Head lice treatment: what works, what doesn't, and why it failed last time

If you have ever had head lice treatment that didn't work you likely haven't had a resistant infestation. It's more likely not enough of the bottle was used, the application missed the scalp, or the second application was skipped. Here's what the trials actually show and how to get it right first time.

Before you buy anything, find a live louse, otherwise you may be wasting your money.

Head lice are about 2 to 3mm long, greyish-brown, and move fast, which is why they're easier to catch with a comb. The reliable way to check is detection combing: wet the hair, work conditioner through it, then comb from the scalp to the ends with a fine-toothed detection comb, wiping the comb onto white kitchen roll after each stroke. If there's an active infestation, you'll find a moving louse after combing a few sections of the hair.

Itching is usually the first sign of an infestation, which is an allergic reaction to louse saliva, not to the lice themselves, and on a first infestation it can take several weeks to develop; plenty of children are carrying lice before you even noticed. The same reaction works in reverse after treatment, so a scalp can stay itchy for a week or two once the infestation has actually gone. Treating again because the itching hasn't stopped is one of the more common ways people end up buying three bottles for one infestation.

Live eggs are laid within about a centimetre of the scalp, as they need the body heat to hatch. Once they hatch, the empty shells stay glued to the hair shaft and grow out with it. Hair grows at roughly a centimetre a month, so white cases sitting several centimetres down the shaft are the remains of an infestation that was dealt with weeks ago. They can sit there until they're combed out or the hair is cut, and they aren't a reason to treat.

Head lice move by direct head-to-head contact, which is why they are easily passed through primary-age children and the adults who sit next to them. Contrary to popular belief they can't jump and they can't fly, they can only crawl; they need a hair shaft to grip.

Away from a scalp a louse dehydrates and dies within a day or so, which is why transmission from hats, pillows, headphones and hairbrushes is uncommon rather than the main route.

They also have nothing to do with hygiene, clean hair is no barrier at all. A louse grips the shaft regardless of how recently and often it was washed.

You also can't catch them from pets, head lice are specific to humans. The lice that live on animals can't establish on a human scalp.

Neurotoxic insecticides were the original approach. Permethrin, a pyrethroid, and malathion, an organophosphate, both act on the louse nervous system. The problem with poisoning an insect is that the insect can evolve around it. Lice that carried mutations in the sodium channel the pyrethroids bind to survived treatment and passed the mutation on. Others developed enzymes that broke the insecticide down before it could act and after decades of heavy use in schools, resistant populations became the norm.

Physical insecticides are the type that most NHS local formularies and health guidelines commonly recommend, and they aren't poisons. Dimeticone, the active in Hedrin, is a silicone that spreads over the louse and into its breathing tubes, killing the lice by suffocation and disrupting their ability to regulate water

Isopropyl myristate with cyclomethicone, the combination in Full Marks Solution, works differently again. A louse is waterproofed by a thin layer of waxy hydrocarbons on its cuticle. The solution dissolves that layer off, and once the waterproofing is gone the louse dries out and dies.

Apply to dry hair. Wet hair dilutes the product and stop it forming the continuous coating it needs to do anything. This is the opposite of wet combing, and it's a genuinely easy mistake to make if you've done both at different times.

Buy enough. The label on each product gives a volume guide: when you've finished applying, the hair should be visibly soaked from root to tip, not damp in patches. If you're rubbing the last of a bottle into the ends to make it stretch, you probably didn't buy enough.

Cover the scalp, not just the hair. Section the hair with clips and work through it part by part, applying at the roots first and then drawing the product down the length. The eggs are laid within a centimetre of the scalp because they need the warmth, so the roots are where the product has to reach. Don't forget behind the ears and the nape of the neck, which are the two spots people consistently miss and the two spots lice favour.

Leave it on for the full contact time. Hedrin 4% lotion needs eight hours or overnight, which is the trade-off for its convenience at application. Full Marks and Lyclear Express are short contact times you can do in an evening, but check the carton you've actually got, because the directed time varies by pack.

Keep it away from heat. Silicone-based products are flammable while they're on the hair. No hairdryer, no straighteners, no smoking, nothing near a gas hob or an open fire until it's washed out. This gets left out of most head lice advice and it shouldn't be.

Comb it through afterwards. None of these products reliably kill every egg, and combing physically removes dead lice and eggs that the treatment didn't get. Use the fine-toothed comb in the pack, on the whole head, section by section.

Do the second application on day seven. This is not optional. Eggs take roughly seven to ten days to hatch, and a newly hatched louse takes another nine to twelve days to mature and start laying eggs of its own. So the second treatment has a window: late enough that the eggs surviving the first application have hatched and the young lice are now exposed, early enough that none of them have started laying. If you treat it too early you won't be killing anything and if you leave it to late there may already be a fresh generation of eggs on the scalp, setting a reminder on your phone for 7 days after the first treatment.

