John M. Clark, M2M and Department of Veterinary and Animal Sciences, says a new study that finds the majority of head lice in the U.S. are now resistant to more over-the-counter treatments, confirms what scientists have observed in recent years. “What it is telling us is that, right now, these over-the-counter products aren’t nearly as effective as they used to be,” he says. This is because the lice have developed mutations to fight off the treatments.