Finasteride
/ Selective 5-alpha-reductase type 2 inhibitor (small molecule)ALIAS · Propecia (trade) · Proscar (trade) · Finasteride
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Tier 1. FDA-approved 1992 as Proscar (5 mg) for benign prostatic hyperplasia and 1997 as Propecia (1 mg) for androgenetic alopecia in men. The Prostate Cancer Prevention Trial (Thompson 2003) is one of the largest chemoprevention trials ever conducted. The Phase 3 Propecia trials (Kaufman 1998) supported the AGA approval.
Finasteride is a synthetic 4-azasteroid that selectively inhibits type 2 5-alpha-reductase, the predominant isoenzyme in prostate, scalp hair follicles, and seminal vesicles. Inhibition reduces conversion of testosterone to dihydrotestosterone, lowering circulating DHT by approximately 70 percent at the 1 mg dose and similarly at the 5 mg dose (the dose-response is shallow above 1 mg for systemic DHT). In androgenetic alopecia, reduced DHT exposure in genetically susceptible scalp follicles slows or partially reverses miniaturisation; in BPH, reduced intra-prostatic DHT shrinks epithelial volume.
Tier 1. The Phase 3 Propecia program (Kaufman 1998) reported increased hair counts and improved global photographic assessment with finasteride 1 mg daily over 12 months in men with androgenetic alopecia. The Prostate Cancer Prevention Trial (Thompson 2003) randomised approximately 18,800 men to finasteride 5 mg or placebo for seven years and reported reduced overall prostate cancer incidence with a complicated finding of increased high-grade tumour incidence in the finasteride arm that was later attributed largely to detection bias. Long-term BPH efficacy is established by the MTOPS study and PLESS.
Decreased libido, erectile dysfunction, ejaculatory disorders, and gynecomastia are recognised adverse events at both 1 mg and 5 mg doses. The post-finasteride syndrome literature describes persistent sexual, neurological, and mood symptoms after discontinuation in a subset of users; the syndrome is recognised in regulatory labelling but its causal mechanism remains contested. Like dutasteride, finasteride distorts PSA interpretation and is contraindicated in pregnancy due to teratogenicity in male foetuses.
Regulatory status
- FDA status:
- FDA-approved
- Compounding:
- Not eligible for compounding (approved, not in shortage)
The post-finasteride syndrome is a load-bearing controversy: regulators have updated labelling to acknowledge persistent sexual adverse events, but the prevalence, mechanism, and reversibility remain debated. The PCPT high-grade-tumour signal generated years of debate about whether the apparent excess reflected detection artefact or a real biological effect, with subsequent re-analyses and the related REDUCE trial of dutasteride generally favouring the detection-bias interpretation. Topical finasteride compounded formulations are increasingly used off-label for AGA but have a smaller controlled-trial base than oral dosing.