Permethrin 1% cream rinse is the most widely recommended over-the-counter treatment for head lice. The process is straightforward: wash your hair with regular shampoo, towel dry, apply the cream, wait 10 minutes, then rinse it out. But the details of each step matter, and a second treatment about nine days later is usually necessary because permethrin doesn’t kill unhatched eggs.
How Permethrin Works Against Lice
Permethrin is a synthetic version of a natural insecticide found in chrysanthemum flowers. It works by disrupting the nervous system of lice, essentially paralyzing them so they can no longer breathe. It kills live lice on contact and continues to kill newly hatched lice for several days after treatment.
The important limitation: permethrin does not reliably kill eggs (nits) that haven’t hatched yet. Lice eggs take about 7 to 10 days to hatch, which is why a second treatment is timed to catch any new nymphs before they’re old enough to lay eggs of their own.
Step-by-Step Application
Start by washing your hair with regular shampoo. Don’t use conditioner, and don’t use a shampoo-conditioner combination. Conditioner coats the hair shaft and can prevent the permethrin from working properly. Rinse the shampoo out thoroughly, then towel dry your hair. Let it air dry for a few minutes so it’s damp but not dripping wet.
Shake the permethrin bottle well. Then apply it generously, working it through the hair until the scalp and all of the hair are thoroughly saturated. Pay special attention to two areas people commonly miss: behind the ears and along the back of the neck. These are prime spots where lice congregate and lay eggs close to the warmth of the skin.
Leave the permethrin in place for exactly 10 minutes. Set a timer. Rinsing too early reduces effectiveness, and leaving it on longer than directed doesn’t improve results. After 10 minutes, rinse your hair thoroughly with warm water and dry it with a clean towel.
Nit Combing After Treatment
Once your hair is dry, comb through it with a fine-toothed nit comb to remove dead lice and any remaining eggs. While permethrin handles the live lice, the eggs glued to your hair shafts won’t fall out on their own. They’re cemented in place with a substance that’s surprisingly resistant to water and shampoo.
Work through small sections of hair at a time, starting as close to the scalp as possible and pulling the comb slowly all the way to the tips. Wipe the comb on a paper towel after each pass so you can see what you’re removing. This step is tedious, especially with long or thick hair, but it reduces the number of nymphs that could hatch before your second treatment. Some schools also have policies requiring nit-free hair before a child can return to class, making thorough combing practically necessary.
The Second Treatment on Day Nine
Plan to repeat the entire process about nine days after the first application. This timing is deliberate. By day nine, any eggs that survived the first treatment will have hatched, but the new lice won’t yet be mature enough to reproduce. The second round kills these nymphs and breaks the life cycle.
If you skip the second treatment and even a few nymphs survive, you can end up right back where you started within a couple of weeks. The re-treatment follows the same steps: shampoo, towel dry, apply permethrin, wait 10 minutes, rinse, and comb.
Cleaning Your Home and Belongings
Lice can’t survive long once they’re off a human head. They typically die within one to two days without a blood meal. Still, it’s worth taking a few precautions on the day you treat.
Wash pillowcases, sheets, towels, and any recently worn hats or scarves in hot water (at least 130°F or 54°C) and run them through a hot dryer cycle. Items that can’t be washed, like stuffed animals or decorative pillows, can be sealed in a plastic bag for two weeks. Vacuum upholstered furniture and car seats. You don’t need to fumigate the house or use insecticidal sprays on furniture, as lice don’t infest homes the way fleas or bedbugs do.
Combs, brushes, and hair accessories should be soaked in hot water (at least 130°F) for five to ten minutes.
Common Side Effects
Mild itching and redness on the scalp are the most common reactions, and they can be confusing because the lice themselves cause itching too. Scalp itching often continues for a few days after treatment even when the lice are dead, simply because the bites are still healing. Other possible reactions include a tingling or numb sensation on the skin and mild rash.
More serious reactions are rare but worth knowing about. Difficulty breathing, persistent skin irritation that worsens instead of improving, or pus-filled sores on the scalp are signs that need medical attention.
What to Do If Treatment Doesn’t Work
If you still see live, moving lice 24 to 48 hours after treatment, permethrin may not have worked. This can happen for a few reasons. The most common is incomplete application, particularly missing areas near the nape of the neck or behind the ears. Another possibility is resistance. Lice in some regions have developed genetic mutations that make them less susceptible to permethrin, and this has become increasingly common across North America.
If a full two-treatment course of permethrin (day one and day nine) fails to clear the infestation, prescription alternatives are available. Your doctor or pharmacist can recommend a different class of treatment that works through a different mechanism, which resistant lice won’t be able to withstand. The key indicator of true treatment failure is live lice that are still crawling after completing both rounds. Finding nits alone isn’t necessarily a sign of failure, as dead or empty egg casings can remain stuck to hair for weeks.
Tips That Make a Difference
- Check the whole household. Examine everyone living in your home before assuming only one person is affected. Treat all infested family members on the same day to prevent passing lice back and forth.
- Don’t double up on products. Using two different lice treatments at the same time doesn’t improve results and increases the chance of skin irritation.
- Skip the conditioner before treatment. For the same reason, avoid leave-in styling products, detanglers, or oil treatments before applying permethrin. These create a barrier between the medication and the lice.
- Use good lighting for nit combing. Natural sunlight or a bright lamp makes it much easier to spot the tiny, oval-shaped eggs, which are usually tan or yellowish before hatching and white or clear after.