What Causes Head Lice? A Dermatologist Explains How to Spot Nits, Tell Them From Dandruff, and Clear Lice From the Whole Home

3 days ago

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Head lice (Pediculus humanus capitis) are wingless parasitic insects 2–3 millimetres long that feed on blood from the human scalp and lay eggs (nits) that stick firmly to the base of the hair, no more than 6 millimetres from the scalp. Lice are not caused by dirt and cannot appear on their own. They spread through direct head-to-head contact, so they are most common in children aged 3–11 years and in people living in the same household.

In this article, a dermatologist explains the life cycle of lice, how to tell nits from dandruff, each lice treatment used in medical practice (generic name, strength, how to use it and the day to repeat it), why lice come back after treatment, and how to deal with the whole household in a single round.

What are head lice, and what really causes them?

Head lice live only on the human scalp. They do not live on dogs, cats or other pets, and they do not carry disease. They differ from body lice, which live in the seams of clothing.

Adult louse: about 2–3 mm long (roughly the size of a sesame seed), grey-brown, with 6 legs ending in claws that grip the hair.
Feeding: it sucks blood from the scalp several times a day. Louse saliva causes the itching.
Movement: it crawls quickly but cannot jump or fly, and it cannot swim from one head to another.
How lice spread: direct head-to-head contact is almost always the route. Sharing combs, hats or pillows is a much less common route.

So, to the question "can lice appear on their own?" the answer is no. Live lice or nits must always come from another person, and washing hair every day does not prevent lice, because lice cling tightly to hair and tolerate water.

The life cycle of lice: the numbers used to plan treatment

Knowing the life cycle is the key to clearing lice completely, because most lice treatments do not kill all the eggs. The day to repeat treatment is calculated from the hatching period.

Egg (nit): about 0.8 mm, oval, cloudy white to yellow-brown, fixed to the base of the hair with a glue-like substance. It hatches in 7–10 days (on average 8–9 days).
Nymph: moults 3 times and becomes an adult in 9–12 days.
Adult: lives on the scalp for about 30 days. A female lays roughly 6–10 eggs a day, and more than 100 over her lifetime.
Off the head: lice that fall off mostly die within 1–2 days, without blood and warmth. Nits that fall off the hair almost never hatch at room temperature.

In practical numbers: if you treat on day 1, the eggs that survive hatch by day 7–10. The second treatment should therefore fall on day 7–10 (most often day 9), before the newly hatched nymphs are old enough to lay new eggs.

Symptoms of head lice: what to look for

An itchy scalp, especially at the nape and behind the ears, often worse at night.
In a first infestation, itching may not start for 4–6 weeks, because the body needs time to develop an allergic reaction to louse saliva. In people who have had lice before, itching starts within 1–2 days.
A feeling of something crawling on the head, poor sleep, irritability, and shorter attention span in children.
Red bumps or scratch marks at the nape, neck and behind the ears.
Swollen lymph nodes at the back of the head or behind the ears when scratching has caused sores and a secondary infection.
Some people have no symptoms at all, so everyone in the household should be checked, not only the person who complains of itching.

How to check for head lice with certainty

To diagnose an active infestation "right now", you must find a live louse. Finding nits alone does not always mean lice are still present.

Wet combing: coat the hair thoroughly with conditioner and comb from root to tip, one section at a time, with a fine-toothed nit comb (gap between teeth 0.2–0.3 mm). Wipe the comb on white paper each time. Conditioner slows the lice down so they are easier to catch.
Visual check in good light: look at the nape, behind the ears and the front hairline, where lice most often lay eggs.
Check how far the nits are from the scalp: nits within 6 mm of the scalp may still be alive. Nits more than 1 cm away are usually empty hatched shells or dead eggs, because hair grows about 1 cm a month.
Dermoscopy by a doctor: separates live eggs (with a cap and solid contents) from empty, translucent shells with an open end, which helps decide whether more treatment is needed.

How nits differ from dandruff: the most common point of confusion

This is one of the most searched questions. The easiest way to tell them apart is to try pulling or blowing them off.

Nits: small oval shapes about 0.8 mm long, stuck to the side of a hair at an angle. They do not blow off or shake off. You need a fingernail or a fine comb to slide them off. They are usually no more than 6 mm from the scalp.
Dandruff: flat flakes of skin of uneven size and shape, white or oily yellow. They come off easily when shaken or blown, do not stick to the hair, and are spread all over the scalp.
Hair casts: white rings wrapped around the hair shaft that can be slid up and down the hair, unlike nits, which stick on one side and cannot be moved.
Hair spray or gel residue: small, irregular beads that wash away with water.

