Though not as much a safety issue, in some patients laser hair-removal treatment has a paradoxical effect of stimulating hair growth [129, 153-156]. It is considered a rare side effect of laser treatment. The most susceptible patients are the ones that have darker Fitzpatrick skin type (III-V), are of Mediterranean descent, and receiving treatments on their face. New terminal hair growth has been observed in areas untreated but close proximity to the treated ones. In most cases, this paradoxical hair growth occurs at a site that has high vellus growth and relatively free of terminal hair. In a 7-year retrospective analysis of 750 patients of Mediterranean ancestry, terminal hair induction was seen in 4% of the subjects [157]. In 28 out of 30 cases, the terminal hair growth was on the face and the subjects were skin type III or IV. Moreover, there was no relation between stimulated hair growth and the laser system, or the energy fluence used. In another study of 543 patients treated over 5 years in a dermatology clinic in Spain, the incidence rate for hair-growth stimulation on the face was somewhat higher at 10.5% [158]. Unlike these studies in Mediterranean populations, a retrospective study from Canada involving a similarly large patient group of 489, the terminal hair stimulation was seen in only 3 (0.6%) subjects who had Type IV skin [159].
Localized hypertrichosis has been observed under conditions that result in dermal trauma, stress, or inflammation. Increased localized hair growth has been reported after fracture cast application [ 160], bug bites [161], site surrounding a burn [162], or scar [ 163], and site of chronic inflammation [164]. Since the laser-induced paradoxical hair growth has mostly been associated with the Mediterranean population, it is not clear what the underlying cause(s) are for this effect.