Nix starts killing lice within minutes of application, but it takes 8 to 12 hours to finish the job. If you still see a few lice moving slowly during that window, that’s normal. The active ingredient, permethrin, disrupts the nervous system of lice, causing paralysis and eventually death. You don’t need to reapply just because some lice are still twitching hours later.
How Nix Kills Lice
Permethrin, the active ingredient in Nix, works by interfering with nerve signaling in lice. It forces their nerve cells to fire uncontrollably, which leads to paralysis. Lice essentially lose the ability to breathe and die. This process doesn’t happen all at once for every louse on the scalp, which is why the 8 to 12 hour window matters.
You apply Nix to damp hair, leave it on for 10 minutes, then rinse. Even after rinsing, the product stays active. Permethrin binds to the hair shaft and continues working for up to 10 days, even through regular shampooing (as long as you skip conditioner, which can strip the residue). This residual effect is designed to catch newly hatched lice that emerge from eggs the initial treatment didn’t destroy.
What to Expect in the First 12 Hours
The CDC recommends checking your hair 8 to 12 hours after applying Nix. A few slowly moving lice at that point is not a sign of failure. They’re dying. Use a fine-toothed nit comb to remove any dead or still-moving lice from the hair.
If, after 8 to 12 hours, lice look just as active as they did before treatment, that’s a different story. It likely means the lice aren’t responding to permethrin, and you should talk to a healthcare provider before trying again with the same product.
Does Nix Kill Eggs Too?
Nix is marketed as effective against both lice and their eggs (nits). In practice, not all eggs are destroyed by a single treatment. Some nits survive and can hatch days later. The residual permethrin on the hair shaft is meant to kill these newly hatched nymphs before they mature, but it doesn’t always catch every one.
This is why a second application is recommended if you still see live lice or new nits seven days or more after the first treatment. That timing is intentional: it targets any lice that hatched after the first round but before they’re old enough to lay new eggs themselves.
The Resistance Problem
Nix was once the gold standard for lice treatment, but resistance has become widespread. A large meta-analysis pooling data from over 24,000 head lice found that 65% of lice carried genetic resistance to permethrin. The trend is accelerating. Before 2004, about 33% of lice populations showed resistance. After 2015, that number jumped to 82%.
These resistant lice, sometimes called “super lice,” have a genetic mutation that makes their nerve cells less vulnerable to permethrin. For these lice, Nix simply won’t work regardless of how long you leave it on or how many times you reapply. The 8 to 12 hour check is the best way to tell: if lice are still moving at full speed after that window, resistance is the most likely explanation.
Resistance rates vary by region, so Nix still works well in some areas. But if your first treatment clearly fails, switching to a different type of product (one that uses a different mechanism, like suffocating lice rather than attacking their nervous system) is more effective than repeating the same treatment.
Scalp Irritation After Treatment
Itching can continue for days or even a week or two after a successful Nix treatment. This doesn’t necessarily mean you still have lice. Permethrin can cause mild scalp irritation, including itching, redness, or a burning sensation. The bites lice left behind also take time to heal and can itch on their own.
The key distinction is between itching with visible live lice and itching without them. Comb through the hair carefully with a fine nit comb under good lighting. If you’re finding only dead lice and empty egg casings, the treatment worked and the itching should fade on its own.
Getting the Most Out of Nix
A few practical details affect how well Nix works. Apply it to freshly shampooed hair that hasn’t been treated with conditioner or a combination shampoo-conditioner. These products coat the hair shaft and can block permethrin from binding properly, reducing both the immediate kill and the 10-day residual protection.
After rinsing Nix out, avoid conditioner for the full 10-day residual period. Comb out dead lice and nits with a fine-toothed comb the same day you treat. Check again daily for the next week, removing any nits you find close to the scalp. If you see live, actively moving lice on or after day seven, apply a second treatment. If that second round also fails, it’s time for a different approach.