Head Lice Treatments Compared
Physical insecticides, malathion and wet combing — how each works, how to apply them so they actually work, and what to do when they have not.
Part of the Complete Head Lice Guide.
Key fact: Whatever you use, repeat it after 7 days. Treatments kill lice far more reliably than they kill eggs, and eggs already laid hatch over the following week. Skipping the second application is the commonest reason head lice appear to come straight back.
The three options
| Physical insecticide | Malathion 0.5% | Wet combing | |
|---|---|---|---|
| Examples | Dimeticone 4%, isopropyl myristate | Derbac-M liquid | Conditioner and a detection comb |
| How it works | Physically coats and suffocates or dehydrates the louse | Chemical insecticide | Physically removes lice |
| Contact time | 10 minutes to overnight, depending on product | 12 hours or overnight | Sessions of 10 to 30 minutes |
| Schedule | Repeat after 7 days | Repeat after 7 days | Days 1, 5, 9 and 13, check day 17 |
| Resistance | Not expected — the action is physical | Reported | Not possible |
| Suits | Most people, including pregnancy | Where a traditional insecticide is wanted or others have failed | Pregnancy, babies, anyone avoiding chemicals, and anyone who will stick at it |
Physical insecticides
Dimeticone is a silicone that coats the louse and blocks its ability to manage water and air. Because the action is physical rather than chemical, lice cannot develop resistance to it in the way they do to insecticides — which is why these products have largely become the routine first choice.
- Applied to dry hair, worked through thoroughly to the scalp and to the ends
- Contact time varies by product — some are 10 minutes, some 8 hours or overnight. Read the leaflet rather than assuming
- Repeat after 7 days
- Flammable — keep away from flames, cigarettes and hair dryers while in the hair
- Licensed in pregnancy, and usable from a young age with appropriate supervision
Malathion
Derbac-M is an aqueous 0.5% malathion liquid, the traditional insecticide option, suitable from six months of age.
- Applied to dry hair and left on for 12 hours or overnight
- Repeat after 7 days
- Aqueous rather than alcoholic, which matters — alcoholic preparations are avoided in asthma and on broken or eczematous skin
- Resistance has been reported, so a failure after correct use is a reason to switch approach rather than repeat it
- It has a strong smell, which is worth knowing before an overnight application on a child
Honest framing: malathion still works for many infestations and remains a recommended traditional option, but resistance is documented, and physical insecticides are not subject to it. If you have used malathion correctly, twice, and live lice remain, changing to a different mode of action is more sensible than a third round.
Wet combing
Free, chemical-free and genuinely effective — but only if done to the schedule, which is where most attempts fail.
| Session | Why then |
|---|---|
| Day 1 | Removes the lice present now |
| Day 5 | Catches lice hatched since, before they are old enough to lay |
| Day 9 | Same again |
| Day 13 | The last of the hatchlings |
| Day 17 | A check. No lice at the final sessions means it has worked |
The logic is that a newly hatched louse takes several days to mature and lay eggs, so combing every four days breaks the cycle without killing anything chemically. Missing a session lets one reach laying age, and the cycle restarts.
Choosing between them
| Situation | Reasonable choice |
|---|---|
| Straightforward case, want it dealt with quickly | A physical insecticide, twice, 7 days apart |
| Pregnancy or breastfeeding | Wet combing or dimeticone 4% first, malathion if needed |
| Baby under 6 months | Speak to a pharmacist or GP first — wet combing is usually preferred |
| Asthma, eczema or sensitive scalp | Avoid alcoholic preparations. Wet combing, dimeticone or aqueous malathion |
| Prefers no chemicals | Wet combing, done to the full schedule |
| A physical insecticide has failed twice, used correctly | Switch to a different mode of action |
Applying a lotion properly
- Use enough — long or thick hair needs considerably more than one bottle's worth for a single application, and under-dosing is a common cause of failure
- Get it to the scalp, section by section, not just over the surface of the hair
- Cover the whole head, including behind the ears and the nape of the neck
- Leave it on for the stated time — rinsing early is the second common cause of failure
- Comb through afterwards to remove dead lice
- Do not use a hair dryer while the product is in — several are flammable
- Do not treat anyone who has not had a live louse found
Pregnancy, babies, asthma and eczema
Check with a pharmacist or GP before treating if the child is under 6 months, if you are pregnant or breastfeeding, if there is asthma or eczema, or if the scalp is broken, weeping or crusted. Wet combing and dimeticone 4% are generally the preferred starting points in pregnancy, breastfeeding and very young children, with aqueous malathion used if needed. Alcoholic preparations are avoided in asthma and on damaged skin.
