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 and absorption.15,16 Despite recognised neurotoxicity, lindane is still widely used, partly because alternatives are not readily available in many countries. Lindane passes transdermally during treatment and other exposures, and may be stored in fatty tissues and excreted in breast milk.17 Acute exposure to lindane during scabies treatment has potentiated seizures in people on medication that reduces seizure threshold.18,19 Therefore, lindane appears to be contraindicated for those undergoing therapy for HIV infection,18 or attention deficit hyperactivity disorder using amphetamine,19 and in those who suffer from epileptiform seizures. Concern has been expressed that lindane may be a risk factor for triggering of seizures in epileptics because it may alter liver cell function. Lindane does cause oxidative stress but does not appear to modify liver microsomal function, and in experimental systems these effects were mitigated by prior treatment with phenobarbital.20 21 Consequently, those being treated with barbiturates may be at lower risk of suffering side-effects from lindane. However, it is not clear whether people receiving anticonvulsant drugs in general are at greater risk of having seizures if exposed to lindane.

Various studies have shown that the solvent vehicle plays an important role in the rate of transdermal absorption of lindane.22 23 Additionally, much of the drug can also be absorbed as the treatment is washed off because a depot of lindane builds up in the stratum corneum.23-25 In many countries, scabicides are still applied after a hot bath but the resultant peripheral vasodilation is likely to enhance transdermal absorption. A related increase in passage of lindane through the dermis has been identified if soap and hot water are used to remove the acaricide at the end of the treatment process. Absorption can be minimised if cool water alone is used to remove residues of lindane products before bathing.25 An investigation of the absorption of permethrin and lindane through human cadaver skin in vitro found that lindane achieved a rate of 2 microgram/hour/cm2 in less than 5 hours, whereas the rate for permethrin was one-tenth of this after 10 hours. However, fresh guinea pig skin absorbed both at the same rate.26

Most RCTs have reported no serious adverse events using these topical insecticide-based products. One RCT reported five serious adverse events, two possibly associated with permethrin (rash and diarrhoea) and three possibly associated with lindane use (pruritic rash, papules and diarrhoea).11 Post-marketing surveillance of permethrin use in the USA from 1990 to 1995 found six adverse events per 100 000 units of product (equivalent to one central nervous system adverse event for each 500 000 units of permethrin used).27 Case series based on community intervention studies have reported a burning paraesthesia as one of the most frequent adverse events following permethrin use, particularly in the immunodeficient.27,28 A burning sensation was the most frequent adverse event, although not significantly so, in the largest RCT, with 23 events in 233 people following application of 5% permethrin, compared with 12/232 after 1% lindane lotion (P = 0-08).11

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