Some laser hair removal treatments are touted to be permanent, but it is truly just a misnomer. After laser hair removal treatment, it is common for hair to regrow.
Why do hairs continue to grow after the the treatment?
This is mainly due to the hair’s growth cycle. Hairs in the anagen, or active phase will be affected by the laser. The hairs in the catagen, or dormant stage will be unaffected during the treatment and grows when it goes to the anlagen phase. Undergoing laser also only forces the hair follicles to go dormant; it does not destroy the hair follicle.
The laser used during the treatment focused a beam of low-energy light on individual hair follicles underneath the skin. This energy absorbs melanin (responsible for the pigmentation in hair) and heats up the follicle and destroy its structure. The growth cycle of each hair is different, which is why some hair will not be damaged by the laser and continue to grow.
At least six cycles of treatment are required to expect a significant reduction in hair, with each session conducted after every four to six weeks. Even then, some hair may regrow and touch-up treatments every six to 12 months. The rate at which the hair regrows depends on the individual, with various conditions affecting the rate of regrowth. This includes: skin tone, hair colour, and where the hair is located. A 10 to 20 percent reduction in hair growth can be expected after each treatment, according to the American Academy of Dermatology. The hairs that regrow usually returns much finer due to most of the melanin removed during the treatment, making them less visible.
Source(s):
http://www.livestrong.com/article/70628-hair-grow-back-after-laser/
http://health.howstuffworks.com/wellness/spa-health/laser-hair-removal-last.ht