How does shampoo against lice work? - briefly
Lice‑combating shampoos contain insecticidal agents such as permethrin, pyrethrins, or silicone‑based dimethicone that penetrate the insect’s cuticle and interfere with its nervous system, leading to rapid paralysis. The formulation also coats and suffocates any remaining parasites, ensuring they cannot breathe or cling to the hair shaft.
How does shampoo against lice work? - in detail
Lice shampoo eliminates infestations by delivering chemicals that target the parasite’s nervous system or physical structure. The product typically contains one or more of the following active agents:
- Pyrethrins or pyrethroids – synthetic analogues that bind to voltage‑gated sodium channels in lice neurons, causing uncontrolled firing, paralysis, and death.
- Dimethicone – a silicone‑based polymer that coats the exoskeleton, obstructing respiration through the spiracles and immobilizing the insect.
- Spinosad – a bacterial metabolite that interferes with nicotinic acetylcholine receptors, disrupting synaptic transmission.
- Benzyl alcohol – a solvent that penetrates the cuticle, leading to dehydration and lethal metabolic stress.
Formulation design ensures adequate contact time and distribution across hair shafts. Surfactants lower surface tension, allowing the active ingredient to spread evenly and penetrate the lice cuticle. Emulsifiers keep oil‑based agents stable in an aqueous base, while preservatives prevent microbial growth in the bottle.
Application follows a precise protocol: the shampoo is applied to wet hair, massaged to the scalp, left on for a specified duration (usually 5–10 minutes), then rinsed thoroughly. The waiting period permits the active compound to reach the central nervous system of the parasite before it is washed away. A second treatment, typically 7–10 days later, targets newly hatched nymphs that survived the first exposure because they were not yet susceptible.
Resistance management relies on rotating products with different modes of action. For example, switching from a pyrethroid‑based formula to a dimethicone‑based one reduces selection pressure on sodium‑channel mutations that confer pyrethroid tolerance.
Safety considerations include avoiding contact with eyes, limiting exposure to broken skin, and following age restrictions indicated on the label. Most agents are approved for topical use because systemic absorption is negligible; adverse effects are generally limited to mild scalp irritation.
In summary, lice‑killing shampoo works through a combination of neurotoxic or physical mechanisms, optimized delivery systems, and scheduled re‑applications to eradicate both adult insects and emerging nymphs while minimizing resistance and ensuring user safety.