Finasteride
Finasteride is a selective 5-alpha-reductase type II inhibitor used in male pattern hair loss (1mg) and benign prostatic hyperplasia (5mg). It reduces dihydrotestosterone production, slowing or partially reversing androgen-driven hair loss in men.
- Chemical formula
- C23H36N2O2
- CAS number
- 98319-26-7
- ATC code
- G04CB01
- Molecular weight
- 372.5 g/mol
- Drug class
- 5-alpha-reductase inhibitor (type II)
- Also known as
- MK-906, Proscar (5mg), Propecia (1mg)
What is it?
Finasteride is a selective inhibitor of 5-alpha-reductase type II, an enzyme that converts testosterone to dihydrotestosterone (DHT). It was approved in 1992 at 5mg per tablet for benign prostatic hyperplasia (BPH) under the brand name Proscar, and in 1997 at 1mg for male pattern hair loss under the brand name Propecia. The molecule was developed by Merck and authorised generics are widely available since the early 2010s. DHT plays a central role in both prostate enlargement and androgenetic alopecia in men.
Mechanism of action
Finasteride binds with high affinity to 5-alpha-reductase type II, blocking the conversion of testosterone into DHT. Serum DHT concentration falls by approximately 65-70% with the 1mg daily dose used for hair loss and by approximately 90% with the 5mg dose used for BPH. Reduced DHT exposure decreases androgen-driven miniaturisation of scalp hair follicles in men with pattern hair loss, and reduces prostate volume in BPH. The molecule does not significantly affect 5-alpha-reductase type I, which is the dominant isoenzyme in the skin.
Pharmacokinetics
After oral administration finasteride is rapidly absorbed, with peak plasma concentrations within approximately two hours. Absorption is not affected by food. Bioavailability is approximately 65%. The molecule is extensively metabolised in the liver via CYP3A4 to inactive metabolites and eliminated through both urine (39%) and faeces (57%). The terminal half-life is approximately 5-6 hours in young adults and longer in elderly patients, supporting once-daily dosing.
Indications
Finasteride is approved at 5mg per day for the treatment of benign prostatic hyperplasia in adult men, where it reduces prostate volume, improves urinary flow and reduces the risk of acute urinary retention and need for surgery. At 1mg per day it is approved for the treatment of male pattern hair loss in men aged 18 to 41 years. The molecule is not approved for use in women or paediatric patients, although off-label use exists for some endocrine indications under specialist supervision.
Safety profile
Common adverse effects include sexual side effects (decreased libido, erectile dysfunction, ejaculation disorders) reported in approximately 1-3% of men in clinical trials. Less commonly, breast tenderness and gynaecomastia may occur. A subset of patients report persistent sexual symptoms after discontinuation, although causation is debated. Finasteride at 5mg has been associated with a small reduction in PSA values, which should be considered when interpreting prostate cancer screening tests. The molecule is contraindicated in pregnancy due to risk of male fetal genital malformation.
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Frequently asked questions
Does finasteride regrow hair? ▾
Finasteride 1mg daily produces visible hair regrowth in approximately 30-65% of men with mild-to-moderate pattern hair loss, with another 30-50% experiencing stabilisation rather than regrowth. According to clinical guidelines, response is best in the crown area; the frontal hairline responds less reliably. Effects begin to appear after 3-6 months of consistent daily use, with full effect typically evaluated at 12 months. Treatment must be continuous to maintain results.
Can women take finasteride? ▾
Finasteride 1mg is not approved for hair loss in women, partly because clinical trials in postmenopausal women did not show efficacy. Some specialists prescribe it off-label for selected female patients with hyperandrogenism-related alopecia, but the evidence base is limited. Finasteride is strictly contraindicated in pregnancy and women of childbearing potential should avoid handling broken or crushed tablets due to risk of male fetal genital malformation.
How is the 1mg hair loss dose different from the 5mg BPH dose? ▾
The 1mg dose used for male pattern hair loss reduces serum DHT by approximately 65-70% — sufficient to slow androgen-driven follicle miniaturisation. The 5mg dose used for BPH reduces serum DHT by approximately 90%, producing more marked prostate volume reduction. Both doses use the same molecule and similar safety profile, but adverse effects (particularly sexual side effects) are reported more frequently at higher doses according to comparative studies.
What is post-finasteride syndrome? ▾
Post-finasteride syndrome refers to a constellation of persistent symptoms — typically sexual, sometimes also cognitive or mood-related — reported by some men after discontinuing finasteride. The syndrome is recognised by patient communities and some regulators (including the FDA), but its incidence, mechanism and direct causation by finasteride remain debated in the medical literature. According to current guidelines, patients should be informed of the possibility before starting treatment.
Does finasteride affect PSA tests for prostate cancer screening? ▾
Yes. Finasteride at 5mg can reduce serum PSA values by approximately 50% after 6-12 months of treatment. To allow accurate prostate cancer screening, measured PSA values should be approximately doubled when interpreting results in men taking finasteride. According to clinical guidelines, use of finasteride should be disclosed to the clinician before any PSA-based screening or interpretation.
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