Because the laser targets the melanin, or the chromophore of the hair, only hair with melanin can absorb the energy and destroy the hair follicle with the conducted heat. Hair only has melanin during the anagen phase of growth—when the hair is actively growing. Not all hairs are in the same growth phase at the same time, so you’ll need to return for additional treatments to ensure that all of the hairs are treated while in the anagen phase.