Finasteride is a selective inhibitor of type II 5-alpha reductase, the enzyme that converts testosterone to dihydrotestosterone (DHT) in the prostate, liver, and skin. At 5 mg (Proscar), it treats benign prostatic hyperplasia (BPH), reducing prostate size and improving urinary symptoms. At 1 mg (Propecia), it treats androgenetic alopecia (male pattern baldness).
Reduces risk of acute urinary retention and need for surgical intervention
Promotes hair regrowth and slows hair loss at 1 mg dose10
What to watch for
Decreased libido
Erectile dysfunction
Decreased ejaculate volume
Women who are or may become pregnant (teratogenic, causes hypospadias in male fetuses)1
Handling of crushed or broken tablets by women of childbearing potential
The bottom line
Evidence rating strong. Most-documented uses: reduces prostate volume by 20-30%, improves urinary symptoms and flow rate in bph, reduces risk of acute urinary retention and need for surgical intervention. 10 sources indexed (1998–2021), with 2 interaction records on file.
The science
How it works, mechanistically.
Core mechanism
Competitively and irreversibly inhibits type II 5-alpha reductase, reducing conversion of testosterone to DHT by approximately 70%. Since DHT is the primary androgen driving prostate growth and hair follicle miniaturization, lowering DHT levels leads to prostate shrinkage (20-30% over 6 months) and promotes hair regrowth in androgenetic alopecia.
Class
5-Alpha Reductase Inhibitor / Urologic
Dosing
Dosing & protocol.
Common range
5 mg once daily for BPH; 1 mg once daily for androgenetic alopecia (as prescribed by your physician)
Recommended form
Oral tablet
Bioavailability ~65%; food does not affect absorption. Effects on prostate volume take 6-12 months to fully manifest; hair regrowth effects may take 3-6 months.
Safety
Full safety detail.
Side effects
Decreased libido
Erectile dysfunction
Decreased ejaculate volume
Gynecomastia and breast tenderness
Post-finasteride syndrome (controversial; persistent sexual side effects after discontinuation)
Depression and mood changes (uncommon)
Contraindications
Women who are or may become pregnant (teratogenic, causes hypospadias in male fetuses)1
Handling of crushed or broken tablets by women of childbearing potential
Known hypersensitivity to finasteride or any component1,2
St. John's Wort can lower finasteride exposure. In healthy men, 14 days of St. John's Wort pretreatment significantly reduced finasteride Cmax, AUC, and half-life, and a PubMed-indexed clinical correspondence describes a BPH patient with a PSA rise after starting 900 mg/day St. John's Wort while controlled on finasteride. The concern is reduced finasteride effect, with possible worsening BPH control or less reliable PSA suppression.
Recommendation: Avoid St. John's Wort while taking finasteride unless the prescriber specifically approves it. If St. John's Wort has already been started or stopped, tell the clinician interpreting PSA or urinary symptom changes because finasteride exposure and PSA suppression may change over several weeks.
Saw palmetto (Serenoa repens) has antiandrogenic and 5-alpha-reductase-related activity, overlapping with finasteride therapy for BPH or androgenetic alopecia. A prostate tissue study compared saw palmetto and finasteride effects on prostatic androgens, and a PubMed-indexed case series described persistent sexual and psychiatric symptoms after Serenoa exposure, including combined Serenoa therapies such as finasteride. The concern is additive sexual, mood, or PSA/DHT interpretation effects rather than an acute toxicity syndrome.
Recommendation: Avoid adding saw palmetto to finasteride unless the prescriber knows you are using both. Tell the clinician interpreting PSA, urinary symptoms, hair-loss response, libido, erectile function, mood, or persistent sexual adverse effects if saw palmetto is started or stopped.
Soares Júnior JM, Guimarães DZ, Simões RDS et al.. Systematic review of finasteride effect in women with hirsutism. Revista da Associacao Medica Brasileira (1992). 2021
Jiménez Cruz JF, Quecedo Gutiérrez L, Del Llano Señarís J. [Finasteride: 10 years of clinical use. Systematic review of the literature]. Actas urologicas espanolas. 2003
Edwards JE, Moore RA. Finasteride in the treatment of clinical benign prostatic hyperplasia: a systematic review of randomised trials. BMC urology. 2002
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