The "shrivelled" or "dried-up" nits many people ask about are hatched shells or dead eggs. They are flat, translucent, clear white and usually more than 1 cm from the scalp. They do not make lice come back, but they should be combed out so they do not cause confusion at later checks.

If you cannot tell dandruff from nits, or there are thick scales as well, read more in What causes dandruff, and how to tell each type apart.

How to tell head lice from other scalp conditions

An itchy scalp is not always caused by lice. A dermatologist will rule out the following conditions before starting a lice treatment.

Seborrheic dermatitis: yellow, oily scales with red patches along the hairline, eyebrows and beside the nose.
Scalp psoriasis: thick, well-defined plaques with silvery scales that often extend past the hairline.
Tinea capitis (scalp ringworm): seen in children, with round patches of hair loss, short broken hairs and scaling. It needs oral antifungal medicine. Read more: Scalp ringworm in children: symptoms and treatment.
Impetigo: honey-coloured crusts, often on top of lice scratch marks. Read more: Impetigo: a bacterial skin infection.
Scabies: also very itchy at night, but the rash is between the fingers, at the wrists and waist, and not on the scalp in adults. Read more: What causes scabies, and what night-time scabies itching feels like.

For an overview of every cause of an itchy scalp, read What causes an itchy scalp.

Types of lice treatment, and how to choose

Lice treatments fall into 2 main groups by how they work. Understanding the difference helps explain why some people do not get better after treatment.

Treatments that damage the louse nervous system (neurotoxic pediculicides): for example permethrin, pyrethrins, malathion, ivermectin and spinosad. They work well, but lice in some areas can become resistant.
Treatments that act physically (physical agents): for example dimeticone and benzyl alcohol. They coat the louse so it suffocates or cannot control its water balance. Lice rarely become resistant to these, because they do not act through nerve receptors.

The doctor chooses a treatment based on age, weight, pregnancy, previous treatments and what is actually available in Thailand. Details of the commonly used treatments follow, one by one.

Each lice treatment: strength, how to use it and the day to repeat it

Permethrin 1% (permethrin 1% lotion / cream rinse)

How to use: wash hair with ordinary shampoo that contains no conditioner and towel-dry until damp. Apply the product thoroughly from root to tip, leave for 10 minutes, then rinse off.
Repeat: day 9 (within days 7–10), because it does not kill all the eggs.
Age: can be used from 2 months of age.
Good to know: do not use conditioner, or shampoo that contains conditioner, before applying the product, and do not wash the hair again for 1–2 days after applying it, because this reduces how well the product stays on the hair. Permethrin-resistant lice are now found more often in many countries, caused by a mutation in the sodium channel of the louse nerve cells (kdr mutation).

Pyrethrins combined with piperonyl butoxide (pyrethrins + piperonyl butoxide)

How to use: apply to dry hair, leave for 10 minutes, then rinse off. Repeat on day 9–10.
Age: from 2 years of age.
Contraindication: people who are allergic to chrysanthemums or ragweed.

Dimeticone 4% (dimeticone 4% lotion)

How to use: saturate dry hair strand by strand, let it dry naturally, and leave on for the time stated on the label (generally 8 hours or overnight), then wash out.
Repeat: day 7.
Age: can mostly be used from 6 months of age.
Advantages: acts physically, lice rarely become resistant, and it is gentle on the skin. It is a good option when permethrin resistance is suspected.
Caution: hair coated with the product is flammable. Keep away from flames, cigarettes and high-heat hair dryers until it has been washed out.

Benzyl alcohol 5% (benzyl alcohol 5% lotion)

How to use: apply to dry hair until saturated, leave for 10 minutes, then rinse off. Repeat on day 7.
Age: from 6 months of age.
Limitation: it does not kill eggs, so it must be repeated every time.

Ivermectin 0.5% lotion (ivermectin 0.5% lotion)

How to use: apply once to dry hair, leave for 10 minutes, then rinse off.
Advantages: in most studies a single application worked, because newly hatched nymphs die before they can feed on blood.
Age: from 6 months of age.

Spinosad 0.9% (spinosad 0.9% suspension)

How to use: apply to dry hair, leave for 10 minutes, then rinse off. Repeat on day 7 only if live lice are still seen.
Advantages: kills both lice and most of the eggs.
Age: from 6 months of age.

Malathion 0.5% (malathion 0.5% lotion)

How to use: apply to dry hair, leave for 8–12 hours, then wash out. Repeat on day 7–9 if lice are still seen.
Age: 6 years and older.
Caution: contains alcohol, is flammable, has a strong smell and is hard to find in Thailand.