Treat everyone who has live lice on the same day. Check every head in the house with a detection comb, then treat everyone who's got them together. Treating one person on Monday and another on Thursday could result in the infestation moving between them.

Check again two weeks after the second application. Detection comb, same method as before, if there are no lice then you have successfully treated the infestation.

1. You didn't use enough. A 100ml bottle on shoulder-length hair coats the top layer and leaves the underside dry, and lice sitting in the hair that didn't get covered survive to repopulate the head.

2. It didn't reach the scalp. People apply like they're applying conditioner, working the product through the lengths and leaving the roots comparatively dry. The lice and the viable eggs are at the scalp; the ends of the hair are the least important part of the head.

3. Skipping the second application, or mistiming it. Either it wasn't done, or you did it too early because you found live lice on day three and panicked, or too late because life got in the way. Any of the three leaves a generation to mature.

4. You treated on itch alone. If you never found a live louse, there may not have been anything to kill. Post-infestation itching and old egg cases both look like ongoing lice and neither is.

5. Someone in the house wasn't treated. One untreated head that nobody checked, or an adult who assumed they couldn't have them, and the infestation cycles back through the family.

6. Genuine reinfection. Two weeks clear, then live lice again. This probably isn't the same infestation, it's a new infestation from the same source, usually school or a friendship group.

7. Actual resistance. Unlikely to be the cause. If you used a physical treatment, this is close to a non-issue; there's no target for the louse to evolve around. If you used an older insecticide product, it moves considerably further up this list.

If a treatment failed, don't immediately switch brands first. Do one course properly, with enough product, before you change anything. Unless you used an older insecticide containing permethrin.

If you've completed two full courses correctly, with enough product and both applications on time, and you're still finding live lice a fortnight later, stop buying treatments and speak to a pharmacist.

Wet combing gets recommended more than it should based on what the evidence supports, usually because it's advertised as chemical-free and that is safer than Hedrin etc.

The method is a fine-toothed detection comb through wet, heavily conditioned hair. The conditioner helps slow the lice down and stops them gripping to the roots and scalp, which is why plain wet hair doesn't work as well. Section the hair, comb from the scalp to the ends, wipe the comb on white kitchen roll after every stroke, and cover the whole head. Then repeat on days five, nine and thirteen, and keep going until you've had three consecutive sessions with nothing on the comb.

You must stick to the schedule, because combing removes lice but not reliably eggs, the sessions have to be spaced so that anything hatching since the last one gets caught before it's old enough to lay. Done properly it takes around half an hour per head per session, over two weeks, for a child who would rather be doing anything else. Roughly one in two people who commit to a fortnight of this clear the infestation, which is not as good as the rate of success from a product used correctly.

When wet combing is the right choice:

1. Babies under six months, where the products aren't suitable
2. Pregnancy or breastfeeding, if you'd rather avoid a product; a pharmacist can tell you which are suitable, but this is a reasonable preference
3. Known sensitivity or reaction to a treatment
4. Cost, if buying enough product for a household genuinely isn't manageable (Though check your local pharmacy, some can offer this on an NHS Care at the chemist scheme)
5. Detection, which is where a comb is unambiguously the best tool; you should be combing to confirm an infestation and to confirm it's gone regardless of how you treat it

When it isn't:

If you have long or thick hair, several children, or a household that isn't going to hold to a four-session schedule across two weeks. A half-finished combing course won't fully clear the infestation and it can take a lot of time to do the proper schedule, compared to using a product properly once, and again on day seven.

Stripping the beds and boil-washing everything. A louse away from a scalp is dehydrating and dying within a day or so. Wash the pillowcase and anything that has touched their head in the last 48 hours, but you can leave washing the bedding and clothing as they aren't how this is spreading.

Keeping children off school. There's no reason to as long as treatment has started. Head lice aren't a public health risk, they don't carry disease.

"No nit" policies. Some schools still send children home for egg cases in the hair. Old cases grow out with the hair and can persist for weeks after a successful treatment; excluding on that basis is excluding children who have already been treated. Speak to the school if they have this

Treating the whole household as a precaution. Check every head, treat the ones with live lice. Applying a product to people who don't have an infestation is money spent for no benefit.

Repellent sprays and conditioning products sold as prevention. These sit next to the treatments and the evidence behind them is thin at best. If you want to prevent an infestation catching hold, combing weekly during an outbreak at school does more than any spray, because it catches it early rather than repelling anything.

Tea tree oil, mayonnaise, vinegar, olive oil and the rest. Some have limited laboratory data against lice in a dish, which is a long way from a treatment that works on a head. Tea tree oil in particular is a common skin sensitiser, so on a child's scalp you're risking a reaction for no established benefit.

Cutting a child's hair. It doesn't treat anything. Lice need very little hair to hold on, and short hair is no protection; it can however make application easier and make it easier to spot lice.