5-alpha reductase inhibitors
Target: 5-alpha reductase enzyme (Types I & II)
- ~0%
- DHT reduction (finasteride)
- ~0%
- DHT reduction (dutasteride)
- 0%
- stable hair or regrowth (finasteride 5y trial)
- ~0 wk
- dutasteride steady-state half-life
What are 5-alpha reductase inhibitors?
5-alpha reductase inhibitors slow the conversion of testosterone to dihydrotestosterone (DHT), the androgen primarily responsible for male-pattern hair loss and benign prostatic hyperplasia. Two FDA-approved drugs: finasteride (Type II) and dutasteride (Types I & II).
How do 5-alpha reductase inhibitors work?
DHT binds androgen receptors in scalp hair follicles, shortening the growth phase and shrinking follicles until they no longer produce visible hair. Suppressing DHT lets follicles return to normal cycling. Effects are typically visible at 3-6 months and require continuous treatment.
History of 5-alpha reductase inhibitors
Finasteride 5 mg (Proscar) was approved 1992 for BPH; 1 mg (Propecia) for hair loss in 1997. Dutasteride (Avodart, 2001) inhibits both Type I and Type II, offering ~3x greater DHT suppression but only off-label use for hair loss in the US (approved in Korea/Japan for AGA).
Drugs in this class
Finasteride (Propecia)
finasteride · Multiple (Merck originated as Propecia) · FDA 1997
Oral DHT blocker. Standard of care for male-pattern hair loss. ~80% slow shedding, ~60% regrow.
Avodart
dutasteride · GlaxoSmithKline (Avodart) · FDA 2001
Stronger DHT blocker than finasteride. Off-label for hair loss. Higher effect, smaller evidence base.
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Related topics
Sources
Primary sources cited above. FDA labeling, peer-reviewed trials, and specialty-society guidelines only.
- Finasteride 5-year Extension Study in Men with Androgenetic Alopecia · Journal of the American Academy of Dermatology, 2002 · PMID 11907492
- Comparative Effectiveness of Finasteride vs Dutasteride for Androgenetic Alopecia: Systematic Review and Meta-analysis · JAMA Dermatology, 2022 · PMID 35107826
People also ask
What's the difference between finasteride and dutasteride?
Finasteride blocks Type II 5-alpha reductase only and reduces scalp DHT by ~70%. Dutasteride blocks both Type I and Type II and reduces DHT by ~90%. Dutasteride is FDA-approved for benign prostatic hyperplasia, not hair loss — it's used off-label for AGA. Half-life differs dramatically: finasteride ~5-6 hours, dutasteride ~5 weeks at steady state.
Does post-finasteride syndrome exist?
A subset of patients report persistent sexual, cognitive, or mood symptoms after discontinuing finasteride. The condition is recognized by some clinicians and contested by others; the literature is mixed on causality, mechanism, and prevalence. The FDA has updated the label to acknowledge reports of persistent erectile dysfunction. Patients with concerns should discuss with a prescriber before starting.
Are sexual side effects of finasteride permanent?
In most patients, side effects (low libido, ED, ejaculatory disorders) resolve within weeks of discontinuation. A minority report persistent symptoms — see the post-finasteride question above. Prospective data shows ~1-4% incidence of any sexual side effect during treatment, with most resolving on stopping.
Can women take 5-alpha reductase inhibitors?
Finasteride and dutasteride are not approved for women and are contraindicated in pregnancy due to teratogenicity (risk to a male fetus). Some dermatologists prescribe finasteride off-label for female-pattern hair loss in post-menopausal women with appropriate counseling.