What is the first‑line therapy for Pediculus humanus capitis (head lice) and what alternative treatments are recommended for infants under 2 months, pregnant patients, or when resistance is suspected?

Treatment of Head Lice (Pediculosis Capitis)

First-Line Therapy

Permethrin 1% cream rinse is the first-line treatment for head lice, applied to damp, towel-dried hair for exactly 10 minutes, then rinsed with warm water, with a mandatory second application 7-10 days later. 1, 2, 3

Application Protocol for Permethrin 1%

• Wash hair with a non-conditioning shampoo first, as conditioning products prevent proper adherence of the medication 1, 2
• Towel-dry hair until damp (not soaking wet) before application 1, 3
• Apply permethrin 1% cream rinse thoroughly to the scalp and hair 1
• Leave on for exactly 10 minutes—not longer or shorter—to ensure effective treatment 1
• Rinse over a sink with warm water (not hot) to minimize systemic absorption 1, 3
• Repeat application in 7-10 days is mandatory because permethrin has only 70-80% ovicidal activity, meaning 20-30% of eggs survive the first treatment 1, 2, 3

Special Populations Requiring Alternative Approaches

For infants under 2 months: Permethrin is not recommended; manual removal with a fine-tooth nit comb is the safest approach, as most pediculicides lack safety data in this age group 2, 3

For pregnant or lactating women: Permethrin 1% or pyrethrins with piperonyl butoxide remain the safest options despite potential resistance concerns 3

Second-Line Therapy for Treatment Failure or Suspected Resistance

If live lice are confirmed 7-10 days after a second properly applied permethrin treatment, switch to malathion 0.5% rather than applying permethrin a third time. 3

Malathion 0.5% Application Protocol

• Apply to completely dry hair and scalp (not damp) 3
• Allow to air dry naturally—do not use heat-producing devices (hair dryer, curling iron) while hair is wet 3
• Leave on for 8-12 hours (typically overnight) 2, 3
• Wash off after the contact time 2, 3
• Malathion has the highest ovicidal activity (~98%) of all available treatments, often requiring only a single application 3
• Reapply in 7-9 days only if live lice are still present on examination 3

Critical Safety Warning for Malathion

• Contains ~78% isopropyl alcohol, making it highly flammable 2, 3
• Do not smoke near the patient during treatment 3
• Avoid open flames, sparks, and heat sources 2
• Approved for children ≥6 months, but contraindicated in children <24 months 3

Third-Line and Alternative Treatments

Spinosad 0.9% Topical Suspension

• Achieves high cure rates (84.6-86.7% lice-free at 14 days) with a single 10-minute application to dry hair 3, 4
• Superior efficacy compared to permethrin 1% (44.9-42.9% cure rate) in head-to-head trials 4
• Well-tolerated with minimal adverse effects 4
• Approved for children ≥6 months 3

Benzyl Alcohol 5%

• Kills lice by asphyxiation 3
• Apply for 10 minutes and repeat after 7 days 3
• Not ovicidal, so the second application is essential 3
• More than 75% of patients are lice-free 14 days after initial treatment 3
• Approved for children ≥6 months 3

Topical Ivermectin 0.5% Lotion

• Requires only a single application 5
• 73.8% of patients lice-free at day 15 (lower than spinosad or benzyl alcohol) 5
• Kills lice by causing pharyngeal muscle paralysis, preventing feeding 5
• Well-tolerated (pruritus in 0.9% of patients) 5
• Should be reserved as third-line treatment due to limited clinical experience and cost 5

Oral Ivermectin

• Single dose of 200 mcg/kg, repeated in 10 days 3
• Contraindicated in children weighing <15 kg due to risk of blood-brain barrier penetration 3
• Not FDA-approved for pediculosis 3

Treatments to Avoid

Lindane 1% should NOT be used due to:

• Low ovicidal activity (30-50% of eggs survive) 3
• Risk of seizures and neurotoxicity 2, 3
• Widespread resistance 3
• Contraindicated in individuals weighing <50 kg 3

Environmental Decontamination Measures

• Wash all clothing, bedding, and linens in hot water and dry on hot cycle on the same day as treatment 1, 2
• Wash hair care items (combs, brushes) in hot water 1, 2
• Remove items that cannot be washed from body contact for at least 72 hours, as lice cannot survive off the human body for more than 2-3 days 1, 2
• Fumigation of living areas is NOT necessary 1

Who to Treat

• Treat only individuals with live lice or nits within 1 cm of the scalp 2, 3
• Treat family members who share a bed with the infected person 2, 3
• Examine all household members, but do not treat prophylactically 3

Common Pitfalls to Avoid

Misinterpreting Post-Treatment Itching

• Post-treatment itching is NOT a sign of treatment failure and can persist for many days after lice are killed due to inflammatory response 1, 2, 3
• Do not retreat based on itching alone 1, 2
• Retreat only if live lice are seen 7-10 days after treatment 1, 2
• Topical corticosteroids or oral antihistamines may relieve post-treatment inflammation 3

Assuming Resistance Before Ruling Out Improper Application

• Most "resistance" is actually due to improper application, not true biological resistance 2, 3
• Before assuming treatment failure, consider: misdiagnosis, noncompliance, reinfestation from untreated contacts, or improper application technique 2, 3
• Common application errors include: using conditioning shampoo before permethrin, not leaving on for full 10 minutes, rinsing with hot water, or skipping the second application 1, 2

Unnecessary Nit Removal

• Nit removal is NOT necessary to prevent spread, but can help decrease diagnostic confusion 2
• Vinegar or vinegar-based products can be applied to hair for 3 minutes before combing to help loosen nits 3