Propecia (also known as finasteride) is used in the treatment of male Propecia and Proscar (used in the treatment of prostate disease) both contain finasteride but in different amounts. Finasteride works by blocking an enzyme that converts testosterone to dihydrotestosterone (DHT). DHT has a number of functions, including beard and hair growth, prostate function and sexual function. Male pattern baldness is thought to be related to the activity of DHT on the hair follicle.

Propecia is generally well tolerated. The side effects of Propecia are directly related to the blocking of DHT. The most common side effects are decreased libido, erectile dysfunction and a decreased volume of semen. DHT is also involved in the development of male sex characteristics in the developing fetus. Women who are capable of becoming pregnant should not handle crushed or broken Propecia tablets. That is because the drug can be absorbed through the skin contact. Studies done in animals suggest there is a risk to a male fetus if the drug is absorbed.