Over the past five years an increase in the number of cases of head lice has been noticed throughout the United States school systems, along with a decrease in the effectiveness of conventional pediculicides. “Products out there just aren’t working like they used to,” says Dr. Teri Meinking, a leading head lice researcher at the University of Miami in Coral Gables, Fla. “I know the lice have developed resistance. Some of these lice will walk around in a dish containing 5% permethrin”.
A 1999 head lice study conducted by Dr. Richard J. Pollack of Harvard showed a roughly 90% survival rate in head lice taken from American children and treated with permethrin, the active ingredient in the leading conventional pediculicides, versus a roughly 1% survival rate in head lice harvested from children outside the US who had never before been treated. Their study also showed that lice resistant to permethrin at low doses are also resistant at high doses—strongly suggesting that strong prescription formulations of permethrin at a high 3-5 percent are unwarranted.