A second application of lice shampoo can kill newly hatched lice, but you may want to wait a week between treatments to prevent scalp irritation.

If you or someone in your household has lice, you’re probably eager to get rid of them.

Maybe you’ve already gone through one round of treatment with over-the-counter (OTC) lice shampoo and still notice a few stray lice. You might wonder if you can shampoo a second time to get rid of the lice right away.

Using lice shampoo twice in a row won’t get rid of lice any faster, though. More importantly, it may even cause some irritation. Lice shampoos contain pesticides such as pyrethrins and permethrin — and while these ingredients are safe to use as directed, they may cause unwanted side effects when used in excess.

We reached out to Dr. Richard Antaya, Director of Pediatric Dermatology at Yale Medicine, for answers to frequently asked questions about applying lice shampoo. Read on to get the details.

You shouldn’t apply lice shampoo twice in a row, Antaya says, as it’s not intended for this purpose.

Keep in mind, it can take at least 8 to 12 hours for the treatment to kill adult lice. If you notice lice moving on your head a few hours after treatment, that doesn’t mean the treatment is ineffective.

That said, OTC lice shampoos don’t kill lice eggs (nits), Antaya says.

“It’s imperative to use it again a week later to kill any newly hatched lice,” Antaya says, going on to explain that using it 2 days in a row won’t kill the new lice.

Instead, reapply lice shampoo about 7 to 9 days later.

The lice treatment you choose will come with directions describing how to apply the product and how long to leave it on your head. This time frame can vary from product to product, but Antaya says you can leave some lice shampoos on for 10 minutes.

You may feel tempted to leave the shampoo on for longer than directed, but doing so won’t help kill off any more lice.

Lice die after absorbing the active ingredients in the shampoo, but this takes time — often up to a day.

Experts have tested the 10-minute time frame and determined it’s enough time for the shampoo to work — without causing unnecessary skin irritation for most people, according to Antaya.

Again, it’s always best to use the shampoo as directed to avoid any unnecessary irritation.

You can wash most lice shampoos off after about 10 minutes, as noted above — there’s no need to leave them on any longer. It’s important not to leave any OTC lice treatments on your head overnight.

If you’re using coconut oil to smother lice, however, you can leave this natural remedy on overnight.

Learn more about natural remedies for head lice.

It’s fine to shampoo your hair as usual before application, Antaya says.

Just skip conditioner, since hair products that coat your strands may make the lice treatment less effective.

“I recommend not using a conditioner a few days after to ensure that it doesn’t interfere with treatment,” Antaya says.

Learn more about why you shouldn’t use conditioner after a lice treatment.

Lice shampoo has FDA approval, and experts generally consider it safe when people use it as directed. It does pose a risk of potential side effects, though, including some mild skin irritation.

Although severe allergic reactions are rare, in some cases, they do happen.

Get medical attention right away if you have symptoms of a serious reaction, including:

  • severe discoloration or a rash that doesn’t improve
  • itchiness or swelling of your face, tongue, or throat
  • serious or lingering eye irritation
  • an infected or pus-filled area on the scalp
  • dizziness
  • difficulty breathing

Applying too much lice shampoo may increase your risk of side effects. For that reason, Antaya advises against using more than directed.

Serious allergic reactions to lice shampoos are rare, as noted above.

But if you have a ragweed or chrysanthemum allergy, the Centers for Disease Control and Prevention advises that you do not use any lice treatment containing pyrethrins. People extract pyrethrins from chrysanthemum flowers, so these treatments may cause breathing difficulties or an asthma attack.

Instead, opt for a treatment that contains permethrin, as evidence suggests permethrin typically doesn’t cause an allergic reaction.

Even if you don’t have any allergies, it never hurts to do a patch test — especially if you have sensitive skin. To do one at home, apply a small drop of the shampoo behind your ear. If you don’t notice any irritation or other unwanted side effects within about a day, you can go ahead and use the treatment as directed.

You can also ask your doctor about getting an in-office allergy test.

You can prevent head lice in the first place by avoiding the following:

  • Engaging in head-to-head contact with others: Lice primarily spread when one head touches another head. Since children often play close together, they generally have a higher chance of getting head lice. Encouraging your kids to keep a bit of physical distance during playtime and sports activities can help you avoid lice.
  • Sharing clothing and accessories that touch your head: In some cases, you can get head lice by sharing items like hats, scarves, brushes, or other hair accessories. Experts consider this less common, but it’s still possible.
  • Resting your head on fabric or furniture: Lice don’t live for long after they fall off their human host, but they may live for up to 2 days on furniture, carpet, stuffed toys, bedding, and similar items.

If someone in your house has head lice, these steps may help prevent additional infestations:

  • Vacuum your furniture and floor thoroughly.
  • Wash all fabric items that may have lice, including sheets, pillowcases, clothing, etc. Use hot water that’s at least 130°F (54.5°C) and dry these items on high heat.
  • Boil combs, brushes, and other washable hair tools for 5 to 10 minutes in water that’s at least 130°F (54.5°C).
  • Dry clean items that can’t be washed, or seal them in a plastic bag for 10 days to suffocate the lice.

Wondering when to contact a doctor about a head lice infestation?

Antaya recommends trying OTC treatments first. Use them as directed, repeating the treatment after 1 week.

But if you find live lice on your head a few days after the second treatment, connect with a dermatologist or another doctor for professional treatment, he says.

A doctor can recommend prescription medication that can help kill any lice resistant to OTC treatments.

If you or someone in your household has lice, Anataya sympathizes: “Anyone who has had lice wants the treatment to work and work fast,” he says.

But for best results, try using lice shampoo as directed. Applying it 2 days in a row or leaving it on overnight won’t make the treatment any more effective — but it may irritate your scalp.