Head lice don’t damage your hair itself. They live on your scalp, feed on small amounts of blood, and use your hair strands as anchors to hold on and lay eggs. The real effects happen on your scalp, where lice saliva triggers itching, inflammation, and sometimes secondary skin infections from scratching. Your hair serves as their habitat, not their food source.
How Lice Use Your Hair
Lice have specialized claws designed to grip individual hair strands. They crawl along the shaft to reach your scalp, where they feed on blood several times a day. Your hair is essentially a ladder and a home, not a meal. Lice cannot burrow into the scalp or tunnel through the hair shaft. They stay on the surface.
Female lice cement their eggs (called nits) directly onto hair strands, typically within a quarter inch of the scalp where body heat keeps the eggs warm enough to develop. Nits take 7 to 12 days to hatch. The glue holding them to the hair is remarkably strong, which is why nits don’t slide off easily when you run your fingers through your hair. This is actually one way to tell nits apart from dandruff: dandruff flakes brush away easily, while nits stay firmly attached even when you tug at them.
The claws of lice common in the United States are better adapted for gripping certain hair types and widths than others. This is a matter of physical fit between the louse’s claw and the diameter of the strand, not hair cleanliness or health.
What Happens on Your Scalp
When lice bite your scalp to feed, they inject saliva that contains proteins your immune system recognizes as foreign. This triggers an inflammatory response, which is what causes the intense itching. If you’ve never had lice before, the itching may not start for several weeks because your body hasn’t yet developed sensitivity to the saliva. With repeat infestations, itching can begin within a day or two.
The itching itself leads to the most visible problems. Persistent scratching can break the skin on your scalp, creating small wounds that are vulnerable to bacterial infection. This is called secondary impetigo, a common complication where bacteria enter through scratched skin and cause crusty, oozing sores. In severe or prolonged cases, the combination of scratching, inflammation, and infection can make the scalp red, tender, and swollen, particularly behind the ears and along the neckline where lice tend to concentrate.
Can Lice Cause Hair Loss?
Lice themselves don’t cause hair to fall out. They don’t weaken the hair shaft or affect the follicle. However, aggressive scratching over weeks can damage hair at the root, and in extreme untreated cases, matting can occur. A rare condition called plica polonica involves hair becoming so severely tangled and matted, sometimes cemented together by dried blood, lice secretions, and secondary infections, that sections of hair must be cut out. This only happens with prolonged, untreated infestations and is not a typical outcome.
Once lice are eliminated and the scalp heals, hair growth continues normally. There’s no lasting structural damage to the hair or follicles from a standard infestation.
Nits vs. Dandruff: How to Tell the Difference
Nits are clear or white and look strikingly similar to dandruff flakes at first glance. The key difference is how they behave. Dandruff is made up of dead skin cells that shed naturally and move freely along the hair or fall off when touched. Nits are glued in place. If you try to slide a small white speck along the hair and it won’t budge, it’s likely a nit.
Location helps too. Nits cluster close to the scalp, usually within that quarter-inch zone near the base of the hair. Dandruff flakes scatter more randomly across the scalp and hair. If you spot small white specks farther from the scalp, they’re more likely to be old, already-hatched nit casings or dandruff rather than viable eggs.
Why Over-the-Counter Treatments Often Fail
If you’ve treated lice with a drugstore product and they came back, you’re not alone. Lice have developed significant resistance to the most common active ingredient in over-the-counter treatments, a class of insecticides called pyrethroids. A meta-analysis of 20 studies found that roughly 59% of head lice populations are now resistant to pyrethroids. The resistance has grown sharply over time: before 2004, about 33% of lice were resistant. After 2015, that number climbed to 82%.
Permethrin, the specific pyrethroid in most drugstore shampoos, faces the highest resistance rate at around 65% overall. This means that for many people, the standard first-line treatment simply won’t kill all the lice. If an over-the-counter product doesn’t work after two applications spaced a week apart, prescription alternatives that use different mechanisms are available and tend to be far more effective against resistant populations.
Regardless of which treatment you use, manual nit removal with a fine-toothed comb remains essential. Because nits are cemented so firmly to the hair shaft, chemical treatments often kill live lice but leave viable eggs behind. Combing through small sections of wet, conditioned hair every few days for two weeks catches newly hatched lice before they can lay more eggs and helps break the cycle.
How Long Lice Survive Without a Host
Lice are obligate parasites, meaning they depend entirely on human blood to survive. Off the scalp, they typically die within 24 to 48 hours because they dehydrate quickly and lose their food source. Nits also can’t hatch without the warmth of a human scalp. This means that while you should wash bedding and recently worn hats in hot water, you don’t need to deep-clean your entire house or bag up belongings for weeks. Lice that fall off a head are already on borrowed time.