The lotion contains ivermectin and is sold under the brand name Sklice by Sanofi Pasteur, which paid for the study. The U.S. Food and Drug Administration used the results to approve topical ivermectin lotion in February.

"The advantage of it is, it's a one-application, one-shot treatment," lead author Dr. David Pariser of Eastern Virginia Medical School, in Norfolk, told Reuters Health.

The treatment "sounds like it has promise in a population itching to get rid of lice," said Dr. Hannah Chow of Loyola University Health System in Illinois, who was not connected with the study.

"Knowing how difficult it is to completely remove all lice despite due diligence with treatment and nit picking, ivermectin has potential to reduce the spread of lice, thus reducing parental anxiety, missed school and work, and social embarrassment," she said.

Hundreds of people worldwide suffer from head lice, which feed off blood and lay up to 300 eggs, which are the nits, during a month-long lifespan.

The cost of treating infestations - including the value of lost school days and parents forced to be out of work - is estimated at $1 billion a year in the United States. There's also been concern about resistance to other insecticide treatments.

Using a lotion with 0.5 percent ivermectin, the researchers found that after 14 days it had worked in 73.8 percent of 141 volunteers - most of whom were children younger than 12. In comparison, 17.6 percent of the 148 kids (and a handful of adults) whose hair was treated with a drug-free form of the lotion were louse-free after two weeks.

Lotions were applied to dry hair and then rinsed out after 10 minutes. The immediate success rate, judged the day after the lotion application, was 94.9 percent in the test group and 31.5 percent in the control group.

"It gets the kids back to school and the parents back to work," said Pariser.

The study, involving children in 11 states, did not compare the ivermectin to any other treatment.

However, in a previous study that did test other drugs as well, Pariser and his colleagues note that ivermectin showed a similar one-day success rate of 92.4 percent while malathion, an insecticide sometimes used to treat lice, cleared 82.4 percent of patients after one day.

"A topical drug formulation is indeed welcome and is expected to have less risk of systemic adverse events," Dr. Oliver Chosidow of Henri-Mondor Hospital in Creteil, France, and Bruno Giraudeau of University Francois-Rabelais in Tours, France, said in an editorial published with the new study in the New England Journal of Medicine.

But they advised that more-established techniques, such as treatments with permethrin or pyrethrin, or even malathion in cases of resistance, should be tried before using ivermectin.

"Ivermectin should be the last choice, whether topical (for still-infested persons) or oral (especially for mass treatment)," they said.

Interviewed by Reuters Health by phone and e-mail, Chow speculated that more than one application of ivermectin lotion might be necessary because the treatment didn't work in all cases.

"Knowing that new lice could re-hatch in seven to 10 days if ivermectin didn't completely kill all nymphs, live lice and newly laid nits, I wonder if a second application will be recommended," she said. "I would still advocate nit picking at this point."

"If you do this, think about retreating and continue to be very vigilant," she said.

SOURCE: bit.ly/Yor2zt New England Journal of Medicine, online October 31, 2012.

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