Permethrin and pyrethrins are two different over-the-counter active ingredients for head lice, and the differences that matter are the age each one is approved for and what the CDC says each one kills. Permethrin lotion, 1% is approved for children two months of age and older. Pyrethrins combined with piperonyl butoxide are approved for children two years of age and older. Neither kills unhatched eggs, and lice can resist both. Here is what that means for the bottle in your hand.
Those facts decide more than the brand on the front of the box. They tell you which product you can use on your youngest child, why a single application is not the whole plan, and when it is time to stop opening packages and ask for help instead.
Permethrin And Pyrethrins Side By Side
Side by side, the two ingredients separate on three points you can act on: the age range each one is approved for, what each one kills, and the exact wording in the active ingredients line. The CDC’s clinical guidance on head lice says every over-the-counter product approved by the FDA for the treatment of head lice contains one of a short list of active ingredients, and it names both permethrin lotion, 1% and pyrethrins combined with piperonyl butoxide among them.
| What the CDC states | Permethrin lotion, 1% | Pyrethrins with piperonyl butoxide |
|---|---|---|
| Approved age | Children two months of age and older | Children two years of age and older |
| Live lice | Kills live lice | Kills live lice |
| Unhatched eggs (nits) | Does not kill unhatched eggs | Cannot kill unhatched eggs |
| After the rinse | Can kill newly hatched lice for several days after treatment | Kills only live lice |
| Resistance | Lice have developed some resistance to 1% permethrin, and the prevalence of resistance is unknown | Efficacy may be reduced by development of resistance, and the prevalence of resistance has not been well studied and is unknown |
Check the active ingredients line before you open any package, and use only a product made for people: the AAP says never to use medicines made for use on animals.
The Age Limits Are Not The Same
For a child under two years old, the age approvals settle the question before anything else does. The CDC says permethrin is approved for use on children two months of age and older, and that pyrethrin is approved for use on children two years of age and older. That gap is the clearest practical difference between the two for a household with a baby or a young toddler in it. The CDC also says to consult your healthcare provider before treating young children, and the AAP says to check with your child’s doctor before beginning any head lice treatment.
Guidance is just as specific about who handles the bottle. The AAP says medicine should be applied by an adult, that children should never apply it themselves, and that a child should not be left alone with medicine in their hair. It says to rinse the medicine off with warm water, not hot water, with the child’s head over a sink, and it warns that placing anything made of plastic on a child’s head is dangerous. The CDC adds that lice medications should be kept out of the eyes and flushed immediately if they get in.
One condition sits in front of both ingredients. The AAP says head lice medicine should be used only when it is certain that a child has living head lice, and the CDC says finding a live nymph or adult louse on the scalp or hair is the best way to diagnose an infestation. If no crawling louse has turned up yet, that is the step to finish first, and it is worth confirming lice before buying a shampoo rather than treating a scalp that may be itching for another reason. A product with an age limit on the label is a poor place to guess, which is the heart of the question about lice shampoo and toddlers under two.
Neither Ingredient Kills Unhatched Eggs
Neither ingredient kills unhatched eggs, and that one fact is why a single bottle and a single rinse are not a plan. The CDC says permethrin kills live lice but not unhatched eggs, and that pyrethrins can only kill live lice, not unhatched eggs. It also says permethrin can kill newly hatched lice for several days after treatment. Eggs already attached to the hair are the part of an infestation a rinse leaves behind, which is the real subject of whether drugstore lice shampoo kills the eggs, and they hatch on their own schedule: the CDC puts hatching at about a week, in a range of six to nine days, and the AAP says seven to twelve days depending on the surrounding temperature.
That is why the directions talk about a second application. For a medicine that kills lice but not eggs, the CDC says to treat again about a week after the first treatment, seven to nine days depending on the drug, after all the eggs have hatched but before new eggs are produced. Use the medicine as directed on the box or label, and talk to a healthcare provider before re-treating.
Killing lice and removing them are two different jobs, which is why the comb earns as much attention as the bottle. The CDC says to use nit combs to remove nits and lice from the hair shaft and, after each treatment, to check the hair and remove nits and lice every two to three days for the next two to three weeks. The AAP describes the same comb-out rhythm over the same two to three weeks, while noting that the comb-out method “usually doesn’t work on its own to get rid of head lice.” Nits may stay in the hair after treatment; the CDC says successful treatment will kill crawling lice, so a hair shaft that still holds specks is not proof of failure, and removing dead lice and leftover nits is its own task.
Resistance Is Real, And Its Reach Is Unknown
Lice can resist both ingredients, and the CDC says the prevalence of that resistance is unknown. The CDC is careful in both directions: lice have developed some resistance to 1% permethrin, but the prevalence of that resistance is unknown, and the efficacy of pyrethrins may be reduced by the development of resistance, though the prevalence of that has not been well studied and is unknown either. The AAP’s review of head lice treatment myths says head lice can be resistant to the active ingredients in over-the-counter treatments in some communities, and that the likelihood of resistance varies across the country.
The AAP says an over-the-counter product sometimes is not the most effective choice, and the CDC says incorrect use of a medicine can reduce its effect just as resistance can.
