PERMETHRIN

Fetal Risk Summary

Permethrin, a pyrethroid, is indicated for the topical treatment of Sarcoptes scabiei (scabies) infestations. The drug is also active against lice, ticks, fleas, mites, and other arthropods. In patients with moderate to severe scabies, systemic absorption was estimated to be 2% or less of the dose (1,2).

Oral reproduction studies with permethrin have been conducted in mice, rats, and rabbits at doses of 200–400 mg/kg/day (1,2). No adverse effects on fertility or evidence of fetal harm were observed in these studies. Species-specific carcinogenesis was observed in mouse studies but negative results were seen in rats. Genetic toxic studies revealed no evidence of mutagenicity (1,2). Only the highest oral concentrations (2500–4000 ppm) of permethrin administered after implantation were found to significantly lower the protein and glycogen contents of rat placentas, but this had no statistical effect on the number of live fetuses (3).

Although no studies of permethrin placental transfer have been located, the molecular weight (about 391) is low enough that transfer should occur. However, because only small amounts are absorbed systemically and this is rapidly metabolized by ester hydrolysis to inactive metabolites, the opportunity for fetal exposure to permethrin appears to minimal, if it occurs at all.

A 1995 case report described the use of multiple courses of permethrin and other agents in a pregnant woman who had developed crusted scabies (4). She was initially treated at 2 months' gestation with monosulfiram (a pesticide not available in the United States). Relapse occurred at 5 months' gestation and she was treated with two total body (24 hour) applications of malathion 0.5% liquid and one total body (24 hour) application of monosulfiram 25% solution. Three weeks later another relapse occurred and she was treated with permethrin 5% cream to the whole body for 12 hours. Because live mites were found the next day, a sulfur/tar/coconut oil soak to the scalp three times daily followed by two total body benzyl benzoate 25% treatments within 24 hours. Then permethrin cream was used weekly for 3 weeks. A healthy male infant was delivered at term (4).

The Centers for Disease Control and Prevention (CDC) considers permethrin or pyrethrins with piperonyl butoxide to be the treatments of choice for pubic lice in pregnant women (5). Although not specifically mentioned, permethrin or pyrethrins with piperonyl butoxide should also be used if other body areas of a pregnant woman, such as the head, are infested with lice. For pregnant women with scabies, permethrin is considered the treatment of choice in the United States and the United Kingdom (5,6).

Breast Feeding Summary

No reports describing the use of topical permethrin during lactation have been located. Although the molecular weight (about 391) is low enough for excretion into breast milk, the minimal systemic absorption and rapid metabolism suggests that little, if any, of the drug will be found in milk. The CDC considers permethrin or pyrethrins with piperonyl butoxide to be the treatment of choice for pubic lice during lactation (5). For lactating women with scabies, permethrin is considered the treatment of choice in the United States and the United Kingdom (5,6).

References

1. Product information. Elimite. Allergan, 2001.
2. Product information. Acticin. Bertek Pharmaceuticals, 2001.
3. Spencer F, Berhane Z. Uterine and fetal characteristics in rats following a post-implantational exposure to permethrin. Bull Environm Contam Toxicol 1982;29:84–8.
4. Judge MR, Kobza-Black A. Crusted scabies in pregnancy. Br J Dermatol 1995;132:116–9.
5. CDC. 1998 Guidelines for treatment of sexually transmitted diseases. MMWR 1998;47:1–116.
6. Clinical Effectiveness Group (Association of Genitourinary Medicine and the Medical Society for the Study of Venereal Diseases). National guideline for the management of scabies. Sex Transm Inf 1999;75 (Suppl):S76–7.