Nix is a 1% permethrin cream rinse that you apply to damp hair, leave on for exactly 10 minutes, then rinse out with warm water. It kills live lice on contact but does not kill unhatched eggs, so proper combing and a possible second treatment are essential to fully clearing an infestation. The process is straightforward, but small details in hair preparation and aftercare make a real difference in whether the treatment works.
Before You Apply: Hair Preparation
Start by washing hair with a plain shampoo, one that does not contain a built-in conditioner. This step matters more than most people realize. Conditioners, cream rinses, and 2-in-1 shampoos leave a coating on each hair strand that blocks permethrin from reaching lice effectively. For the same reason, skip any detangling sprays or leave-in products. After shampooing, rinse thoroughly with water and towel dry until the hair is damp but not dripping wet.
You should also avoid using conditioner for two weeks after treatment. Products marketed as “nit looseners” can also interfere with Nix and are best left on the shelf.
Step-by-Step Application
Shake the bottle well before opening it. Then follow these steps:
- Start behind the ears and at the nape of the neck. These are the warmest spots on the scalp and where lice concentrate most heavily. Work outward from there.
- Saturate completely. Every strand of hair and every inch of scalp needs to be covered. For thick or long hair, you may need more than one bottle.
- Protect the eyes. Drape a washcloth or towel across the forehead to keep the product from dripping into the eyes.
- Leave it on for 10 minutes, but no longer. Set a timer. Leaving it on longer does not improve effectiveness and can increase skin irritation.
- Rinse with warm water. Then towel dry and gently comb out any tangles with a regular comb before moving on to nit removal.
How to Comb Out Nits
Nix kills live lice but not unhatched eggs (nits), which are tiny oval-shaped specks glued to individual hair strands close to the scalp. Lice lay their eggs within about a quarter inch of the scalp surface, roughly the thickness of a pencil. Any nits you find farther down the hair shaft have likely already hatched or are dead. Removing nits by combing is one of the most important parts of the process, because every viable egg left behind can hatch into a new louse within a week or so.
Wet the hair and separate it into small sections, each about the width of the fine-toothed nit comb included in the kit. Comb each section starting right at the scalp and pulling all the way to the tip of the hair. Run the comb through each section several times before clipping that section out of the way and moving to the next one. A spray bottle of water helps keep the hair damp and manageable as you work through it.
After each pass, rinse the comb in a bowl of warm soapy water and wipe it clean with a paper towel. This prevents you from redepositing eggs or lice back onto the hair. For short hair that is hard to section, comb all the hair to the right, then to the left, then from back to front, repeating several times.
Plan to repeat this combing process every other day for two full weeks. This catches any nits you missed and any newly hatched lice before they are old enough to lay eggs themselves.
When to Apply a Second Treatment
Because Nix does not kill eggs, a second application is sometimes necessary. The CDC recommends retreating 7 to 9 days after the first application if you still see live lice. Permethrin does continue to kill newly hatched lice for several days after the initial treatment, which is why the waiting period exists: it gives remaining eggs time to hatch while the residual product on the hair can still kill them. If you spot live, crawling lice a week or more after your first treatment, go through the full application process again.
Possible Side Effects
Most people experience only mild reactions. A slight burning, stinging, or itching sensation at the scalp is common and usually fades quickly. Some people notice temporary numbness or tingling where the product was applied, or a mild rash. It is also normal for itching to temporarily increase right after the first application, even as the treatment is working. This happens because dead and dying lice can trigger an inflammatory response in the skin.
Severe burning, intense redness, or significant swelling of the scalp are not typical and warrant a call to your doctor. Signs of a true allergic reaction, such as hives, difficulty breathing, or swelling of the face, lips, or throat, require emergency medical attention.
Who Should Not Use Nix
Nix is safe for children two months of age and older. It should not be used on infants younger than two months. If you have a known allergy to chrysanthemums, ragweed, or synthetic pyrethroids, Nix can trigger a serious reaction, including breathing difficulty or an asthmatic episode. Permethrin is derived from a class of compounds related to natural chrysanthemum extracts, which is why this cross-reactivity exists. People who are pregnant or nursing should check with a healthcare provider before using it, as controlled safety studies in those groups have not been conducted.
A Note on Permethrin Resistance
Permethrin has been a first-line lice treatment for decades, but resistance is a growing concern. In some populations studied, nearly 60% of lice survived prolonged exposure to permethrin in laboratory settings. This resistance comes from genetic mutations that change the way lice nerve cells respond to the chemical, essentially making it less toxic to them. In countries where resistance is widespread, sales of permethrin-based products have declined while alternative treatments have gained ground.
In practical terms, this means Nix may not work for everyone on the first try. If you complete two full treatments with proper technique (no conditioner, full saturation, correct timing) and still find live lice, the issue is likely resistance rather than user error. At that point, a different treatment approach is needed, and your pharmacist or doctor can recommend alternatives that work through a different mechanism.
Cleaning Your Home After Treatment
Lice cannot survive long off a human head, typically no more than one to two days. Still, basic cleaning reduces the chance of picking them back up. Wash any bedding, pillowcases, towels, and recently worn clothing in hot water and dry on high heat. Items that cannot be washed, like stuffed animals or decorative pillows, can be sealed in a plastic bag for two weeks. Soak hair brushes, combs, and hair accessories in hot water (at least 130°F) for 5 to 10 minutes. Vacuum upholstered furniture and car seats. Extensive deep cleaning or fumigating sprays are unnecessary, as lice need human blood to survive and will die on their own without a host.