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What is Permethrin?

Mary McMahon
Updated Feb 05, 2024
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Permethrin is a broad-spectrum contact insecticide which kills a wide variety of arthropods on contact, including fleas, ticks, scabies, lice, and bees, among many others. This product is sold for a variety of purposes, ranging from flea control in dogs to treatments for wood to deter damage from boring insects. Some caution is advised when using permethrin, as it is fatal to cats and fish, and it has been linked with tumors in laboratory animals.

Historically, permethrin was derived from chrysanthemums, but by the 1970s, it was being produced synthetically. This insecticide works as a neurotoxin, overstimulating the nervous system of insects which come into contact with it. Once of the problems with permethrin is that it is an indiscriminate pesticide, killing all insects which come into contact with it, including beneficial arthropods. Mammals are also at risk of developing health problems if they ingest this insecticide or are exposed to large amounts of it, and some people experience violent allergic reactions when they use it to treat parasitic infestations.

Several companies manufacture permethrin in a wide variety of formats. Sprays can be used to treat a wide area for insects, and they are sometimes used by campers and aid organizations working in developing nations to keep insects out of beds and clothing. Permethrin is also used in the treatment of some wood to prevent insect infestation.

In medical use, topical applications of permethrin are used to prevent fleas in dogs, although pet owners should be aware that these products should never be used on cats. Permethrin creams and lotions are also available to treat lice, scabies, and other parasitic infestations in humans. Typically, only one application is required, as the insecticide will last through several washings.

Before someone uses a permethrin product to treat a parasitic infestation, he should discuss his health with a medical professional. Permethrin can interact poorly with some medications, and it is also not advised for use by pregnant women and nursing mothers. A history of allergies, asthma, and some other conditions may also contraindicate its use.

When using permethrin to treat wood, garments, tents, and so forth, people should be aware that this type is not intended for topical use in humans and other animals, because it is very strong. The material should be allowed to dry completely after an application of spray to avoid coming into direct contact with the permethrin, and wearing gloves during the spraying process can be advisable as well. An application will typically last for two weeks or two washings, whichever comes first.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a WiseGeek researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments
By anon338329 — On Jun 12, 2013

Permethrin is very dangerous to cats and the warnings often say so. It is common for dogs in flea collars. The military has been treating its jungle clothing with it for more than 30 years. And yes, it is, in fact, toxic to fish. Treat whatever you need away from cats and lakes and once it dries it is OK.

I take teams out to the Amazon jungle to give free medical and dental care to the poor. All our stuff is treated: hammocks, clothes, suitcases, duffel bags -- anything fabric. Once it dries, it it totally odorless even when the item gets wet form rain.

It is also normal to use it in shampoo for lice and scabies, so you can touch it when wet but do not rub eyes. Once dry it is not very toxic; I have spent time looking into it. Humans do not absorb it into the skin easily, but it works. The cheapest way to get it is buy fire ant or termite treatment. A quart of 13 percent solution gives you 26 quarts and costs the same as a quart of premixed .05 percent sold in a camp store. It lasts a long time because it bonds to fabric, not because it is so highly toxic -- unless you are a cat or fish. For keeping mosquitoes away it is better than getting malaria, dengue fever, yellow fever, Lyme disease or whatever else ticks can carry.

By Perdido — On Sep 19, 2012

@DylanB – Did you know that you can actually buy clothing that has been pretreated with permethrin? I was in a sporting goods store one day when I saw these, and I just shook my head.

The shirts cost nearly $50! Do people not know that they can buy a $10 shirt somewhere and just spray it with their own permethrin? It's amazing what some stores come up with to get more money from their customers!

By DylanB — On Sep 18, 2012

I've heard that permethrin spray is very effective. I have friends who like to go camping, and they spray their clothes, hiking boots, and tents with it a day before they go to give it time to dry.

It's amazing to me that the spray can survive the washing machine! You know it has to be powerful in order to do this.

My friends like to camp near a lake, and they are not allowed to use the spray near the water, because it can kill fish. However, they usually just spray everything before arriving, since it needs to dry before they touch it, anyway.

By OeKc05 — On Sep 17, 2012

You can buy permethrin treatments for dogs in the supermarket at a low price. However, just because it's inexpensive doesn't mean it is the ideal treatment.

My sister used it on her fifty-pound German shepherd, and it had a bad reaction. It became very lethargic and ill, so she couldn't use it on her anymore.

I've even heard of permethrin causing horrible problems in cats. My neighbor's cat had to be hospitalized because of it for two days, and she was one of the lucky ones who survived. Many don't make it through.

By orangey03 — On Sep 17, 2012

To me, it sounds like this permethrin insecticide does more harm than good. It might get rid of bugs, but there is just too much risk involved.

It is tragic that it could harm the very animals you might be trying to protect by using it. I believe I will stay away from permethrin products, for the good of my family and my pets.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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