Nix (permethrin 1%) is designed to be used up to two times per infestation, with the second application on day nine after the first. The product label says to apply a second treatment if you still see live lice seven days or more after the initial use. The American Academy of Pediatrics is more specific, recommending you always plan on a second treatment at the nine-day mark whether or not you spot live lice.
Why Two Treatments Nine Days Apart
Nix kills live lice on contact, but it does not kill unhatched eggs (nits). Those eggs take roughly seven to ten days to hatch. The second treatment is timed to catch any lice that emerged from surviving eggs before they’re old enough to lay new eggs of their own. If you skip the second application, a fresh generation of lice can restart the cycle.
Permethrin does have a residual effect, meaning it stays active on the hair shaft for several days after application and can kill some newly hatched lice on its own. But that residual protection isn’t strong enough to guarantee every new hatchling will die, which is why a second full application is the standard recommendation.
What Happens if Two Treatments Don’t Work
If you still see live, crawling lice after two properly applied Nix treatments, the issue is usually one of two things: application errors or resistant lice.
Common mistakes that reduce Nix’s effectiveness include applying it to wet or damp hair, which dilutes the active ingredient so lice are exposed to a weaker dose. Using conditioner or shampoos with silicone-based ingredients before treatment also causes problems. These products coat the hair shaft and prevent permethrin from sticking, which undermines its ability to keep working in the days after application. For best results, wash hair with a plain shampoo (no conditioner, no 2-in-1 formulas), towel dry thoroughly, and then apply Nix to hair that is as dry as possible.
The other possibility is resistance. Lice in many parts of the United States have developed genetic resistance to permethrin and similar compounds. If you’ve applied Nix correctly twice and live lice persist, repeating the same product a third or fourth time is unlikely to solve the problem. At that point, switching to a different treatment class makes more sense.
Alternatives After Nix Fails
Several other options work through different mechanisms, so permethrin-resistant lice are still vulnerable to them:
- Spinosad 0.9% topical suspension: A prescription product that kills both live lice and is more effective against eggs. A repeat treatment is only needed if you see crawling lice seven days later.
- Benzyl alcohol lotion 5%: A prescription treatment that suffocates lice rather than attacking their nervous system. It requires a second application after seven days.
- Malathion lotion 0.5%: A prescription option with a different chemical pathway. A second treatment is recommended if live lice remain seven to nine days after the first.
Your pediatrician or primary care provider can help you choose among these if over-the-counter Nix hasn’t cleared the infestation after two rounds.
Side Effects of Repeated Use
Permethrin 1% is generally mild. The most common side effects are minor scalp reactions: itching, redness, stinging, or a tingling sensation. These are uncommon and typically resolve on their own. If your scalp is already severely inflamed or broken out before treatment, permethrin can make that irritation worse, so it’s worth addressing any existing scalp condition first.
The product is approved for adults and children two years and older. There is no indication that using it twice within a nine-day window causes cumulative toxicity. The concern with using it more than twice isn’t really about safety to your skin or body. It’s that additional applications of the same product are a sign the lice may be resistant, and continuing to use an ineffective treatment just delays getting rid of them.
Tips for Getting the Most From Each Application
Since you ideally want to limit yourself to two treatments total, making each one count matters. Start with clean, conditioner-free hair that’s been towel dried until it’s barely damp. Apply enough Nix to fully saturate the hair from roots to tips, and leave it on for the full ten minutes before rinsing. Don’t rewash with shampoo immediately afterward, as this strips the residual permethrin that helps kill newly hatching lice in the days between treatments.
Combing with a fine-toothed nit comb after each treatment removes dead lice and loosened eggs. This won’t replace the second chemical treatment, but it reduces the number of viable eggs sitting on the hair shaft waiting to hatch. Comb through small sections methodically, wiping the comb on a paper towel between passes so you can see what you’re removing.
Mark day nine on your calendar and apply the second treatment regardless of whether you see live lice. The AAP emphasizes that two treatments are always needed. Skipping the second round because things look clear is the most common reason lice return a week or two later.