Finasteride blocks the enzyme 5α-reductase, which normally converts testosterone into dihydrotestosterone (DHT) — the androgen most responsible for shrinking scalp hair follicles in genetically susceptible men. By lowering scalp and serum DHT (typically 60–70% reduction at the 1 mg dose), it slows follicle miniaturization and can partially regrow hair, without directly altering testosterone levels themselves.

‍

• Male pattern hair loss / androgenetic alopecia (1 mg daily)
• Benign prostatic hyperplasia (5 mg daily)
• Off-label: combined with topical or oral minoxidil in dermatology hair-loss protocols

• Works upstream on hormone metabolism rather than supplying or replacing a hormone — distinct mechanism from testosterone or SERM-class agents in this atlas
• Carries FDA-mandated labeling on sexual side effects (reduced libido, erectile dysfunction), which can persist after discontinuation in a subset of users (post-finasteride syndrome, still debated in the literature)
• Contraindicated for use by pregnant women or those who may become pregnant, due to risk of abnormal genital development in a male fetus — even from handling crushed/broken tablets