Clinical evidence of lice resistance to over-the-counter products

BACKGROUND: Over-the-counter insecticidal solutions continue to be the mainstream treatment for most cases of head lice. Recorded insecticidal activity 15 years ago was at 100%, with ovicidal kill at 80%. Recently scattered accounts of possible resistance have surfaced. OBJECTIVE: This article is to document the general consensus that head lice are becoming resistant to standard over-the-counter therapies. METHODS: Direct testing of lethality of adult lice with permethrin and pyrethroid was assessed. RESULTS: Present over-the-counter products proved to be successful in only 9 of 32 head lice tested (28% insecticidal activity). CONCLUSIONS: Our series of cases further documents that over-the-counter head lice preparations no longer achieve high levels of insecticidal activity, suggesting increased resistance to these insecticides.