Intended for healthcare professionals

Clinical Review

How to advise a patient when over the counter products have failed

BMJ 2003; 326 doi: (Published 05 June 2003) Cite this as: BMJ 2003;326:1257
  1. Ian Burgess1, director (ian{at}
  1. Insect Research and Development Limited, Cambridge CB1 5EL

    First it is important to know whether treatment really has failed, so it is important that you or a nurse know how to reliably identify live lice at all developmental stages, and that you can differentiate hatched eggshells (nits) from potentially viable eggs. Nobody can reliably identify viable eggs, even with the aid of a microscope, unless the young louse can be seen moving inside the shell.

    Failure with over the counter products may have two causes: improper use and insecticide resistance.

    Improper use

    Inadequate, incomplete or inappropriate applications are the most commonly encountered causes of treatment failure. These result from:

    • Not using enough of the product to adequately cover the scalp and hair

    • Not repeating the treatment after 7–10 days if louse eggs survive the first application of the product

    • Inappropriate use of the product because the person does not have lice.

    The presence of nits is insufficient evidence of infestation, and many items of debris, such as dandruff, dried hair spray, or pseudo-nits (peripilar keratin casts) have been mistaken for nits.

    Pyrethrins and permethrin have a similar mode of activity. Consequently, permethrin may give no benefit if used on lice that have not responded to pyrethrins. In some cases, however, lice may be susceptible to pyrethrins even after permethrin has been unsuccessful.

    Insecticide resistance

    In recent years, resistance to insecticides has become an important factor in treatment failure. Resistance has been confirmed as affecting permethrin treatments. Similar resistance is suspected for pyrethrins but is not yet confirmed. Resistance to lindane has been identified in the Netherlands, Israel, Malaysia, and Denmark but has not yet been confirmed in the United States. Malathion is apparently resistance-free, except in the United Kingdom and France.

    If a patient has adequately used over the counter products but still has lice, the most reliable prescription treatment is malathion. This product is less easy to apply than a shampoo because it is a freeflowing liquid and must be left on for at least eight hours. It has a pungent odour due to a high level of monoterpene components in the essential oils that were included to mask the odour of malathion. The monoterpenes contribute much of the pediculicidal and ovicidal activity of the product. Evidence for activity of malathion is good. About 2-3% of malathion applied to the scalp is absorbed transdermally.

    In some cases, lindane may be prescribed instead of malathion. Evidence for lindane's efficacy is relatively poor, and lindane is about 4–10 times more toxic than malathion. A high proportion of lindane applied to the skin is absorbed transdermally.


    • Competing interests IFB has been a consultant to various makers of pharmaceutical products, alternative therapies, and combs for treating louse infestation.

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