“Conflicting information about the proper treatment of head lice has given rise to uncertainty among patients and treating personnel. For example, the reported efficacy of permethrin fell from 97% in the 1990s to 30% in 2010.”
Source: Laura Meister, Dr. med. and Falk Ochsendorf, Prof. Dr. med. Dtsch Arztebl Int. 2016 Nov; 113(45): 763–772.
See full article: “Head Lice Epidemiology, Biology, Diagnosis, and Treatment.”