Ivermectin, a pill prescribed for the skin disease known as scabies, also gets rid of hair lice that are resistant to conventional lotions, a study published on Thursday says.
Lice affects over 100 million people worldwide each year, especially children of primary school age, according to the paper, appearing in the New England Journal of Medicine.
The main treatments are diluted forms of an insecticide called permethrin and malathion, but since the 1990s lice have becoming more and more resistant to these chemicals.
That has left parents with little choice other than to be, literally, nit-pickers -- to go through their children's hair with a fine comb to haul out the parasites.
Ivermectin is a promising alternative, say French researchers who led the study.
They tested it in a trial involving 812 adults and children in 376 households in Britain, France, Ireland and Israel.
Half of the patients were treated with malathion and half with ivermectin, which was administered twice, seven days apart, at a dose of 400 microgrammes per kilo (2.2 pounds).
After two weeks, 95.2 percent of the ivermectin group were lice-free, compared with 85 percent in the malathion group.
"Ivermectin is more effective than the best anti-lice lotion, but it should be reserved for difficult, resistant cases," the study's coordinator, Olivier Chosidow of France's National Institute of Health and Medical Research (INSERM), told AFP.
Over-using the drug could make it succumb to resistance, following the same path as the lotions, he said.