Only minor adverse effects have been reported for most insecticides. The exception is lindane, for which there are extensive reports of effects related to overdosing (treatment of scabies), and absorption (treatment of head lice). Lindane passes transdermally during treatment of head lice,9 but we found no reports of adverse effects in this setting.
We found no confirmed reports of adverse effects from therapeutic exposure to the organophosphorus compound, malathion. A recent randomised open volunteer study (32 people) examined transdermal absorption of malathion from four head louse treatment products available in the UK.10 Urinalysis for malathion metabolites found 0-2-3-2% of the applied dose was eliminated in urine before decreasing to baseline values by 96 hours. Erythrocyte cholinesterase levels were clinically unaffected, irrespective of dose or whether the skin was excoriated.
Pyrethroid insecticides are listed as contraindicated for people with ragweed allergy
How successful are treatments for head lice?
but we found only one report of an anaphylactoid reaction to a head louse treatment product.11
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