Over-the-counter cures for lice are mostly safe if you follow the directions, but they do contain low doses of poison. Most shampoos and cream rinses sold in drugstores contain pyrethrum (a natural insecticide), pyrethrin (derived from pyrethrum), or permethrin (a man-made version of pyrethrum).

Pyrethrum or pyrethrin products (such as Pronto, Rid, A-200, Triple-X, and R&C) kill lice but do not kill unhatched nits. Permethrin products, such as Nix, last longer; they don't kill nits, but they may kill newly hatched eggs for about a week after treatment. None of these medicines works all the time, and you may need more than one treatment.

Prescription medicines for lice should always be used with the utmost care. They contain higher levels of poison than over-the-counter medicines, so they're generally more effective. But if they're used the wrong way, they can also cause more harm.

The insecticide malathion (also used as an aerial spray to kill mosquitoes) has recently returned as a lice medicine. Malathion first appeared as a lice treatment in 1956 but was pulled off the market in 1994 because of poor sales. In 1999 the FDA approved it as a prescription drug (called Ovide) to treat lice. Malathion damages the nervous systems of lice, and it works very well. But it has drawbacks. It can sting when applied to open sores. It easily catches fire, so you must take extra care to avoid heat sources such as hair dryers, curling irons, and cigarettes when you use it. More disturbing, research suggests that malathion may be linked to some cancers and to nerve damage, but these problems seem to be rare and there's no evidence that the amount of malathion in lice medicines causes them. More studies are under way. Pregnant women, nursing mothers, and children younger than six should not use malathion.

Another poison, lindane, was once used in over-the-counter lotions, but is no longer. Lindane is strong enough to cause serious side effects, such as seizures, in a small number of people, mostly babies and young children. It was recently banned in California for fouling the water supply. The FDA has not banned lindane, but doctors no longer prescribe it for children. Tell your doctor if you prefer to use a lotion without lindane. Products with lindane have many names, so be sure to ask your doctor or pharmacist what you're getting.

If you are confused about which medicine is best for you, speak with your doctor. People who have seizures should not use any of these products without talking to a doctor first.

Don't use a lice treatment, either over-the-counter or prescription, more than once every ten days. Repeated treatments can put you or your child at risk of toxic exposure. Keep in mind that treatment can dry out your skin, so you may still itch for a week or two after the lice are dead.