Pyrethrins and piperonyl butoxide for lice

Examples

These pyrethrin medicines are available without a prescription as shampoos or gels that are left on the hair for 10 minutes and then rinsed out. A second treatment is needed 1 week after the first, to kill newly hatched lice.

How It Works

Pyrethrins kill lice and some of the eggs (nits) at the time it is applied. Unlike permethrin, these products do not continue to work after they have been rinsed out of the hair.

Why It Is Used

Pyrethrin medicines are another common, useful treatment for head and pubic lice.

People who are allergic to ragweed or chrysanthemums should not use products containing pyrethrins.

How Well It Works

Pyrethrin products are useful against lice, but treatment failures are getting more and more common.¹ Some countries have reported an increase in resistance to pyrethrin.² If this occurs, other treatments (such as malathion or permethrin) or a combination of treatments (such as permethrin cream along with trimethoprim sulfamethoxazole) can be used instead, after waiting at least a week.

Side Effects

Pyrethrin products have few side effects.

See Drug Reference for a full list of side effects. (Drug Reference is not available in all systems.)

What To Think About

These products can be less effective than permethrin (Nix) for treating head and pubic lice. Pyrethrin products usually require two treatments in order to get rid of lice.¹

Overuse of lice medicines (such as reapplying the shampoo too soon) can irritate the skin and may increase the risk of side effects from some products. It is common for itching to persist for 7 to 10 days after treatment. Itching is not a reason to treat the person again.

Complete the new medication information form (PDF) (What is a PDF document?) to help you understand this medication.

References

Citations

1. Drugs for head lice (2005). Medical Letter on Drugs and Therapeutics, 47(1215/1216): 68-70.

2. Frankowski BL, et al. (2002). Head lice. Pediatrics, 110(3): 638-642.

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