How many times should a partner with a lice infestation apply permethrin (1% shampoo)?

Permethrin 1% Treatment for Lice in Partners

Partners with lice should apply permethrin 1% shampoo twice: once initially and again 7-10 days later, with many experts recommending routine re-treatment on day 9. 1

Initial Application Protocol

• Apply permethrin 1% to damp hair that has been shampooed with a non-conditioning shampoo and towel-dried 1
• Leave on for 10 minutes, then rinse thoroughly 1
• For pubic lice specifically, apply to affected areas and wash off after 10 minutes 1

Mandatory Second Application

• Repeat application in 7-10 days is essential because permethrin kills only 70-80% of eggs with the first treatment 1, 2
• The American Academy of Pediatrics specifically recommends routine re-treatment preferably on day 9 1, 2
• This second application targets newly hatched nymphs that survived the initial treatment 1

Why Two Applications Are Necessary

• Permethrin's residual effect on hair is impaired by conditioners and silicone-based additives present in modern shampoos, reducing its ability to kill emerging nymphs 1
• The 20-30% of eggs not killed by the first application will hatch within 7-10 days, requiring the second treatment 1
• An alternate schedule of days 0,7, and 13-15 has been proposed for non-ovicidal products, though this is not standard 1

When to Consider Additional Treatment

• Evaluate after 1 week if symptoms persist 1
• Re-treatment may be necessary if live lice are found or if eggs are observed at the hair-skin junction after the two applications 1
• If live lice are confirmed 7-10 days after a properly applied second permethrin treatment, switch to malathion 0.5% rather than applying permethrin a third time due to likely resistance 2

Critical Management Points

• All sex partners within the previous month must be treated simultaneously 1
• Sexual contact should be avoided until both partners have been treated and re-evaluated 1
• Bedding and clothing must be machine-washed/dried on heat cycle or dry-cleaned, or removed from body contact for at least 72 hours 1

Common Pitfalls to Avoid

• Do not assume treatment failure if itching persists after treatment—this is common and not an indication for re-treatment unless live lice are present 2
• Consider improper application as the first cause of treatment failure before assuming resistance 2
• Ensure non-conditioning shampoo is used before application, as conditioning products impair permethrin's effectiveness 1