Mechanism
Pattern hair loss is driven by DHT (dihydrotestosterone) shrinking susceptible follicles over years.
Pattern hair loss is driven by DHT (dihydrotestosterone) shrinking susceptible follicles over years. Finasteride and dutasteride work by blocking the enzyme that makes it.
Why DHT matters for hair loss, in plain language.
Testosterone is converted into a more potent androgen called dihydrotestosterone (DHT) by an enzyme called 5-alpha-reductase. In follicles that are genetically susceptible (the ones at the temples, crown, and central scalp in men, and the central scalp in women), DHT progressively shrinks the follicle over many growth cycles. Each new hair from that follicle is a little thinner and shorter than the last. Eventually it stops producing visible hair at all.
This is why pattern hair loss is gradual, why it follows a predictable distribution, and why blocking DHT halts and partly reverses it.
Finasteride blocks one of the two 5-alpha-reductase enzymes (type 2). It lowers scalp DHT by around 60 to 70%.
Dutasteride blocks both type 1 and type 2. It lowers scalp DHT by around 90%. That is why some men who plateau on finasteride respond when switched to dutasteride.
Full comparison: regrowthindex.com/articles/finasteride-vs-dutasteride