DRUGS FOR HEAD LICE — Since most available pediculicides are not ovicidal, retreatment is usually necessary 7-10 days after the first treatment to kill newly hatched lice before they lay eggs (nits).

Topical permethrin 1% and pyrethrins (see Table 1) are available OTC, but resistance to these products is widespread.2 Topical formulations of benzyl alcohol (Ulesfia), malathion (Ovide), and spinosad (Natroba) are available by prescription and have been effective for treatment of head lice, but they are expensive.3 Off-label use of oral ivermectin is an inexpensive option that has been effective for treatment of head lice resistant to topical drugs.4

MECHANISM OF ACTION — Ivermectin is a fermentation product of Streptomyces avermitilis, a soil-dwelling actinomycete. It binds to glutamate-gated chloride channels in parasites such as lice, inducing paralysis and death. Ivermectin 0.5% lotion is not directly ovicidal, but lice that hatch from treated eggs die within 48 hours, so retreatment is usually not necessary.

CLINICAL STUDIES — Two double-blind trials compared a single application of ivermectin 0.5% lotion to its vehicle alone in a total of 765 patients ≥6 months old with head lice. In a combined analysis, the percentage of patients who were free of live lice after treatment (without manual removal of nits) was significantly higher with ivermectin than with its vehicle alone on day 2 (95% vs 31%), day 8 (85% vs 21%), and day 15 (74% vs 18%).5 Resistance of lice to ivermectin is rare.6

ADVERSE EFFECTS — Adverse reactions to ivermectin lotion, reported in <1% of patients in clinical trials, have included conjunctivitis, ocular hyperemia, eye irritation, dandruff, dry skin, and a burning sensation.