How does lice shampoo work? - briefly
These shampoos contain neurotoxic agents such as permethrin that bind to sodium channels in lice nerves, causing paralysis and death. Surfactants and solvents in the formulation also loosen the cuticle and help detach nits from hair shafts.
How does lice shampoo work? - in detail
Lice shampoo contains a specific insecticidal agent dissolved in a detergent base that enables uniform distribution across the scalp and hair shaft. The active compound, commonly a synthetic pyrethroid such as permethrin or a silicone‑based surfactant like dimethicone, targets the nervous system of the parasite. Pyrethroids bind to voltage‑gated sodium channels on nerve membranes, forcing prolonged opening of the channels, leading to uncontrolled neuronal firing and paralysis. Dimethicone, by contrast, coats the cuticle of the louse, creating a physical barrier that blocks respiration through spiracles, resulting in suffocation.
The detergent component reduces surface tension, allowing the solution to infiltrate the hair cuticle and reach lice hidden near the scalp. Surfactants also detach the adhesive cement that secures nits to hair shafts, facilitating mechanical removal. Once the active ingredient contacts the insect, the following sequence occurs:
- Penetration of the exoskeleton through the wet medium.
- Binding of the neurotoxic molecule to sodium channels (pyrethroids) or formation of a continuous silicone film (dimethicone).
- Disruption of nerve impulse regulation or blockage of gas exchange.
- Rapid onset of paralysis, followed by death within minutes to hours.
- Loss of adhesion of eggs, allowing manual combing to eliminate remaining nits.
Formulation pH is adjusted to approximately 5–6, matching scalp conditions and enhancing the stability of the active ingredient. Some products incorporate chelating agents that bind metal ions in the hair, preventing precipitation of the insecticide and maintaining efficacy throughout the recommended exposure period, typically 10–15 minutes.
Resistance management is addressed by rotating active ingredients or employing combination shampoos that pair a neurotoxic agent with a physical suffocant. This approach reduces the likelihood of lice populations developing mutations in sodium‑channel genes that confer pyrethroid tolerance.
Safety considerations include limited systemic absorption due to the large molecular size of the active compounds and the brief contact time. Toxicity assessments confirm low risk for human skin and mucous membranes when instructions are followed. The shampoo is contraindicated for infants under two months because of immature dermal barriers.
Effective use requires thorough saturation of the hair, adherence to the prescribed contact duration, and subsequent removal of dead insects and loosened nits with a fine‑toothed comb. A repeat application after seven to ten days eliminates any newly hatched lice that escaped the initial treatment, ensuring complete eradication. «The combination of neurotoxic action and physical suffocation provides a dual mechanism that maximizes mortality across all life stages of the parasite».