Efficacy and safety of a combination regimen of phenothrin and ivermectin lotion in patients with head lice in Okinawa, Japan.

In Japan, pyrethroid-resistant head lice have been increasing; however, only 0.4% phenothrin, a pyrethroid drug, is available as an over the counter formulation. In recent years, Sumithrin® Lotion containing 5% phenothrin (PHT) was approved for scabies. In the USA, Sklice® Lotion containing 0.5% ivermectin (IVM) is used for the treatment of pyrethroid-resistant head lice. Therefore, to enhance the treatment of head lice in Japan, we conducted a clinical study to confirm the efficacy and safety of a combination regimen of PHT and IVM (PI regimen). Twelve cases were enrolled and PHT was applied to all patients on day 1. On day 8, five patients (41.7%) were lice free, and PHT was applied again. Notably, seven patients were not lice free and were switched to IVM. The rate of patients who were lice free on the PI regimen, which was the primary end-point, was 75.0% on day 15 and 91.7% on day 22. No adverse events were reported. A genetic analysis of the head lice collected at each visit revealed a kdr mutation in all patients. These results suggest that the PI regimen is safe and effective for the treatment of pyrethroid-resistant head lice in Japan.