Oral ivermectin (oral ivermectin): use only as prescribed by a doctor

Dose: 200–400 micrograms per kilogram of body weight, taken once, then taken again on day 8 (within days 7–10).
Used when: several topical treatments have not worked, or topical treatment cannot be used.
Contraindications: children under 15 kilograms, pregnant women and breastfeeding women. The doctor must check other medicines being taken to look for drug interactions first.

A treatment dermatologists no longer recommend: lindane (lindane) or gamma benzene hexachloride

It is still sold and some websites still recommend it, but it is toxic to the nervous system and can cause seizures, especially in young children, people with low body weight and people with sores on the scalp. Current clinical guidelines therefore no longer use it as a treatment option for lice.

How to know the treatment is working: 8–12 hours after applying, you should not see lice moving normally. If lice are still running about as actively as before, the treatment may not have worked. Do not immediately repeat the same product. Talk to a doctor about changing to a different group of treatment. The choice of treatment, dose and total duration is a decision for the doctor, based on each patient's condition.

The 17-day plan to clear lice completely

Suitable for most treatments that need to be repeated. Adjust the repeat day to the treatment the doctor prescribes.

Day 1: apply the first treatment as directed for that product, then comb out lice and dead nits with a fine-toothed comb. Check everyone in the household on the same day.
Day 2–3: wet-comb again. If lice are still moving normally, the treatment may not have worked, so talk to a doctor.
Day 5: wet-comb to remove newly hatched nymphs.
Day 7–10 (most often day 9): apply the second treatment, before the nymphs are old enough to lay eggs.
Day 13: wet-comb again.
Day 17: wet-comb to confirm. If no live lice are found at all, the treatment has succeeded.

Can wet combing alone work? For people who do not want to use medicine

Yes, and it is the main method for babies under 2 months old, but it must be done with discipline.

Equipment: a metal or rigid plastic nit comb with teeth 0.2–0.3 mm apart, ordinary hair conditioner, and white tissue paper.
Method: divide the hair into small sections, apply conditioner, and comb from root to tip on every section, wiping the comb each time. It takes about 15–30 minutes for short hair and longer for long hair.
Frequency: every 3–4 days, that is, on days 1, 5, 9 and 13, with a confirming check on day 17.
Limitation: the cure rate is lower than with medicine. Skipping even one session may leave some lice behind.

Why lice come back after treatment: 6 common reasons

The second treatment was missed, or was repeated too early or too late, so eggs that survived the first round hatched and went on to lay more.
The treatment was not used correctly, for example applied to soaking wet hair so it was diluted, not applied all the way to the roots, too little used for long hair, or conditioner used before applying it.
Re-infestation from someone close: household members, classmates or relatives who were not checked and treated at the same time.
Resistant lice, especially to permethrin and pyrethrins.
Mistaking dead nits for lice that are still there, which leads to repeated treatment until the scalp becomes irritated.
Using herbal or home remedies instead of evidence-based treatment, which kill lice inconsistently.

Head lice in school-age children

What it looks like: most common in children aged 3–11 years. Girls are affected more often because of long hair and close play. Nits are usually found at the nape and behind the ears.

Cause: head-to-head contact during play, napping close together, or posing close together for photos.

Severity/urgency: not dangerous and not a serious disease, but it should be treated within 1–2 days of being found to reduce spread in the classroom.

When to see a doctor: live lice are still found after 2 complete rounds of treatment, scratch sores are infected, or the child is under 2 years old.

On school attendance, the guidance of many paediatric societies is not to keep children home because of lice. A child can go back to school after the first treatment, and does not have to wait until every nit is gone.

Head lice in adults

What it looks like: usually parents, guardians, teachers or carers of children. Itching may be milder than in children and may be overlooked.

Cause: catching them from a child or a child in their care, sharing pillows, or close cuddling.

Severity/urgency: not dangerous, but untreated adults are an important reason children are re-infested.

When to see a doctor: if nits or lice are found on body hair away from the scalp, such as pubic hair or underarm hair, because this may be pubic lice. These are a different type of louse and need a check for sexually transmitted infections too.

Head lice in pregnant and breastfeeding women

What it looks like: the symptoms are the same as in anyone else, but the treatment choices are more limited.

Cause: usually caught from an older child.

Severity/urgency: not harmful to the baby in the womb, but it should be treated so a newborn does not catch it.

When to see a doctor: talk to a doctor before using any treatment. Commonly used options are permethrin 1% or dimeticone 4% together with wet combing. Avoid oral ivermectin, malathion and lindane.

