Permethrin is an insecticide in the pyrethroid family. Pyrethroids are synthetic chemicals that act like natural extracts from the chrysanthemum flower. Permethrin is used in a number of ways to control insects. Products containing permethrin may be used in public health mosquito control programs. They may be used on food and feed crops, on ornamental lawns, on livestock and pets, in structures and buildings, and on clothing. Permethrin may also be used in places where food is handled, such as restaurants. Permethrin was first registered with the United States Environmental Protection Agency (U.S. EPA) in 1979, and was re-registered in 2006.

Products containing permethrin may be liquids, powders, dusts, aerosol solutions, sprays, and treated clothing. Permethrin is used in cattle ear tags and flea collars, or in spot-on treatments for dogs. There are currently more than 1400 registered products containing permethrin. Some products are used to treat scabies and head lice on people. These products are drugs and are regulated by the U.S. Food and Drug Administration (FDA).

Permethrin can affect insects if they eat it or touch it. Permethrin affects the nervous system in insects, causing muscle spasms, paralysis and death. Permethrin is more toxic to insects than it is to people and dogs. This is because insects can't break it down as quickly as people and dogs. Cats are more sensitive to permethrin than dogs or people because it takes their bodies a long time to break it down.

People can be exposed to pesticides by eating them, breathing them in, getting them on their skin, or getting them in their eyes. Permethrin may be breathed in if a spray or fogger is used indoors, or if wind causes a spray or dust to be blown in someone's face. Dogs may be exposed to permethrin in products that are applied to their skin for flea and tick treatments. People can have skin exposure or breathe in products containing permethrin while applying the products, or during public health mosquito control efforts. Permethrin may be eaten if people forget to wash their hands after using products that contain permethrin. Exposure to permethrin can be limited by reading the pesticide label and following all of the directions.

Less than 1% of the permethrin put on the skin of people was taken into the body. If permethrin is eaten, most of it is quickly absorbed. The pyrethroids easily pass through the lungs into the body if inhaled, but no specific data on permethrin was found.

Once permethrin is absorbed, it quickly moves throughout the body. The greatest amounts of permethrin have been found 3 to 4 hours after it was eaten. Permethrin leaves the body mainly in the urine, but may also be in the feces. In laboratory tests in rats, half of the permethrin was gone from the animals' bodies within a day.

Permethrin was classified by the International Agency for Research on Cancer (IARC) as "not classifiable as to its carcinogenicity to humans" in 1991. This means that IARC could not decide whether or not permethrin can cause cancer. The U.S. EPA decided that permethrin was "likely to be carcinogenic to humans" if it was eaten. This decision was based on the structure of permethrin, what happens to it in the body, laboratory tests that caused tumors in mice and evidence of tumors in rats.

Researchers fed dogs and mice permethrin for up to 2 years and found that their livers increased in weight. The dogs fed permethrin had more tremors than dogs that did not eat it. Rabbits that had permethrin on their skin for 21 days had skin irritation, but no other signs of toxicity. Researchers are not sure whether long-term exposure to permethrin can affect the body's hormone system.

A group of women used a product containing 4% permethrin over their entire bodies to treat a scabies problem during pregnancy. Another group of pregnant women used a product containing 1% permethrin to treat head lice. Exposure to permethrin did not affect the pregnancies of any of the women in either group.

Rats that were fed permethrin when they were pregnant had offspring that weighed less, and some of their offspring developed extra ribs more often than control rats. Pregnant rabbits that were fed permethrin lost their fetuses more often and the offspring that lived had less bone growth.

While children may be especially sensitive to pesticides compared to adults, there are currently no data to suggest that children have increased sensitivity specifically to permethrin.

Permethrin is highly toxic to fish and other animals that live in either salt water or fresh water. Permethrin is low in toxicity to birds, but some aerosol products made with permethrin may also contain other ingredients that can harm birds if they inhale it. Permethrin is highly toxic to bees and other beneficial insects.