Is paradoxical hair growth caused by low-level radiant exposure by home-use laser and intense pulsed light devices?

Introduction and objectives: This qualitative review of paradoxical hair growth, following professional treatments reviews, clarifies whether low fluence is the most probable cause of unwanted hair regrowth after at home light-based treatments. Materials and methods: The proposed causes of unexpected hair regrowth are examined, and our scientific understanding of absorption and scattering of light in turbid tissue is reviewed. Published reports of paradoxical hair growth are assessed. Results: Early laser hair removal studies failed to record the occurrence of hair induction despite the significant numbers of subjects treated. Neither published paradoxical hair growth studies following home-based laser or intense pulsed light (IPL) hair removal treatments, nor randomised or controlled studies documenting paradoxical hair growth following professional treatments could be found. Several authors directly proposed inflammatory response to be the primary cause of hair growth induction. Conclusions: It is unlikely that hair regrowth several centimetres or more away from the irradiated tissue can be attributed to the laser or IPL used. In many cases of paradoxical hair growth, other causes may be responsible for the unexpected hair growth. The primary cause of instances of true' paradoxical hair growth is probably limited to darker phototypes with one or more other characteristics including polycystic ovarian syndrome or other androgen hormonal irregularities following high energy treatments with the corresponding inflammatory sequelae.