Permethrin

Permethrin is sold over-the-counter under the brand name NIX. It’s prominently displayed in every drugstore. It’s usually the first thing parents see when they rush to the pharmacy after finding out their kid has head lice … so they snap it up and rush home to treat with it.

NIX has been used so much, in fact, that more and more head lice have now developed resistance to this chemical. They survive treatment.

According to the CDC (Centers For Disease Control And Prevention), "Treatment failures can be common, depending whether lice are resistant to permethrin in the patient’s geographic location.”

Even for head lice that happen to be part of the dwindling population that is not yet resistant to this chemical, there is no good reason to use it on a child’s head. There are some very good reasons not to.

Multiple treatments needed
(even if lice aren’t resistant)

Even before head lice developed resistance to it, this pesticide had limited effectiveness. According to the CDC, "Permethrin kills live lice, but not unhatched eggs.” This means that even if a treatment worked well, the child would still have nits all over her head containing eggs preparing to hatch days later.

So in order to get rid of a head lice infestation with a Nix treatment, even if the hatched lice all died, you would still need to do a lot of careful nit-combing to get rid of each and every nit before it hatched.

When treating head lice, some chemical residue remains on the head even after rinsing out the Nix. If the lice are not resistant, this residue may continue to kill newly hatching lice for a few days after the initial treatment.

Lice hatching after that window remain unaffected. As the CDC explains, “A second treatment often is necessary in 9-10 days to kill any newly hatched lice before they can produce new eggs.” The Nix instructions call for intensive nit-combing after each treatment.

Neurotoxins are dangerous

Permethrin is a synthetic chemical that kills insects by damaging their nervous systems. It has complex effects on the nervous system in mammals. It‘s is poisonous to dogs, and it can kill cats.

The EPA has also classified it as a likely human carcinogen.

According to the Journal of Pesticide Reform’s Insecticide Fact Sheet, permethrin “is a neurotoxin. Symptoms include tremors, incoordination, elevated body temperature, increased aggressive behavior, and disruption of learning. Laboratory tests suggest that permethrin is more acutely toxic to children than to adults.”

The official NIX website itself warns that using the product may cause scalp irritation, eye irritation, breathing difficulty or asthmatic episodes. It cautions users not to leave the product on for more than ten minutes, and not to re-apply within seven days.

The CDC states that this chemical is not approved for use on children under two years old. It’s hard to see why anyone would want to use it in older children or adults, given the probability of treatment failure, limited effectiveness, and potential toxicity.

There are safe and effective treatments for head lice available. Permethrin is not one of them. (A good lice comb is what you really need to get rid of head lice, and an enzyme shampoo can make it a lot easier.)

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