The Bottom Line

Head lice are tiny parasites that spread by direct head-to-head contact — they have nothing to do with how clean your hair is. Older over-the-counter treatments like permethrin now fail more than 50% of the time due to resistance, so newer prescription options such as ivermectin lotion or spinosad often work much better. With the right treatment and a little household cleanup, most infestations clear up within two to three weeks.

What Are Head Lice?

Head lice (Pediculus humanus capitis) are small, six-legged insects that live on the scalp and feed on blood. They are the most common parasitic infestation in the world, affecting 6 to 12 million people every year in developed countries alone — and far more in developing nations.

Despite what many people believe, getting lice has nothing to do with how often you wash your hair. They spread the same way regardless of someone's socioeconomic background, race, or hygiene habits. The only real risk factor is close head-to-head contact with someone who has lice.

Who Gets Head Lice?

Lice most often affect children between the ages of 3 and 12, though anyone can get them. Girls get them about three times as often as boys — likely because of longer hair and closer head contact during play. Close family members and classmates of an infested person are at highest risk.

A single fertilized female louse can start a full infestation. During her 30-day life she lays 150 to 300 eggs (called nits), which hatch in about a week and reach adulthood in 7 to 10 more days.

Signs and Symptoms

The main symptom is itching, but it often takes 4 to 6 weeks to appear — that is how long it takes your body to react to louse saliva. When itching does start, it tends to be worst at the back of the head, behind the ears, and at the nape of the neck, where lice concentrate most.

When you look closely, you may see:

  • Live lice — tan to brown, about 2–3 mm long (about the size of a sesame seed)
  • Nits — tiny oval eggs cemented tightly to hair shafts, usually within 1–2 cm of the scalp
  • Empty nit casings — these stay on hairs for weeks even after treatment, so they alone do not mean the infestation is still active

Scratching can break the skin and sometimes lead to a secondary bacterial infection with crusting, sores, or swollen lymph nodes in the neck. If that happens, antibiotic treatment is also needed.

How Head Lice Are Treated

Older Treatments — Still Available, But Often Unreliable

Permethrin 1% (the ingredient in many drugstore lice products) used to be the go-to treatment. You apply it for 10 minutes, rinse, and repeat 7 to 10 days later to kill any newly hatched nymphs. The problem: resistance to permethrin now exceeds 50% in many parts of North America and up to 60–90% in parts of Europe. Pyrethrin-based products have the same resistance problem. If you have tried an over-the-counter permethrin product and lice persist, resistance is the likely reason.

Newer, More Effective Options

Several prescription treatments have much better track records today:

  • Ivermectin 0.2% lotion (Sklice): Applied once for 10 minutes, repeated at day 9–10. Achieves about 95% effectiveness with very low toxicity risk.
  • Spinosad 0.9% suspension: Applied once for 10 minutes. It kills both lice and their eggs (ovicidal), so a second treatment is often not needed — a major convenience advantage.
  • Benzyl alcohol 5% lotion (Ulesfia): Applied for 10 minutes, repeated 9–10 days later. About 70–80% effective.
  • Malathion 0.5% lotion (Ovide): Applied overnight (8–12 hours). Kills both adults and eggs, reducing the need for retreatment.
  • Oral ivermectin: Taken by mouth at 0.2 mg per kg of body weight on days 1 and 9, sometimes combined with a topical treatment for hard-to-treat cases.

Helpful Steps Alongside Medication

Medication alone does most of the work, but a few extra steps help ensure complete clearance:

  • Use a fine-tooth nit comb every 3 to 4 days for at least two weeks after treatment to remove eggs and catch any re-infestation early.
  • Wash bedding, clothing, and hats in hot water (above 60°C / 140°F) and dry on high heat — this kills lice and nits.
  • Items that cannot be washed can be sealed in a plastic bag for two weeks.
  • Check and treat all household members who are infested to prevent passing lice back and forth.

After treatment, some itching can continue for 1 to 2 weeks. That is normal — it is your skin reacting to residual louse proteins, not a sign treatment failed. A mild 1% hydrocortisone cream can help with residual itching.

When to See a Dermatologist

  • Over-the-counter permethrin or pyrethrin treatments have not worked after two full applications
  • The infestation keeps coming back despite treatment and household cleanup
  • You see signs of skin infection (crusting, pus, swollen lymph nodes) from scratching
  • You are unsure whether what you see are live lice or just empty nit casings
  • You need prescription-strength treatment for a young child or infant

Frequently Asked Questions

Does having lice mean someone has poor hygiene?

Not at all. Head lice infestations happen to people of every background, hygiene level, and income level. Lice only care about body heat and hair to hold onto — cleanliness plays no role. Please do not feel embarrassed if your child gets lice; it is extremely common.

Why do over-the-counter treatments sometimes stop working?

Lice have developed resistance to the chemicals in older products — particularly permethrin. Resistance rates now exceed 50% in North America, meaning the bug's nervous system has evolved to withstand the drug. Switching to a newer prescription product like ivermectin or spinosad usually solves this problem.

How long does it take to fully clear a lice infestation?

The killing phase is quick, but it takes time to confirm success. Most people are clear of live lice within 24 hours of effective treatment. With a second application 7 to 10 days later to catch any newly hatched nymphs, clinical cure typically happens 2 to 3 weeks after the final treatment. Empty nit casings may still be visible after that but are harmless.

Can lice come back after treatment?

Yes — re-infestation is possible if there is renewed contact with an untreated person. About 10 to 15% of people get re-infested within 3 months, usually from a family or classroom contact who was not treated. Checking all close contacts and treating those who are infested dramatically reduces this risk.

References

  1. Meinking TL, Serrano LF. Head lice resistance to OTC insecticides and treatment with prescription products. J Fam Pract. 2002;51(12):1042–1048.
  2. Burgess IF, Brown CM, Lee PN. Head lice prevalence in schoolchildren and effectiveness of treatments in the UK. Arch Dis Child. 2005;90(12):1231–1235.
  3. Hipolito RB, Mallorca FG, Zuniga-Maglaan CL, et al. Head lice infestation: single dose ivermectin versus permethrin in mass treatment. J Am Acad Dermatol. 2001;44(2):322–326.
  4. Dodd CS. Epidemiology and diagnosis of head lice infestations. J Sch Nurs. 2006;22(4):215–221.
  5. Burkhart CG, Burkhart CN. Assessment of frequency, transmission, and genitourinary complications of head lice. Int J Dermatol. 2005;44(10):811–815.

Trusted Resources

Always consult a board-certified dermatologist for personal medical advice about your skin, hair, or scalp condition.