Finasteride is a hair loss treatment for men that helps to counter male pattern baldness by reducing the action of DHT (dihydrotestosterone) on hair follicles. DHT is a naturally occurring molecule that interacts with hair follicles in a way that causes the growth phase of the hair cycle to gradually decrease. This shrinks the follicle resulting in thinner, shorter hairs until the growth phase is so short that the hairs don’t grow from the scalp at all.
Finasteride stops the production of DHT. It inhibits 5-alpha-reductase, the enzyme which converts testosterone into DHT, meaning that less DHT is present in the scalp (around 60% less). This helps to prevent further hair loss, and in many cases, promotes hair regrowth.