Finasteride blocks the conversion of testosterone to dihydrotestosterone (DHT). Male pattern baldness (MPB) or androgenic alopecia, is caused not by testosterone, but by DHT. It is DHT that causes hair follicles to shrink and lose the ability to create new hairs. This is called “miniaturization.” Testosterone is changed into DHT by an enzyme called 5 alpha-reductase. That enzyme comes in different forms and one form of the enzyme converts testosterone into DHT in the prostate and on the scalp. Finasteride blocks 5 alpha-reductase and therefore blocks the conversion of testosterone to DHT. This is effective at stopping or slowing hair loss in most men.