David L. J. Wardle, MD

Lasers used for hair removal rely on a beam of high intesity light (Light Amplification by Stimulated Emission of Radiation) focused to a specific wavelength (pigment) and target.  Important to note that hair grows in cycles or phases (anagen, telogen and catagen) or "growing", dorment, or transitional. Hair in different parts of the body grow at different rates, and also go through growth phases/dormency at different rates.

Lasers are only effective if the hair is in the active growth phase--it doesnt kill the hair in dorment or transitional phases. Therefore, each laser treatment kills only those hairs in active growth phase (only a percentage of all hairs present in a particular area) and therefore requires multiple treatments over time to get an appreciable result.

Shaving is recommended between treatment because the process of shaving does not throw off the growth cycle process, as does plucking and electrolysis. As such, more hairs will be in active growth phase when you get your treatments. If you pluck or undergo electrolysis, hairs in active growth phase are disrupted in their cycles and my throw off the number of hairs that will be in active growth when it is time for your next treatment with the laser. This results in less effective treatment and more required treatments to get to the desired reduction in hair growth.