We thank Weil for his thoughtful letter. He has expressed a number of concerns about the recommendation that malathion be considered as first-line therapy for head lice. Please allow us to allay these concerns. Although malathion and chlorpyrifos are in the same chemical class, pure malathion has a different toxicity profile than other organophosphates. Specifically, chlorpyrifos would seem to inhibit carboxylesterase more than malathion, resulting in greater toxicity. In addition, it is the “-oxon” impurities, which are potent carboxylesterase inhibitors, of both malathion and chlorpyrifos that make them harmful. The malathion in Ovide is pure and contains no malaoxon.

It is true that the American Academy of Pediatrics (AAP) Committee on Infectious Diseases, which exclusively referenced the 2002 AAP guidelines for head lice, recommends permethrin as first-line treatment for head lice. We cited ample evidence from literature that postdated these guidelines in support of malathion...

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