as Propecia at 1 mg for hair loss and as Proscar at 5 mg for benign prostatic hyperplasia.
How does finasteride work?
DHT binds androgen receptors in scalp follicles and progressively miniaturizes them in genetically susceptible men. Finasteride inhibits the type 2 isoform of 5-alpha reductase, which reduces serum DHT by roughly 65 to 70 percent. With less DHT signaling, miniaturized follicles can recover and stable follicles are protected.
How is finasteride dosed?
The standard hair loss dose is 1 mg orally once daily. Some clinicians use 1.25 mg every other day, which produces similar DHT suppression in many men at lower total exposure.
The 5 mg Proscar tablet is for prostate use and is not the right dose for hair loss; some patients quarter it to approximate 1 mg.
How long until finasteride works?
Shedding may briefly increase in the first one to three months as miniaturized hairs are pushed out and replaced by terminal hairs. Visible thickening typically appears between months three and six. Peak effect on regrowth is usually seen between months nine and twelve. Continued use is required; stopping returns DHT to baseline and miniaturization resumes.
Side effects of finasteride
Sexual side effects (reduced libido, erectile difficulty, reduced ejaculatory volume) are reported in roughly 2 to 4 percent of men in randomized trials, with similar but lower rates in placebo arms. Most are reversible within weeks of discontinuation. Mood changes, including depression in a small subset, have been reported and warrant attention.
Post-finasteride syndrome describes a controversial pattern of persistent symptoms after discontinuation. The epidemiology is still debated. The honest position is that most men tolerate finasteride well, a minority experience reversible side effects, and a smaller minority report persistent issues.
Read our finasteride side-effects breakdown for a deeper look.
Who should not take finasteride?
Contraindicated in pregnancy (relevant to partners) and in known hypersensitivity. Use caution in men with active mood disorders or fertility planning, since DHT plays a role in sperm parameters. Men with elevated PSA should know that finasteride lowers PSA by approximately 50 percent, which must be accounted for in prostate screening.
Finasteride vs the alternatives
Minoxidil works through a different mechanism (vasodilation and follicle stimulation) and is often combined with finasteride for additive effect.