Finasteride is an oral medication used to prevent hair loss. It is also known as Propecia. The mechanism of action is that it reduces 5α-Reductase which is an enzyme that converts testosterone into DHT (dihydrotestosterone). DHT is the main hormone the causes hair loss in men. It causes damage to the hair follicles over time, this is called hair follicle miniaturization. The damage causes hair to grow in thinner and shorter until the hair follicle stops growing hair altogether and it disappears. There is no way to bring a hair follicle back once it is gone.