Finasteride — Drug Monograph
Brand names: Proscar, Propecia
Drug class: Hormonal Agent
Mechanism of Action
Finasteride is a competitive, specific inhibitor of type II (and, at higher doses, type III) 5-alpha-reductase, the intracellular enzyme that converts testosterone to the more potent androgen dihydrotestosterone (DHT). By blocking this conversion, finasteride markedly lowers serum and intraprostatic DHT (serum DHT falls roughly 65-70%) while serum testosterone remains largely unchanged or modestly increased. Because prostatic epithelial growth is DHT-dependent, DHT suppression induces glandular atrophy, reduces prostate volume, and lowers PSA. In the chemoprevention context, sustained DHT…
FDA Indications
- Benign prostatic hyperplasia (BPH) — to improve symptoms, reduce risk of acute urinary retention, and reduce the need for BPH-related surgery (Proscar 5 mg)
- BPH in combination with the alpha-blocker doxazosin to reduce risk of symptomatic progression
- Androgenetic alopecia (male pattern hair loss) in men (Propecia 1 mg)
Common Side Effects
- Erectile dysfunction
- Decreased libido
- Ejaculation disorder / decreased ejaculate volume
- Gynecomastia and breast tenderness
- Testicular pain
- Dizziness (more common when combined with alpha-blockers)
Clinical Pearl
Finasteride roughly halves serum PSA within 6-12 months, so multiply the measured PSA by ~2 to interpret it against standard reference ranges in men on therapy; any sustained rise from the on-treatment nadir should prompt prostate cancer workup. In the PCPT, finasteride reduced overall prostate cancer detection but was associated with a higher rate of high-grade tumors — later analyses suggested this partly reflected improved detection in a smaller gland, but the FDA-labeled class warning…
Related Therapies & Mechanisms
- Abiraterone Acetate (Zytiga) — Hormonal Agent · Prostate
- Apalutamide (Erleada) — Hormonal Agent · Prostate
- Bicalutamide (Casodex) — Hormonal Agent · Prostate
- Darolutamide (Nubeqa) — Hormonal Agent · Prostate