Resistance also does not justify stacking products. The CDC says the drugs used to treat lice are insecticides that can be dangerous if misused or overused, that the same person should not be treated more than two or three times with the same medication when it is not working, that different head lice drugs should not be used at the same time unless a healthcare provider or pharmacist directs it, and that extra amounts of any lice medication need that same direction. MedlinePlus says it in one line: use only one product at a time unless a health care provider tells you to use two. The AAP notes that parents sometimes treat their children with over-the-counter products up to five times before asking their pediatrician for help, and that pattern is worth interrupting early.
When A Treatment Fails, Resistance Is One Of Five Reasons
When a treatment fails, the CDC’s treatment page lists five reasons, and lice resisting the medication is only one of them. Here are all five:
- Misdiagnosis, meaning there were no live lice to kill.
- Combination shampoo or conditioner, or conditioner alone, applied to the hair before the medicine.
- Treatment instructions that were not followed.
- Head lice resisting the medication.
- Reinfestation after a treatment that worked.
Misdiagnosis is first on that list. The CDC says misdiagnosis of head lice infestation “is common,” and that nits get confused with dandruff, hair spray droplets and dirt particles. The AAP’s test is physical: nits are firmly attached to the hair, while dandruff, dirt and other particles are not. A magnifying lens and a fine-toothed comb help, and a brightly lit room with the hair parted section by section is where the answer shows up.
The conditioner line is easy to miss. The CDC says not to use a combination shampoo and conditioner, or a conditioner, before using lice medicine, and not to re-wash the hair for one to two days after the medicine is removed. The AAP gives that window as twenty-four to forty-eight hours with no shampoo. Instructions matter at the same level of detail: the AAP says it is essential for parents to read and follow manufacturer instructions exactly as written, and the CDC points out that shoulder-length hair or longer might need a second bottle.
Irritation is its own trap. The CDC says some medications may cause itching or a mild burning sensation, and the AAP says itching caused by head lice can last for weeks even after the lice are gone. An itch after a treatment is therefore not proof that the treatment failed, and our post on lice shampoo allergic reactions is worth reading before another round.
Reinfestation closes the loop. The CDC says to check all household members and other close contacts, and to treat everyone who is infested, including anyone who shares a bed, at the same time. One treated head in a house of unchecked heads is not a finished job.
If the bottle has already had its turn, the next step is a person, not another package. MedlinePlus says that if an over-the-counter treatment does not work for you, you can ask your health care provider for a prescription product, and the AAP says a pediatrician can help choose an appropriate treatment option, with several prescription treatments safe to use on children as young as six months of age and several calling for only one application. Reliable options are professional Lice Lifters treatment and Lice Lifters products, and our clinics work from a non-toxic, enzyme-based solution and a careful comb-out rather than from either insecticide on the shelf.
Frequently Asked Questions
Is permethrin the same as pyrethrin?
No, the CDC lists them as two separate active ingredients with different age approvals. Permethrin appears on that list as permethrin lotion, 1%, for children two months of age and older. Pyrethrins appear combined with piperonyl butoxide, for children two years of age and older. Two names that look alike in the same small print is exactly why that line deserves a slow read.
Does pyrethrin kill lice eggs?
No. The CDC says pyrethrins can only kill live lice, not unhatched eggs, so the eggs already stuck to the hair are not part of what that application reaches. Hatching happens on its own clock, which is why the label carries timing for a repeat application and why combing continues for weeks. Ask a healthcare provider before re-treating, and follow the label on the box.
Can I use a pyrethrin spray around the house?
No, the CDC says not to use fumigant sprays or fogs, because they are not necessary to control head lice and can be toxic if inhaled or absorbed through the skin. The AAP says not to spray pesticides in your home. What the CDC does recommend is washing and drying the items used in the two days before treatment, and vacuuming the floor and furniture where the infested person sat or lay.
Is a pyrethrin shampoo for humans the same as a product made for animals?
No, and the AAP is emphatic about it: never use medicines made for use on animals, and never use dangerous products like gasoline or kerosene. A product meant for people carries its own age limits and directions, and those are the ones to follow. One crossover is fine, though. The CDC says flea combs made for cats and dogs are also effective for combing hair.
How do I know whether the treatment worked?
Look for dead lice and for lice that have slowed down. The CDC says it takes time to kill all the lice, and that a few lice still moving slowly eight to twelve hours after treatment do not call for another application. If you find no dead lice at all, or the lice seem as active as before, the medicine may not be working, and that is the point to call a healthcare provider.
How long should I wait before treating again?
About a week, and only if live crawling lice are still there. The CDC’s position for a medicine that does not kill eggs is a second treatment seven to nine days out, timed so newly hatched lice are killed before they can produce new eggs. It also says to stop at two or three rounds of the same medication when it is not working, and to talk to a provider first.
What should I do if neither ingredient gets rid of the lice?
Stop adding rounds on your own and get a second set of eyes on the scalp. The AAP says to ask your child’s doctor if treatments you have tried have not gotten rid of lice, and that a third treatment is sometimes needed when live lice persist after the second. A professional check also settles the first item on the CDC’s failure list, which is whether live lice are there at all. You can find a Lice Lifters clinic near you.
Let A Lice Lifters Clinic Settle It
Our clinics exist for the moment you may be in right now: a bottle that did not finish the job, a child who still itches, and no one in the house certain what is on that scalp. A Lice Lifters treatment is non-toxic and safe for children of all ages, built around an enzyme-based solution and a meticulous comb-out by certified technicians in one same-day visit. Our 30 Day Guarantee applies when the whole family is checked. Find a Lice Lifters clinic near you and hand the question to someone who reads scalps all day.
