A head lice infestation can last indefinitely if left untreated, since lice continuously lay new eggs that hatch and mature into adults. With proper treatment, though, most infestations can be resolved within one to three weeks. The timeline depends on how quickly you start treatment, whether you follow up with a second application, and how thorough your nit removal is.
The Lice Life Cycle: Egg to Adult
Understanding how lice develop helps explain why infestations persist and why treatment requires more than one round. The full life cycle from egg to egg-laying adult takes roughly two to three weeks.
Eggs (called nits) are glued to individual hair shafts close to the scalp, where body heat keeps them warm enough to develop. They hatch in about 6 to 9 days. The newly hatched louse, called a nymph, goes through three molts over the next 7 days before reaching adulthood. Once mature, an adult louse can live up to 30 days on a person’s head and lays several eggs per day throughout its life. This overlapping cycle of hatching and egg-laying is what makes lice so persistent: at any given time, your scalp may host eggs, nymphs, and adults all at different stages.
Why You Might Not Notice Right Away
One reason lice infestations drag on is that they’re easy to miss early. If you’ve never had lice before, it can take 4 to 6 weeks before itching starts. That’s because the itch is an allergic reaction to louse saliva, and your immune system needs time to become sensitized. During those weeks, the lice are actively reproducing, which means by the time you start scratching, the infestation may already be well established with multiple generations on your scalp.
People who’ve had lice before tend to notice itching sooner, since their immune system already recognizes the allergen. Either way, itching alone isn’t a reliable way to detect lice. The most definitive sign is finding a live, crawling louse on the scalp.
How Long Treatment Takes
Most over-the-counter lice treatments kill live lice on contact but don’t kill all the eggs. That’s why a second treatment is recommended 7 to 9 days after the first, timed to catch any nymphs that hatched from surviving eggs before they’re old enough to lay new eggs themselves. If you follow this two-treatment schedule and combine it with regular combing to remove nits, the infestation is typically eliminated within two to three weeks of starting treatment.
Here’s what a realistic treatment timeline looks like:
- Day 1: First treatment kills most live lice. You can expect to see a dramatic reduction in crawling lice immediately.
- Days 2 through 8: Surviving eggs may continue to hatch. Wet combing with a fine-toothed nit comb every few days helps remove nymphs and remaining nits.
- Days 7 to 9: Second treatment kills any newly hatched nymphs that the first round missed.
- Days 10 to 14: Continue checking and combing. If no live lice are found after two weeks, the infestation is likely over.
If you’re still finding live lice after two full treatment rounds, the product you’re using may not be effective. Some lice populations have developed resistance to common over-the-counter ingredients like permethrin. The exact prevalence of resistant lice in the U.S. isn’t well documented, but if standard treatments aren’t working after two proper applications, a prescription option from your doctor is the next step.
How Long Lice Survive Off Your Head
Lice are obligate parasites, meaning they need human blood to survive. An adult louse that falls off a person’s head will die within 1 to 2 days without a blood meal. Nits that detach from the scalp are also unlikely to survive, since they need the warmth of the scalp to hatch.
This means your home doesn’t harbor a long-term reservoir of lice. You don’t need to deep-clean every surface or bag up stuffed animals for weeks. Washing bedding and recently worn clothing in hot water and drying on high heat is enough. Lice don’t live on pets, and they don’t jump or fly. They spread almost exclusively through direct head-to-head contact.
Why Some Infestations Keep Coming Back
When lice seem to “last for months,” the problem is usually one of three things: missed nits, skipped second treatments, or reinfection from close contacts. A single missed nit can hatch, mature, and start laying eggs again within two weeks, restarting the cycle. Skipping the second treatment is especially common and gives surviving eggs a free path to adulthood.
Reinfection is the other major factor. If a family member, classmate, or close friend still has untreated lice, head-to-head contact can reintroduce them even after successful treatment. Checking and treating all household members at the same time significantly reduces this risk.
Lice and School Attendance
Children with lice do not need to miss school. CDC guidance is clear: a child who has lice can finish their school day, get treated at home that evening, and return to class the next day. Both the American Academy of Pediatrics and the National Association of School Nurses recommend against “no-nit” policies, which require children to be completely free of nits before returning. These policies lead to unnecessary absences, and the reasoning is straightforward. Nits found more than a quarter inch from the scalp are unlikely to be viable. They’re cemented to the hair shaft and virtually unable to transfer to another person. Misdiagnosis during school nit checks is also common, since dandruff, hair debris, and empty egg casings all look similar to untrained eyes.
How to Know the Infestation Is Over
The most reliable way to confirm lice are gone is wet combing. Saturate the hair with conditioner, then use a fine-toothed nit comb to section through the entire scalp. If you find no live lice or nymphs after combing every three to four days for two full weeks past your last treatment, the infestation is resolved. Finding empty nit casings stuck to the hair doesn’t mean lice are still active. Those shells can remain attached to the hair for months after the infestation ends and will eventually grow out.