Permethrin has a residual effect that will continue to kill nits for several days after the first application. While one application should be sufficient to kill lice and nits, some experts suggest a second treatment one week after the first. Although resistance to permethrin has been reported from other countries, no resistance has been reported in the United States. Shampoos which contain pyrethrin kill lice quickly but do not leave a residual that will continue to kill nits, resulting in a less effective treatment. Two applications of these types of shampoos 7-10 days apart are recommended to kill nits.
Shampoos containing lindane are available by prescription only.
Lindane is not recommended for young children, or pregnant or nursing women, and should only be used if other approved therapies did not work or cannot be tolerated. Misuse or overuse of lindane may be toxic. There are widespread reports from countries other than the United States of lice being resistant to lindane. Alternate treatment methods such as the use of tub butter or margarine, mayonnaise, Vaseline and olive oil, designed to smother head lice and nits can be used with discretion for those persons for whom lice-killing medications have failed. The effectiveness of alternate treatments is unclear.