How fast does a lice shampoo work? - briefly
Lice shampoos start killing adult insects within 5–10 minutes, yet complete elimination usually needs a repeat treatment after 7–10 days. The second application removes newly hatched nymphs that survived the initial exposure.
How fast does a lice shampoo work? - in detail
A lice‑removing shampoo typically begins to kill adult insects within minutes of contact. Most formulations contain a neurotoxic agent such as pyrethrin, permethrin, or dimethicone that penetrates the exoskeleton and disrupts nerve function. Laboratory tests show mortality rates of 90‑100 % after 5–10 minutes of exposure.
The practical timeline for a single application is as follows:
- 0–5 min: Shampoo is applied to wet hair, lathered thoroughly, and left on the scalp. The active ingredient starts to act on live lice.
- 5–10 min: Majority of adult insects are immobilized or dead. Some eggs (nits) may survive because they are protected by the cement surrounding them.
- 10 min: Recommended rinse time is reached. Hair is washed out, and the scalp is rinsed completely to remove residual product.
- 24 h: Any remaining live lice are unlikely to survive; the environment on the scalp is hostile due to residual chemical activity.
- 7‑10 days: A second treatment is advised to target hatchlings that emerged from eggs that survived the first application. This interval aligns with the average 7‑day incubation period of Pediculus humanus capitis.
Factors influencing the speed of action include:
- Concentration of the active ingredient – higher percentages produce faster knock‑down.
- Hair length and density – thicker hair may require longer contact time to ensure full coverage.
- Resistance patterns – populations with documented resistance to pyrethroids may show delayed or reduced efficacy, necessitating alternative agents such as dimethicone.
- Application technique – thorough saturation of the scalp and scalp hair, followed by the manufacturer‑specified dwell time, maximizes performance.
Typical instructions from manufacturers require the shampoo to remain on the scalp for 5–10 minutes before rinsing. Failure to meet the minimum contact period can extend the period needed for complete eradication, often resulting in the need for additional treatments.
In summary, a properly applied lice‑shampoo eliminates the majority of adult parasites within ten minutes, but a follow‑up application after one week is essential to address newly hatched nits and achieve full clearance.