Male pattern hair loss (MPHL) is a common, androgen-induced, progressive disorder in genetically predisposed subjects. Human skin, sebaceous glands and hair follicles contain the 5-alpha reductase (5-AR) needed to convert testosterone to dihydrotestosterone (DHT), the primary androgen responsible for MPHL. Preclinical and clinical data from Phase Ⅰ, Ⅱ, and Ⅲ trials conducted in MPHL patients indicated that dutasteride is a selective inhibitor of both Type Ⅰ and Type Ⅱ 5-AR and is well tolerated at doses of 0.5 mg daily for six months. 0.5 mg of dutasteride was shown to significantly improve hair counts after 12 and 24 weeks compared with baseline. The adverse event has been known to be characterized as either mild or moderate in severity and often resolved with continuation of the medication. Recently conducted open label, multi-center clinical study of dutasteride for over 2 years also showed that it is effective and well tolerated in 712 MPHL patients.