LOS ANGELES — Head lice are itchy, nasty nuisances that can be hard to get rid of. Can a pill provide relief?

A new study found in tough cases, an oral medication kills the parasites more effectively than a prescription lotion applied to the scalp.

The study, published Thursday in the New England Journal of Medicine, compared ivermectin — an antiparasitic drug used for human cases of river blindness and for animal parasites — with a lotion containing the insecticide malathion.

Ivermectin is not approved for use in the U.S. for head lice. Malathion, a topical insecticide available by prescription, is usually used as a second-phase head-lice treatment after over-the-counter topical products have failed.

Researchers studied 812 people in 376 households in seven areas around the world. All participants had louse infestations even after treatments with over-the-counter lotions or malathion-containing prescription lotions.

The households were randomly assigned to get two doses of an ivermectin pill or malathion lotion. In the ivermectin group, 95.2 percent of the participants were lice-free after two weeks, compared with 85 percent of the malathion group. Of households, 171 of 185 using ivermectin were head-lice-free, compared with 151 of 191 using malathion.

Ivermectin ends up in the blood head lice feed on and interferes with the ability of louse nerve cells to communicate, killing the creatures.

The 10 percent difference in effectiveness between ivermectin and malathion lotion is significant, said lead author Dr. Olivier Chosidow, head of dermatology at Hopital Henri Mondor in Creteil, France: "If you still have one or two lice you'll be reinfested one or two weeks later," he said

But he added ivermectin should not be used as a first-line treatment, even though studies have shown head lice are developing resistance to over-the-counter products and malathion. Lice can become resistant to ivermectin, too, he said.

The medication, like malathion, can cause problems for some people, he added. In the study, seven people out of 398 taking ivermectin developed impetigo (a skin infection), nausea, vomiting, gastroenteritis or convulsions.