The majority of hair loss falls in the category of androgenic alopecia – also called male pattern baldness. This is an inherited condition that makes a man's hair follicles stop producing aesthetically acceptable hair in reaction to hormone dihydrotestosterone (DHT).
Finasteride is an FDA-approved hair loss drug that works by blocking an enzyme that would otherwise change testosterone to DHT. It has to be taken every day and may take at least three months to take effect. It can have some side effects, including sexual problems, and it's worth noting that if a man stops taking finasteride, the hair he grew back will probably be lost.