Are head lice resistant to the medications (insecticides) in lice shampoos and creme rinses?
Lice shampoos and creme rinses contain either pyrethrin or permethrin as the active ingredient. Insecticide resistance (see What is Insecticide Resistance?) to permethrin and pyrethrin has been detected in some populations of head lice but the extent of the resistance is not well known. What we do know is that resistance to any pesticide tends to be patchy, high in some areas and low or non-existant in other areas. Patterns of resistance tend to follow patterns of overuse of the pesticide. And, the frequency of resistance can decline when the use of the pesticide is reduced. Therefore, even with some reports of insecticide resistance in head lice, it is likely that in most treatment failures probably have another cause. I believe that resistance is spotty and of relatively low intensity in most head lice populations. Under these conditions pyrethrin and permethrin-based treatments are still the best approach when combined with thorough nit removal by combing.
