After how many days should the head be retreated for lice? - briefly
A second application is generally recommended 7–10 days after the initial treatment to eradicate newly hatched lice.
After how many days should the head be retreated for lice? - in detail
The interval between the initial lice treatment and the follow‑up application is determined by the parasite’s life cycle. Female lice lay eggs that hatch in about 7 to 10 days; the first dose eliminates live insects but does not affect unhatched eggs. A second dose applied after the hatching period removes the newly emerged nymphs before they mature and begin reproducing.
- Standard recommendation: 7–10 days after the first treatment.
- Permethrin (1% or 0.5%) products: 7 days is commonly advised.
- Pyrethrin‑based shampoos: 9 days is frequently cited.
- Spinosad (0.9%) formulations: 7 days is typical, though some protocols allow 10 days.
- Malathion (0.5%) treatments: 9–10 days is often recommended.
The precise timing may vary according to the specific medication, the severity of infestation, and local health authority guidance. Applying the second dose within the 7‑to‑10‑day window maximizes efficacy by targeting the cohort of lice that emerged from eggs after the initial exposure. Failure to retreat within this period increases the risk of resurgence, as surviving nymphs can reproduce and re‑establish the infestation.