Permethrin is a synthetic insecticide used to kill lice, scabies mites, ticks, and mosquitoes. It comes in two main forms for people: a 1% lotion sold over the counter for head lice and a 5% prescription cream for scabies. Beyond medicine, it’s widely used to treat clothing and gear for tick and mosquito protection, and in public health programs to control disease-carrying insects.
How Permethrin Works
Permethrin targets the nervous system of insects. It binds to sodium channels in nerve cells and forces them to stay open longer than normal, flooding the insect’s nervous system with uncontrolled electrical signals. This causes hyperexcitation, tremors, paralysis, and death. The reason permethrin is so useful for humans is selectivity: it’s highly toxic to insects but practically nontoxic to mammals and birds in acute exposures. Your body breaks it down quickly through liver enzymes, while insects cannot.
Treating Scabies
The 5% permethrin cream is the standard treatment for scabies in adults and children two months and older. It’s also considered the treatment of choice during pregnancy, classified as category B by the FDA, meaning animal studies haven’t shown fetal risk. For nursing mothers, it’s considered safe given that only about 2% of the cream is absorbed through skin, though breastfeeding should be paused during the 8 hours the cream is on.
To use it, you apply a thin layer from your neck down to the soles of your feet, making sure to work it into skin folds: between fingers and toes, around your waist, behind ears. You may need the entire tube. For babies and adults over 65, the cream also goes on the scalp, hairline, temples, and forehead. Leave it on for 8 to 14 hours, then wash it off in the shower. One application is usually enough, but if you see live mites two weeks later, you’ll need a second round.
Itching often continues for days or even weeks after successful treatment. This doesn’t mean the scabies are still alive. It’s your skin reacting to the dead mites and their waste as it heals.
Treating Head Lice
The 1% permethrin lotion is available without a prescription and has been a first-line treatment for head lice for decades. After applying it to clean, towel-dried hair, you leave it on for 10 minutes and rinse. A second treatment on day nine kills any lice that hatched after the first application, before they’re old enough to lay new eggs.
However, permethrin resistance in lice is now widespread. In some populations, over 90% of lice carry genetic mutations that make them resistant to the chemical. A study from Iran found that after a full hour of exposure to 1% permethrin, nearly 96% of lice were still alive. Resistance has been documented across many countries, driven by the same mutations in the insects’ sodium channels that prevent permethrin from binding effectively.
If permethrin doesn’t work after two treatments, alternatives include silicone-based suffocating agents like dimeticone, which kill lice physically rather than chemically, and prescription oral medications for stubborn cases. When switching products, choosing one from a different chemical class is more effective than trying another pyrethroid.
Clothing and Gear Treatment
Permethrin is widely used to treat clothing, boots, tents, and other outdoor gear for protection against ticks, mosquitoes, and other biting insects. It has been incorporated into military uniforms and is popular among hikers, hunters, and anyone who spends time in tick-heavy areas. The insecticide bonds to fabric fibers and kills or repels insects on contact.
There’s a significant difference between home-treated and factory-treated clothing. Spray-on products you apply yourself typically last about six washings or six weeks, whichever comes first. Professional factory treatment, where clothing is sent to a service that binds permethrin into the fabric, lasts through roughly 70 washings or an entire spring-to-fall season. The factory process creates a stronger bond with the fibers, which is why it survives so many more wash cycles.
Permethrin-treated clothing works differently from skin-applied repellents like DEET. It doesn’t repel insects at a distance. Instead, when a tick or mosquito lands on treated fabric, contact with the permethrin disrupts its nervous system within seconds, causing it to fall off or die before it can bite through to your skin.
Mosquito Control in Public Health
Public health agencies use permethrin to control adult mosquito populations, particularly in areas where mosquito-borne diseases pose a transmission risk. In south Florida, for example, it’s one of two insecticides used for flying mosquito control. Truck-mounted sprayers drive through residential neighborhoods after sunset, releasing ultra-low-volume mists that kill adult mosquitoes in flight. The after-dark timing targets the hours when mosquitoes are most active while reducing exposure to people.
Risks to Cats, Fish, and Bees
Permethrin’s safety profile has one critical exception among household pets: cats. Dogs tolerate permethrin well, and many flea and tick products for dogs contain it. But cats lack a specific liver enzyme (glucuronide transferase) needed to break permethrin down. Even small exposures, such as a cat rubbing against a recently treated dog, can cause tremors, seizures, and death. Permethrin-containing products intended for dogs should never be applied to cats or used in households where cats groom dogs.
In the environment, permethrin is highly toxic to fish and aquatic invertebrates. The EPA classifies agricultural and wide-area permethrin products as restricted use because spray drift and rainwater runoff can carry the chemical into streams and estuaries at concentrations harmful to aquatic life. Both freshwater and saltwater organisms are affected, and modeling shows that most agricultural and public health spraying scenarios exceed safe thresholds for fish and aquatic invertebrates.
Honeybees and other beneficial insects are also highly vulnerable. Product labels are required to warn against applying permethrin to blooming crops or weeds while bees are actively foraging. This is one reason mosquito spraying is done after sunset, when pollinators are less active.