Propecia is a medication (Finasteride) used to encourage hair growth and retention of existing hair. It works by blocking the enzyme that converts testosterone to dihydrotestosterone (DHT). It may take 6 months or longer to see results.
Wikipedia reports that "In a 5-year study of men with mild to moderate hair loss, 2 out of 3 of the men who took Propecia (finasteride 1 mg) daily regrew some hair, as measured by hair counts. In contrast, all of the men in the study who were not taking finasteride lost hair. "
Wikiepdia also reports side effects may include impotence, abnormal ejaculation, gynecomastia, erectile dysfunction, ejaculation disorder, testicular pain and depression. It is not recommended for women, and women, especially pregnant women should not handle the tablets since it can be absorbed through the skin. Wikipedia reports that many sports organizations have banned finasteride because it can mask use of steroids.