Can You Treat Head Lice with Permethrin a Third Time?
Yes, you can apply permethrin a third time if live lice are still present, but treatment failure after two properly applied treatments strongly suggests permethrin resistance, and you should switch to an alternative agent like malathion 0.5% rather than continuing with permethrin. 1, 2
Understanding Treatment Failure vs. Resistance
Before applying permethrin a third time, you must first rule out the most common causes of apparent "treatment failure":
- Improper application technique is the leading cause of perceived resistance—permethrin must be applied to damp (not soaking wet) towel-dried hair, left on for exactly 10 minutes, then rinsed with warm water 1, 2
- Misdiagnosis of nits as active infestation—only live lice or nits within 1 cm of the scalp indicate active infection requiring treatment 1, 2
- Reinfestation from untreated household contacts or contaminated fomites rather than true treatment failure 3
- Post-treatment itching persisting for days is normal inflammatory response, not treatment failure, and should not trigger re-treatment unless live lice are documented 1, 2
When to Switch Agents Instead of Third Application
If you have confirmed live lice 7-10 days after a second properly applied permethrin treatment, resistance is likely and you should switch to malathion 0.5% rather than applying permethrin a third time. 3, 1, 2
The evidence for this recommendation:
- Permethrin resistance is widespread in the United States, with documented knockdown resistance (kdr) mutations causing 4-8 fold resistance 4, 5
- Studies show 87% treatment failure rates with permethrin in resistant populations 6
- Malathion 0.5% kills permethrin-resistant lice approximately 10 times faster than permethrin and has the highest ovicidal activity of all available treatments 3, 2, 7
If You Proceed with Third Permethrin Application
If you choose to apply permethrin a third time (perhaps due to cost constraints or malathion unavailability), follow this protocol:
- Apply to damp, towel-dried hair after washing with non-conditioning shampoo 1, 2
- Leave on for exactly 10 minutes, then rinse thoroughly with warm water 1, 2
- Schedule the third application on day 13-15 if using the alternate treatment schedule for non-ovicidal products 3
- Simultaneously treat all household members who share a bed with the infected person 1, 2
- Wash all bedding, clothing, and hair care items in hot water on the same day as treatment 1, 2
Recommended Alternative: Malathion 0.5%
When permethrin fails after two proper applications, malathion 0.5% is the evidence-based alternative:
- Apply to dry hair, allow to air dry naturally (never use hair dryer—78% alcohol content makes it highly flammable), then wash off after 8-12 hours 3, 1, 2
- Has high ovicidal activity, often requiring only a single application 3, 2
- Reapply in 7-9 days only if live lice are still present 3, 2
- Approved for children ≥24 months of age 2
Critical Safety Considerations
Never use lindane as an alternative—it has severe safety concerns including seizure risk, low ovicidal activity (only 30-50% of eggs killed), and widespread resistance 1, 2
For pregnant or lactating women experiencing treatment failure, permethrin or pyrethrins with piperonyl butoxide remain the safest options, though resistance may limit efficacy 3, 8
Common Pitfall to Avoid
The most critical error is assuming resistance when the real problem is improper application technique or misdiagnosis. Before labeling a case as "resistant" and switching agents (or applying a third treatment), verify that: