How Do Common Flea Medications Work?

By Jennifer Kvamme, DVM on May 20, 2011

By Jennifer Kvamme, DVM

Treating your pet for fleas, or trying to prevent flea infestations, can be confusing. This is partly because there are so many different options available and the fact that they work in various ways. Here, a basic review …


Certain species of the chrysanthemum flower have been used for hundreds of years as insecticides and repellants. The naturally occurring chemicals obtained from these flowers are called pyrethrins.

Pyrethrins are one of the most common ingredients used in flea and tick control products for pets today. They work by disrupting the normal function of an insect’s nerve cell, causing it to fire impulses non-stop — ultimately leading to the insect’s death. Pyrethrins are low toxicity, making them quite safe for use in mammals. They are applied directly to the animal’s skin or hair to control not only fleas, but also ticks, lice, mites, and mosquitoes.

Pyrethrins are not able to withstand prolonged exposure to light, air, or moisture. Because of their instability, they are usually combined with other chemicals that will protect them from breaking down.

Pyrethroids are the manufactured, or synthetic versions of pyrethrins. They work similarly to pyrethrins, but are more stable, and thus slightly more toxic. Synthetic pyrethroids last for some time and are typically used in topical medications that are meant to cover the animal’s body (called spot-on products). Pyrethroids can also be found in sprays that are used to treat the household for insects. They are not safe for use on cats or kittens.

Other Plant Extracts

A few other plant-derived flea deterrents include rotenone, d-limonene, and linalool. Rotenone is a chemical that can be extracted from the roots of several types of tropical and subtropical plants. It works by paralyzing insects and preventing oxygen uptake to the cells. It is fatally toxic to fish, but generally considered safe for use in small amounts with small animals.

D-limonene and linalool are both obtained from citrus fruit pulp. They work by softening the hard outer shells of insects, causing them to dry out and the insects to die. Citrus products are usually used in flea shampoos and dips. Care must be used when treating cats, as they may be sensitive to the oils from citrus.

Citrus may help repel fleas, but will probably not eliminate a full-on infestation of fleas on your pet and in your home. If you have an infestation, you may need to use a combination of citrus along with more potent chemicals to mount a large-scale attack that will eradicate all of the insects.   

Commonly Used Flea Treatment Chemicals

Imidacloprid is a topical insecticide that works by blocking nervous system conduction in insects. The majority of adult fleas are reportedly killed within 24 hours of application, reducing the chance for them to lay eggs. Imidacloprid is typically mixed with an oily carrier, so that when it is applied directly to the animal’s body, it spreads out over the body and collects in the hair follicles, where it continues to work for approximately one month’s time to kill adult fleas, larvae, and eggs.

Fipronil and metaflumizone are both used in spot-on products. Fipronil can also be found in a spray formula. These chemicals also target the chemical functions in the nervous system of insects, resulting in paralysis and death.

Selamectin is a chemical that kills both internal and external parasites by blocking nerve signal transmissions. It is applied as a spot-on and enters the bloodstream via the skin. It not only kills intestinal parasites, it also protects against heartworm disease as well as fleas (adults and eggs), ticks, and some mites. Dinotefuran is a chemical that kills insects on contact. It kills all stages of the flea life cycle by interfering with nerve signal conduction.


Oral Flea Control Chemicals

Lufenuron is an oral product, meaning that it is ingested by the animal, rather than applied to the outer body as with the above examples. The chemical is then stored in the animal’s fat and transmitted to adult fleas when they bite the animal. Any larvae produced by these adults will not have the ability to produce an exoskeleton, causing them to die. It will not kill adult fleas, however, so another chemical will be needed to control home infestations. 

Spinosad kills adult fleas only and is approved for oral use only in dogs. The active chemical is found in bacteria living in the soil. Spinosad works by over stimulating the insect’s nervous system, causing death. It should be used carefully in animals with epilepsy.

Nitenpyram is an oral flea control product that is approved for use in both cats and dogs. It works by blocking nerve receptors in the insect, killing adult fleas on the animal in about 30 minutes. It does not have a long-term effect, so it cannot be used for continuous flea control. This product is good for short-term trips to areas where fleas might be present -- such as dog parks, shows, trials, or boarding kennels.


Remember that drugs are never without risk. Whichever product or products you choose to use for parasite control, be sure to read the label carefully and get advice from your veterinarian if your animal is very young or old, is sick, or is debilitated. In addition, if your pet experiences mood or behavioral changes after being given parasite prevention products, or if s/he becomes ill, contact your veterinarian immediately.

Image: fairuz othman / via Flickr


Jennifer Kvamme, DVM


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