Proscar is prescribed to treat enlargement of the prostate gland and male pattern baldness. It is a type of 5-reductase inhibitor and an enzyme that prevents the development of testosterone to dihydrotestosterone. The latter, also called DHT, is involved with benign prostatic hyperplasia. Proscar decreases the amount of DHT, a natural body hormone which causes prostate growth. Proscar is not for the prevention of prostate cancer but increases slightly the risk of developing a serious prostate cancer form.