Why do treatments not work for lice? - briefly
Resistance to common pediculicides and incomplete coverage let lice survive, while rapid reinfestation from untreated contacts undermines efficacy. Incorrect dosage, short treatment duration, and egg survival further reduce success.
Why do treatments not work for lice? - in detail
Treatments frequently fail against head‑lice infestations for several scientifically documented reasons.
First, many products target the nervous system of adult insects, yet the lice life cycle includes eggs (nits) that are protected by a cement‑like shell. Conventional sprays and shampoos cannot penetrate this shell, leaving a reservoir of viable embryos that hatch after the chemical effect has worn off.
Second, extensive use of common insecticides, such as permethrin and pyrethrins, has created widespread resistance. Genetic mutations alter the target receptors, reducing binding affinity and rendering the chemicals ineffective at the concentrations recommended on labels. Laboratory assays routinely demonstrate that resistant populations survive doses several times higher than those that kill susceptible strains.
Third, user error undermines efficacy. Incomplete application, insufficient contact time, or failure to treat the entire scalp and surrounding hair result in surviving lice. Instructions often require a precise duration of exposure (e.g., 10 minutes) and thorough combing of wet hair with a fine‑toothed nit‑comb; skipping either step leaves live insects.
Fourth, reinfestation is common in environments where close contact occurs (schools, daycare centers). Even after successful eradication, untreated contacts can reintroduce lice within days, creating the appearance of treatment failure.
Fifth, product stability can be compromised by storage conditions. Heat, light, and moisture degrade active ingredients, decreasing potency. Labels advise keeping products in a cool, dry place; failure to follow these guidelines leads to sub‑therapeutic dosing.
Finally, some over‑the‑counter formulations contain only low concentrations of active agents, designed to minimize irritation. These dilute preparations may reduce symptoms but lack sufficient lethality to eradicate an established infestation.
Addressing these factors—using resistance‑aware agents, following precise application protocols, treating all contacts, and ensuring product integrity—significantly improves outcomes where standard treatments have previously proved ineffective.