Head lice in babies and children under 2 years

What it looks like: less common than in older children, but it can happen when a sibling or carer has lice.

Cause: being held or cuddled, or sleeping in the same bed as someone with lice.

Severity/urgency: a baby's skin is thin and absorbs medicine readily, so extra care with medicines is needed.

When to see a doctor: every time, before using a lice treatment. Babies under 2 months are treated mainly with wet combing. From 2 months, the doctor may choose permethrin 1%, and from 6 months there are more options, such as dimeticone. Never use lindane or malathion.

Resistant or recurring head lice

What it looks like: you have completed 2 rounds of treatment on the right days, but 8–12 hours after applying you still see lice moving normally.

Cause: lice resistant to the neurotoxic treatments, or re-infestation from someone close.

Severity/urgency: not dangerous, but repeating the same treatment many times increases scalp irritation without helping the lice go away.

When to see a doctor: when treatment has failed twice. The doctor will review how the product was used, confirm with dermoscopy whether the nits are still alive, and then switch to a physical treatment such as dimeticone, or another option such as ivermectin lotion, spinosad, or oral ivermectin in suitable cases.

Head lice with an infected scalp sore

What it looks like: scratch sores, pus, honey-yellow crusts, matted hair, an odour, and swollen, tender lymph nodes at the back of the head.

Cause: skin bacteria such as Staphylococcus aureus entering scratch marks.

Severity/urgency: see a doctor within a few days. If there is a fever or the sores spread quickly, see a doctor as soon as possible.

When to see a doctor: as soon as you see pus or have a fever. The doctor may prescribe a topical antibiotic such as mupirocin 2% or fusidic acid 2% applied 2–3 times a day for 5–7 days, or an oral antibiotic if the sores are wide, and may postpone irritating lice treatments until the sores improve.

Pubic lice and body lice: how they differ from head lice

Pubic lice (Pthirus pubis): short, stocky, crab-like lice that live in pubic hair and may also be found in underarm hair, chest hair or eyelashes. They are mostly spread through sexual contact. Testing for other sexually transmitted infections is advised, and sexual partners should be treated at the same time. If pubic lice are found on a child's eyelashes, a doctor should assess them further.
Body lice (Pediculus humanus corporis): live and lay eggs in the seams of clothing and come up to feed on the skin. They occur where changing clothes or bathing is difficult and can carry certain diseases. The main treatment is washing clothes and bedding in hot water.
Head lice: live only on the scalp, do not carry disease and have nothing to do with sexual contact.

Dealing with lice in the whole home, bedding and belongings

Check everyone in the household on the same day by wet combing, but treat only people who have live lice or nits close to the scalp. For a person who sleeps in the same bed as the patient, the doctor may consider treating them at the same time.
Bedding, hats and towels used in the 2 days before treatment: wash in hot water of at least 60 degrees Celsius, or tumble dry on high heat for at least 15 minutes.
Items that cannot be washed, such as soft toys and helmets: seal in a plastic bag for 2 weeks.
Combs, brushes and hair clips: soak in hot water of about 60 degrees Celsius for 5–10 minutes.
Floors, sofas and car seats: ordinary vacuuming is enough.
No need to spray insecticide around the home, because it does not help and increases the risk of breathing in chemicals.
No need to cut the hair short or shave the head. It would help, but it is not necessary if the treatment is used correctly.

Myths about head lice you should stop believing

"Lice are caused by dirt": not true. Anyone can get lice, even if they wash their hair every day.
"Lice can jump or fly": not true. Spread is almost entirely through direct head contact.
"Lice come from dogs and cats": not true. Human head lice live only on humans.
"Vinegar kills nits": there is no good evidence that it kills lice or loosens nits. It cannot replace treatment.
"Mayonnaise, olive oil or coconut oil kills all the lice": they may slow the lice down for a while and make combing easier, but they do not kill lice and eggs reliably.
"Pounded custard apple leaves on the head are safe because they are natural": substances in custard apple seeds and leaves can kill lice to some extent, but if they get into the eyes they can cause severe conjunctivitis, and they can irritate the scalp.
"Kerosene, insecticide or dish soap kills lice well": very dangerous, with a risk of fire, burns and poisoning through absorption. Do not use.
"You must wait until all the nits are gone before going to school": not necessary. A child can go back to school after treatment has started.

How to prevent head lice

Teach children to avoid putting their heads together when playing or taking photos.
Tie up long hair or braid it, to reduce contact with other people's hair.
Do not share combs, hats, headscarves or pillows while there is an outbreak at school.
Check your child's head by wet combing once a week while classmates have lice.
Do not use lice treatments as prevention in people who do not have lice, because they do not work and increase the chance of resistance.

