Why doesn't lice shampoo work? - briefly
Many over‑the‑counter lice treatments contain insufficient concentrations of insecticide, allowing surviving nits to repopulate the scalp. Incorrect application—such as inadequate contact time or failure to repeat treatment after hatching—further reduces efficacy.
Why doesn't lice shampoo work? - in detail
Lice treatment shampoos often fail because the active ingredient does not reach the parasite in sufficient concentration. Many products rely on neurotoxic compounds such as permethrin or pyrethrins, which require direct contact with the louse’s nervous system. If the shampoo is rinsed too quickly, diluted by hair products, or applied to a wet scalp, the dose falls below the lethal threshold.
Resistance is another major factor. Repeated exposure to the same chemical class selects for mutations in the voltage‑gated sodium channel of lice, rendering them less susceptible. Surveys worldwide report resistance rates above 60 % for permethrin‑based formulations, making them ineffective in many infestations.
Application errors reduce efficacy further. Common mistakes include:
- Applying less than the recommended amount.
- Missing the scalp and hair shaft where lice reside.
- Skipping the required waiting period before rinsing.
- Failing to repeat treatment after 7–10 days, when newly hatched nymphs emerge.
Formulation limitations also play a role. Some shampoos contain surfactants that bind to hair proteins, decreasing the availability of the insecticide. Others lack sufficient residual activity, so any lice that survive the initial exposure can repopulate the scalp within hours.
Finally, reinfestation from untreated contacts or contaminated objects can give the appearance of treatment failure, even when the shampoo works against the original population.
In practice, successful control combines a properly applied, resistance‑aware shampoo with a second‑generation product (e.g., ivermectin or benzyl alcohol) and thorough removal of nits using a fine‑toothed comb. Re‑treatment after one week eliminates survivors that develop from eggs missed during the first application.