Decreasing malathion application time for lice treatment reduces transdermal absorption.

OBJECTIVE: Head lice are the most common parasitic infestation in the United States requiring topical treatment with pediculicides. Ovide, the 0.5% malathion formulation used in treatment of head lice requires placement on dry hair for 8-12 h. Malathion, however, is effective at killing lice and nits in 10 min. Our concern of over exposing children to malathion has led us to examine whether significantly more malathion will penetrate transdermally when applied for the recommended 8 h than for a shorter but apparently equally effective period.

METHODS: In vitro absorption studies were performed across haired rat skin and human abdominal skin to determine whether reducing malathion application time decreased skin absorption.

RESULTS: A 0.5 h exposure caused 0.36+/-0.14% of the donor malathion to penetrate through human skin after 24 h and 2.1+/-0.6% remained in the skin after washing with shampoo. After 8 h of topical applications penetration was approximately three-fold greater (1.02+/-0.41) and 3.4+/-0.5% remained in the skin (p<0.05 versus 0.5 h). The relationship between absorption and exposure time also occurred for haired rat skin (p<0.05). This differential continued for 72 h even after removal of the source.

CONCLUSIONS: Significantly less malathion penetrated from Ovide after 0.5 h versus the suggested 8 h application, without decreasing the product's efficacy. Further clinical studies in children are warranted to confirm the efficacy of this shortened application time.