There have been studies showing how dangerous pesticides are in treating lice, but they are still FDA approved. However, the factor that may limit or eliminate the use of NIX, RID or other permethrin based shampoos for the control of head lice, may not be warnings of toxicity, but rather the growing resistance of the lice to its effectiveness. According to a study in the journal Archives of Dermatology, head lice are becoming resistant to NIX, in particular in south Florida, where the tests were conducted. And researchers at the Harvard School of Public Health, in 2002 reported that head lice collected from children in Massachusetts and Idaho are no longer killed by permethrin.
"Head lice sampled from children who were chronically infected and treated multiple times with pyrethroid shampoos tend to be resistant to permethrin." "Tests have shown that lice from American children are solidly resistant to permethrin."
