Commonly used treatments for head lice will not work for many people, researchers have warned.
Lice have developed a resistance to certain treatments
A study at 30 schools in Wales found a high level of resistance in lice to one of the main chemicals used in lotions.
Two-thirds of the lice tested after checks on 2,800 children were resistant to pyrethroid-based insecticides.
Liverpool's School of Tropical Medicine and the National Public Health Service for Wales conducted the study. NPHS recommended malathion-based products.
Head to head
Head lice live close to the scalp, where there is warmth, food and shelter.
They cannot fly, jump or hop and are spread when people's heads touch each other.
Most children are affected at some point in their life, but the majority of head lice infections are not caught at school.
Insecticides are recommended when live lice have been found in hair.
If a school gets an infestation, teachers write to parents and recommend they use an insecticidal lotion to treat their children.
Researchers visited 30 primary schools in Wales and checked just under 2,800 children for head lice.
One in 10 had head lice, with girls and children from deprived areas most likely to be affected.
Any lice which were found were sent to Liverpool for tests.
It was found that around two-thirds of the lice carried a gene variant which meant they had a resistance to pyrethroid-based treatments.
'Check your hair'
Dr Daniel Thomas, of the National Public Health Service for Wales, said: "Our recommendation is that the chemical malathion is more likely to control head lice.
"Parents should ask the chemist for advice about which products to use. And it will say on the side of the box what chemicals the product contains."
He said parents of affected children should also check their own hair, and use a lotion if they found live lice there.
Professor Janet Hemingway, director of the Liverpool School of Tropical Medicine, said: "We were able to show that there was a pyrethroid resistance.
"That means the lotion doesn't work as well as it should do, and you don't clear the infestation.
"It's not a major health problem, but it's a very significant social issue for parents and their children."