If it has not worked
| Possibility | What to do |
|---|---|
| The second application was skipped | The commonest reason. Complete the course |
| Not enough product, or rinsed early | Reapply properly, using enough to reach the scalp everywhere |
| Someone at home untreated | Check every household member and treat anyone with live lice, on the same evening |
| Reinfection from a close contact | Tell close contacts so they can check — head-to-head contact is the only route |
| Resistance | Switch to a different mode of action rather than repeating the same product |
| There were never live lice | Nits alone do not need treating. See how to check |
Resistance
Resistance to traditional insecticides is real and has changed practice. Permethrin resistance is widespread enough that permethrin products are no longer recommended for head lice at all, and malathion resistance has been reported.
Two things follow. Physical insecticides and wet combing are not subject to it, because neither works by a mechanism lice can adapt to. And repeat treating without confirming live lice — on the strength of nits, itching, or a school letter — is exactly the behaviour that selects for resistance, which is the practical reason the "find a live louse first" rule matters beyond any individual household.
Because treatments kill lice far more reliably than they kill eggs. Eggs already laid hatch over the following week, so a single application leaves a new generation to emerge a few days later. Repeat after 7 days unless the product specifically says otherwise. There is no single best one. Physical insecticides such as dimeticone coat and suffocate the louse and are not subject to resistance; malathion 0.5% is the traditional insecticide option; and wet combing removes lice mechanically with no chemicals. All three work if used properly, and the choice depends on age, pregnancy, skin conditions and preference. Wash and condition the hair, detangle, then comb with a fine-toothed detection comb from the scalp to the ends, checking the comb after each stroke. Repeat the whole head at least twice per session, and do sessions on days 1, 5, 9 and 13 with a check on day 17. Derbac-M is malathion 0.5% and is suitable from six months of age. In pregnancy, breastfeeding and very young children, wet combing or dimeticone 4% is generally tried first, with aqueous malathion used if needed. Check with a pharmacist or GP before treating a baby under six months or during pregnancy. Check the obvious causes first: the second application skipped, not enough product used, rinsed off early, or an untreated household member. If it was used correctly twice and live lice remain, switch to a different mode of action rather than repeating the same product, because resistance to traditional insecticides is documented.Frequently Asked Questions
Why do I need to treat head lice twice?
What is the best head lice treatment?
How do you do wet combing for head lice?
Is Derbac-M safe in pregnancy or for babies?
The treatment did not work. What now?
References
- NHS. Head lice and nits. nhs.uk
- National Institute for Health and Care Excellence. Clinical Knowledge Summary: Head lice. cks.nice.org.uk
- Welsh Medicines Information Centre. Head Lice: Questions and Answers for Healthcare Professionals. wmic.wales.nhs.uk
- UK Health Security Agency. Health protection in children and young people settings, including education. gov.uk
- Burgess IF et al. Randomised, controlled, assessor blind trial comparing 4% dimeticone lotion with 0.5% malathion liquid for head louse infestation. PLoS ONE 2007. journals.plos.org
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Head lice treatment should only be used when a live louse has actually been found, and product instructions vary, so always read the leaflet supplied. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.