A literature review of head lice treatments in the U.S. found a significant decline in the effectiveness of the over-the-counter (OTC) permethrin and synergized pyrethrins (collectively pyrethroids) treatments, indicating a need for judicious use of effective treatments by clinicians.

Researchers searched three major biomedical databases (Embase, Medline, and STN) for literature on head lice treatments from 1985 – when current products were first available – to 2014. Their search yielded 579 articles.

Early studies showed that pyrethroids were 96- to 100% effective, however recent clinical studies have demonstrated that this effectiveness has declined to 25% (a level described as no better than placebo). This resistance is likely due to a shared site of action with dichlorodiphenyltrichloroethane (DDT), and widespread use linked to easy OTC availability. Furthermore, the authors state that the prescription medication Lindane should be limited as a treatment method due to its toxicity. Lindane includes a box warning of serious adverse events, including death, at the recommended dose rate, and it is banned in California. 

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Prescription products that the authors highlight as safe and effect are malathion, benzyl alcohol, and the recently FDA approved spinosad and topical ivermectin. The increased resistance to current OTC head louse treatments suggests that they may no longer be effective treatment options. In addition, home remedies such as mayonnaise and essential oils have not been proven to be safe or effective.

Lead author of the study, Dr. Ellen Koch said, “The lesson we should learn is that those products that do remain effective, which are available by prescription, should be used judiciously so that they do not suffer the fate that has befallen the pyrethroids.”

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