Is permethrin (pyrethroid) still effective for treating pediculosis (lice)?

Effectiveness of Permethrin for Lice Treatment

Permethrin 1% lotion remains effective as a first-line treatment for head lice, but resistance has been reported, requiring consideration of alternative treatments when initial therapy fails. 1

Current Effectiveness of Permethrin

• Permethrin 1% lotion is still recommended by the American Academy of Pediatrics as a first-line treatment for head lice due to its effectiveness and safety profile 1
• However, resistance to 1% permethrin has been reported, though the prevalence of this resistance is not known 2
• Studies have shown variable eradication rates with permethrin, ranging from 13% to 75% depending on the geographic region, likely due to development of resistance 3

Proper Application for Maximum Effectiveness

• Permethrin should be applied to damp hair that is first shampooed with a non-conditioning shampoo and then towel dried 2
• It should be left on for 10 minutes and then rinsed off 2
• A second application is recommended in 7-10 days to kill newly emerged nymphs, as permethrin is not 100% ovicidal 2, 1
• Many experts now recommend routine re-treatment, preferably on day 9 2

Factors Affecting Treatment Success

• Conditioners and silicone-based additives present in almost all currently available shampoos impair permethrin adherence to the hair shaft and reduce its residual effect 2
• When faced with treatment failure, consider these possible explanations before assuming resistance:
  • Misdiagnosis (no active infestation)
  • Lack of adherence to treatment protocol
  • Inadequate application (not using sufficient product)
  • Reinfestation from untreated contacts 2

Alternative Treatments When Permethrin Fails

• Malathion 0.5% can be used in people who are 24 months of age or older when resistance to permethrin is documented or when treatment fails despite correct use 2, 1
• Malathion has high ovicidal activity, and a single application is adequate for most patients 2
• In five randomized trials, malathion was more effective than permethrin or phenothrin, achieving eradication rates of 80% to 98% 3
• Dimeticone, a silicone compound with physical action against lice, has shown 70-97% effectiveness in three randomized trials and may be preferred due to minimal adverse effects and low likelihood of resistance development 3

Safety Considerations

• Permethrin has extremely low mammalian toxicity and does not cause allergic reactions in individuals with plant allergies 2
• Malathion has a high alcohol content (78% isopropyl alcohol), making it highly flammable and requiring careful handling 2
• Malathion is contraindicated in children younger than 24 months due to safety concerns 2

Treatment Algorithm

1. Start with permethrin 1% lotion applied correctly to damp, non-conditioned hair for 10 minutes 2, 1
2. Re-treat in 7-10 days (preferably day 9) 2, 1
3. If live lice are still present after proper application and re-treatment:
   • Consider resistance to permethrin 2
   • Switch to malathion 0.5% for patients ≥24 months of age 2, 1
   • Consider dimeticone-based products as an alternative with fewer adverse effects 3
4. For all treatments, supplement with wet combing to remove remaining lice and nits 2

Permethrin resistance appears to be geographically variable, with one study showing that head lice in the United States were less susceptible to permethrin than those in Borneo, where pediculicides were essentially unknown 4. Despite these concerns, permethrin may still be effective for newly recognized infestations before resistance develops 4.