The hormone is DHT. Genetically prone follicles on top of the scalp shrink each cycle under dihydrotestosterone, the hair going fine, then wispy, then invisible. Finasteride blocks the enzyme that makes DHT and drops scalp levels by about two-thirds ¹; dutasteride blocks both forms and cuts blood DHT by around ninety percent ². Follicles fully scarred over are past saving, so the clock matters.

It works, and the answer has held for thirty years. At two years, 48% of treated men had measurably more crown hair and 83% held steady or improved, against 7% and 28% on placebo ³; at five years the effect held ⁴. Dutasteride does the same harder at similar side-effect rates ⁵, but it's on-label for hair loss only in South Korea and Japan, so a willing prescriber is harder to find ⁶.

One pill a day, and don't quit at the shed. The first month or two brings a wave of shedding that looks like the drug making things worse. It's the opposite: dormant follicles waking up in sync and pushing old hairs out first. The rebuild peaks at one to two years ³. It suppresses rather than cures: stop and the decline resumes within a year ⁴.

The years, for the half who respond. Months one to three look alarming. By six to twelve the part holds and your barber asks if you grew it out. By year two nobody mentions it ⁴. The pill won't give back the hair you had at twenty; it changes which photo you take at forty.

Minoxidil is the standard add-on. Topical 5% works on blood flow rather than DHT, and a pill plus minoxidil beats either alone ⁷. A topical finasteride spray gets similar hair counts with far less blood DHT ⁸. A transplant puts hair where there's none but doesn't stop native thinning, so surgeons keep patients on the pill. Saw palmetto, the "natural" version, hits the same enzyme too weakly to count on.