Hair growth in each root occurs in a cycle independent of the other roots. The growth phase lasts an average of 4 to 5 years, after which the follicle enters the resting phase, which lasts about 2 to 4 months. At the end of the resting phase, the hair falls out naturally and is replaced by a new hair. Consequently, some hair loss is a normal part of the hair growth cycle. In fact, on a typical day, about 50 to 150 scalp hairs are lost.
Androgenetic alopecia occurs much more frequently in men than in women.
It affects roughly 40 million men in the United States. While some types of hair loss are easily reversible, male pattern baldness is more permanent. Hair loss may continue until the 2 sections become joined, leaving a horseshoe-shaped area of hair on the sides and back of the head. Balding may begin at any age after puberty, even in the middle teens, and can range from partial loss to complete baldness. Male pattern baldness progresses slowly and is not associated with redness, itching, or pain.