Inhibits 5α-reductase type II, the enzyme that converts testosterone to DHT. DHT is the primary driver of androgenetic alopecia. Finasteride slows or halts loss in most men; some regrowth possible.
Dose: 1mg oral daily, or 0.5-1mg/mL topical for those concerned about systemic effects.
Caveats: Post-finasteride syndrome is rare but real. Sexual side effects, mood changes that may persist after discontinuation. Consider topical formulation. Not for pregnant women (teratogenic).