The most common types of treatment for head lice in the UK are organophosphates (malathion) and pyrethroids (permethrin and phenothrin), which act directly on the insect's nervous system.
These are both available as over the counter preparations, with pyrethroids the most widely used, because they require shorter application time and have a less strong smell than malathion.
The authors used nit combs on a random sample of almost 300 000 primary schoolchildren at 31 schools spread across the five health authorities of Wales.
All lice collected as a result of the screen were tested for resistance to pyrethroids, including measuring increased amounts of enzymes, such as glutathione transferases, monooxygenases, or esterases.
Around 80% of 316 lice tested were resistant to the treatment, equating to resistance to four out of five head lice affecting primary schoolchildren in Wales, say the authors.
Whether this pattern is reflected elsewhere in the UK will depend on how head lice treatments are used, they add.
But they suggest that where resistance develops, a newer silicone based lotion might be a suitable alternative.