Results from a new study may leave you scratching your head.
Head lice populations in at least 25 states have become immune to most over the counter treatments.
Researchers at Southern Illinois University gathered lice samples from 30 states. Nearly all of the populations they tested showed high levels of gene mutations, making them more resistant to the active ingredient in lice shampoos.
Samples from 25 states contained all three mutations and four states had lice with at least one mutation.
The resistant lice can be treated with different chemicals, but some are only available by prescription.