Paradoxical effects of hair removal systems: a review

Although a variety of lasers have proven to be clinically effective for long-term hair removal, the use of these lasers has also been associated with undesirable side effects, such as hyper- and hypopigmentation, crusting, erythema, and edema. One notable side effect that seems to be underreported in the literature is the growth of fine dark hair in untreated areas close to the treated ones. This contradictory hypertrichosis is known as the paradoxical effect. In this paper, we review the published reports of the paradoxical effect and offer some possible explanations for this effect. The paradoxical effect has been documented most commonly after the use of induced pulse light and alexandrite lasers. One possible explanation is the activation of dormant hair follicles by suboptimal fluences. Another mechanism may be the synchronization of hair growth cycles by direct light stimulation.