Reportedly, 99% of the lice in the United States are resistant to pyrethrins and permethrins, chemicals used in many lice-fighting brands, including Rid and Nix. (2014 study Journal of Medical Entomology).
Further, Lindane, used in Kwell shampoo for lice, is a highly toxic pesticide related to the banned DDT. Cancer-causing, it can also damage the liver, immune system and nervous system, triggering seizures in sensitive people. In the past, it killed lice but it did not kill the lice eggs (nits). Pediatrician Louis Pottkotter cautions that Kwell shampoo has been linked to childhood brain cancer. Lindane becomes lodged in the fat tissue and vital organs (like the brain), where it can damage a person’s health for decades because it breaks down slowly. Alan Green, MD, notes that in 2003, the Food and Drug Administration cautioned that lindane should not be used by anyone weighing less than 100 pounds.