Most lice shampoos need to stay on the hair for about 10 minutes, and lice should be dead within 20 minutes to a day after treatment. But “working” has two timelines: how long you leave the product on, and how long until the infestation is actually gone. A single application rarely finishes the job because most over-the-counter shampoos don’t kill eggs, so the full process takes about 9 to 10 days.
How Long to Leave It On
The standard over-the-counter lice shampoos, which contain either permethrin or pyrethrin, require 10 minutes of contact time on dry or towel-dried hair. That’s it. Leaving the product on longer than directed doesn’t improve effectiveness and can irritate the scalp. Use a timer rather than guessing.
Prescription options follow a similar pattern. Ivermectin lotion also calls for a 10-minute application. Silicone-based treatments (dimethicone) work by physically coating and suffocating lice rather than using a chemical insecticide, and research has shown a single 15-minute application can kill both lice and eggs.
When Lice Actually Die
If the treatment is working, lice should be dead within 20 minutes of application. You can check by combing through wet hair with a fine-toothed nit comb after rinsing. Dead lice won’t move when you place them on a white paper towel or tissue. Some may still twitch slightly right after treatment due to residual nerve activity, but they should be clearly immobile.
If you find lice that are still crawling normally a full day after treatment, the product likely didn’t work. This is a sign of resistance, which has become increasingly common with permethrin and pyrethrin products. At that point, switch to a different type of treatment rather than repeating the same one.
Why One Treatment Isn’t Enough
Killing the live lice is only half the problem. Lice eggs (nits) are glued to hair shafts close to the scalp, and most over-the-counter shampoos don’t kill them. These eggs hatch in 6 to 9 days, producing tiny nymphs that will grow into egg-laying adults within another week or two if left untreated.
This is why a second treatment is essential. The goal is to catch newly hatched lice before they’re old enough to lay eggs of their own. The timing depends on which product you used:
- Permethrin or pyrethrin products: Repeat 9 to 10 days after the first treatment.
- Spinosad: Repeat in 7 days if you still see lice.
- Malathion: Repeat in 7 days if you still see lice.
- Oral ivermectin: Repeat in 7 to 10 days if needed.
So while the shampoo itself works in minutes, the full treatment timeline is closer to 10 days from start to finish. Some people need that second application, and some don’t, but skipping it is the most common reason infestations come back.
What to Do Between Treatments
The days between your first and second treatment aren’t passive waiting time. Combing through wet, conditioned hair with a fine nit comb every two to three days removes both dead nits and any nymphs that hatch before your next application. This dramatically improves your odds of clearing the infestation in one cycle.
Focus on the hair within a quarter inch of the scalp, since that’s where viable eggs are laid. Nits found further down the hair shaft are usually empty shells or non-viable. Work in small sections under good lighting, wiping the comb on a paper towel after each pass so you can see what you’re pulling out.
Signs the Treatment Worked
After your second application, check the scalp carefully over the following week. If you find no live, moving lice during comb-throughs at days 2, 4, and 7 after the second treatment, the infestation is cleared. Finding empty egg casings or dead nits still attached to hair is normal and doesn’t mean treatment failed. Those will grow out with the hair over the coming weeks.
If you’re still finding live lice after two rounds of the same product, resistance is the likely culprit. Don’t simply repeat the same treatment a third time, and don’t combine multiple products at once. Switch to a product with a different active ingredient, or try a silicone-based suffocant, which works through a physical mechanism that lice can’t develop resistance to.