Signs that you should see a dermatologist

You have completed 2 rounds of treatment on the right days and still find live lice.
You are not sure whether it is nits, dandruff or another scalp condition.
There are sores, pus, yellow crusts, or swollen and tender lymph nodes.
The patient is a child under 2 years old, or is pregnant or breastfeeding.
The scalp is red, stinging or has a rash after using a lice treatment (this may be an irritant rash from the product).
Lice or nits are found on the eyelashes, eyebrows or pubic hair.

A dermatologist can confirm the diagnosis with dermoscopy, rule out other conditions, and plan treatment suited to each person's age and circumstances.

Quick summary

Lice spread through head contact with someone who has lice. They are not caused by dirt, and they cannot jump or fly.
Nits stick tightly beside the hair and do not blow off, whereas dandruff is made of flakes that shake off easily.
Nits hatch in 7–10 days. Most treatments do not kill all the eggs, so treatment must be repeated on day 7–10 (most often day 9).
Commonly used treatments: permethrin 1%, dimeticone 4%, benzyl alcohol 5%, ivermectin 0.5% lotion and spinosad 0.9%. Lindane is no longer recommended.
Check everyone in the household at the same time, treat only those with live lice, and wash bedding in hot water at 60 degrees Celsius.
See a dermatologist if 2 rounds of treatment have not worked, if there are infected sores, or if the patient is a baby or is pregnant.

Frequently asked questions about head lice

How do you get rid of nits?

Nits go away with 2 things used together: a lice treatment repeated on the right day, and wet combing with a fine-toothed nit comb. Most treatments, such as permethrin 1%, do not kill all the eggs, so a second application is needed on day 7–10 after the first, to kill newly hatched nymphs before they lay new eggs. In between, apply conditioner and comb every 3–4 days until day 17. Nits more than 1 cm from the scalp are usually hatched shells and do not make lice come back, but they can be combed out for tidiness.

How many days can lice live?

On the scalp, an adult louse can live about 30 days, but once it falls off the head it mostly dies within 1–2 days, because it needs to feed on human blood several times a day and needs the warmth of the scalp. Nits that fall off the hair almost never hatch at room temperature. For this reason, home cleaning should focus only on items that touched the head in the 2 days before treatment, such as pillowcases, blankets, hats and combs, washed in hot water at 60 degrees Celsius or sealed in a bag for 2 weeks. There is no need to spray insecticide around the whole house.

How do dandruff and nits differ?

Nits are small oval beads about 0.8 mm long, stuck beside the hair with a glue-like substance. They do not blow off or shake off, and you need a fingernail or a fine comb to slide them off. They are usually close to the root at the nape and behind the ears. Dandruff is flat skin flakes of uneven size that fall off easily when shaken and are spread all over the head. Another look-alike is the hair cast, a white ring around the hair that can slide along it. If you still cannot tell them apart, a dermatologist can check with a dermoscope in a few minutes.

Which treatments can kill nits?

Treatments that kill nits better than the others are spinosad 0.9% and malathion 0.5%. Ivermectin 0.5% lotion does not kill eggs directly but causes newly hatched nymphs to die before they feed, so it is often used just once. The most commonly used treatments, permethrin 1%, pyrethrins and benzyl alcohol 5%, do not kill all the eggs, so they must be repeated on day 7–10. Dimeticone 4% kills some of the eggs and is repeated on day 7. The choice depends on age, pregnancy and any history of resistance, and a doctor should decide.

Where do head lice come from?

Head lice come only from another person who has lice. They do not appear on their own from sweat, dirt or unwashed hair, and they do not come from pets. The most common route is head-to-head contact, for example children playing together, napping side by side, or cuddling a child. Sharing combs, hats and pillows is a less common route, because lice can survive off the head for only 1–2 days. When one child has lice, everyone in the household should be checked at the same time, and the school should be told so other parents can check their children too.

How many eggs can one louse lay?

A female louse lays about 6–10 eggs a day, and more than 100 eggs over her roughly 30 days on the head. Eggs hatch into nymphs in 7–10 days, and the nymphs need another 9–12 days to be old enough to lay eggs. Without treatment, the number of lice therefore rises very quickly. This is why a single application is often not enough, and why treatment must be repeated exactly on day 7–10 to break the cycle before the next generation starts laying eggs, together with continued wet combing until day 17.

Written by Dr. Patara Voravutinon, Dermatologist · Last reviewed October 4, 2026

The information in this article is general knowledge and is not a diagnosis or an individual prescription. The choice of treatment, dose and duration should be assessed by a doctor.