Several shampoos and lotions effectively kill head lice, but which one works best depends on whether the lice in your area are resistant to common over-the-counter ingredients. The most widely available option is permethrin 1% cream rinse (sold as Nix), but resistance to this ingredient now exceeds 98% in many parts of the United States. That means if you’ve tried permethrin and it didn’t work, you’re not doing anything wrong. The lice have simply evolved past it.
Why Standard Lice Shampoo Often Fails
Permethrin and pyrethrins (the active ingredients in most drugstore lice shampoos like Nix and RID) belong to a class of insecticides called pyrethroids. They work by attacking the louse’s nervous system. The problem is that lice have developed genetic mutations that make them nearly immune. A systematic review of resistance data found that pyrethroid resistance in North American head lice climbed from about 84% in the early 2000s to over 98% by 2016. Some studies have measured resistance as high as 99.6%.
These resistant strains are sometimes called “super lice,” and they’re now the norm rather than the exception. Permethrin is still considered a reasonable first try because it’s inexpensive, widely available, and safe for children as young as two months old. But if lice are still crawling after a full course of treatment, the product hasn’t failed. The lice are simply resistant, and you need a different approach.
Over-the-Counter Options That Still Work
Because resistance is tied specifically to pyrethroids, products that kill lice through a physical mechanism rather than a chemical one bypass resistance entirely. These are worth knowing about.
Dimethicone (silicone-based products): Lotions containing dimethicone coat lice in a thin layer of silicone that blocks the tiny breathing holes along their bodies. Contrary to what many people assume, the lice don’t simply suffocate. The silicone prevents them from excreting water, which causes a buildup of internal pressure that eventually ruptures their gut. Because this is a physical process, lice can’t develop genetic resistance to it. Several dimethicone-based products are available without a prescription.
Ivermectin 0.5% lotion: Originally prescription-only, this was approved for over-the-counter sale in late 2020 (previously sold as Sklice). It’s applied to dry hair, left on for 10 minutes, then rinsed. The major advantage is that it requires only a single application and no nit combing. It’s approved for anyone six months and older.
Prescription Treatments for Stubborn Cases
If over-the-counter products haven’t cleared an infestation, prescription options offer stronger results.
Spinosad (Natroba): This is derived from a naturally occurring soil bacterium. It kills both live lice and eggs, which means many people need only one treatment. Like ivermectin lotion, it doesn’t require nit combing, which is a significant time-saver for parents dealing with long or thick hair.
Malathion 0.5% lotion: This is a more intensive treatment. You apply it to dry hair, leave it on uncovered for 8 to 12 hours, then wash it out with regular shampoo and comb out the dead lice and eggs. A second application may be needed 7 to 9 days later if live lice are still present. One important safety note: malathion lotion is flammable. You cannot use a hair dryer, curling iron, or go near open flames while it’s on your hair or while your hair is still wet with the product. It’s approved for children six and older, and safety hasn’t been established for younger children.
Why the Second Treatment Matters
Most lice treatments kill live lice effectively but don’t kill all the eggs. Lice eggs (nits) are glued to hair shafts close to the scalp and hatch in about 7 to 9 days. If you only treat once, newly hatched lice from surviving eggs restart the infestation. That’s why the CDC recommends a second treatment 7 to 9 days after the first for any product that doesn’t reliably kill eggs. The timing is designed to catch newly hatched lice before they’re old enough to lay eggs of their own.
The exceptions are treatments like ivermectin lotion and spinosad, which kill eggs or newly hatched nymphs effectively enough that a single application is often sufficient.
What About Tea Tree Oil?
Tea tree oil is one of the few natural remedies with lab data behind it. In laboratory testing, a 1% concentration of tea tree oil killed 100% of head lice within 30 minutes. Its effect on eggs is weaker, though. It took a 2% concentration to prevent half of eggs from hatching, and combining tea tree oil with another plant compound called nerolidol improved the ovicidal effect significantly.
The catch is that lab results don’t always translate to real-world effectiveness. Applying tea tree oil evenly to every louse on a head of hair is harder than coating lice on a glass dish. There are no large clinical trials comparing tea tree oil head-to-head with approved lice treatments in actual patients. If you want to try it, it’s reasonable as a supplement to other methods, but it’s not a reliable standalone treatment.
The Wet Combing Method
If you want to avoid chemicals entirely, wet combing with a fine-toothed nit comb is the oldest and most reliable non-chemical approach. It works, but it’s labor-intensive. You saturate the hair with conditioner (which immobilizes the lice temporarily), then methodically comb from the scalp to the tips in small sections. This needs to be repeated every 3 to 4 days for at least two weeks to catch lice at every stage of their life cycle. Many families use wet combing in combination with a treatment product to improve their odds.
Choosing the Right Product by Age
Age restrictions matter, especially for young children. Permethrin 1% is approved for infants two months and older, making it the go-to for the youngest children. Ivermectin lotion is approved starting at six months. Malathion can’t be used on children under six. For babies under two months, manual wet combing is the only recommended approach.
Permethrin is also considered the preferred treatment during pregnancy. If you’re pregnant and dealing with lice, it remains the safest well-studied option despite resistance concerns, because resistant lice are annoying but not dangerous, and a second-line treatment can be discussed with a healthcare provider if needed.
Practical Tips That Affect Whether Treatment Works
Whichever product you use, a few details make or break the outcome. Apply treatment to dry hair unless the product label specifically says otherwise, since water dilutes the active ingredient. Don’t use regular conditioner before or after treatment, because it coats the hair shaft and can interfere with the product’s ability to reach the lice. Use enough product to fully saturate the hair and scalp. For thick or long hair, one box may not be enough.
After treatment, wash bedding, hats, and pillowcases in hot water (at least 130°F) and dry on high heat. Items that can’t be washed can be sealed in a plastic bag for two weeks. Lice can’t survive more than one to two days without a human host, so anything left off the head for 48 hours is no longer a risk. Vacuuming furniture and car seats is reasonable, but fumigating the house or bagging every stuffed animal in sight is unnecessary. The infestation lives on the head, not the house.