Finasteride is a hair treatment for men that helps to counter male pattern hair loss by reducing the activity of DHT (dihydrotestosterone) on the hair follicles. DHT is a molecule that activates a receptor in the hair follicle which causes the growth phase of the hair cycle to gradually decrease. This shrinks the follicle resulting in finer, shorter hairs until the growth phase is so short that hairs do not grow from the scalp at all.
Finasteride stops the production of DHT. It works as an inhibitor of 5-alpha-reductase, the enzyme which converts testosterone into DHT. This means that less DHT is present in the scalp (around 60% less), preventing further hair loss and in many cases, promoting hair regrowth.