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Fipronil is a phenylpryazole insecticide that was registered for use in 1996 (#NPIC). It is a nervous system disruptor effective on contact or ingestion. Fipronil is often used to treat rice seeds, and can be found in several tick and lice control medications for pets.

Chemical Description

Fipronil is a white powder with a moldy odor (#NPIC). Technical fipronil has a low solubility in water and does not bind strongly with soil (#CDPR). In aerobic soils, fipronil has a half-life of 122 to 128 days, where soil organisms degrade it into fipronil-sulfone. The half-life of fipronil-sulfone is 34 days (#NPIC).


Fipronil is used to control a variety of insects including ants, beetles, cockroaches, fleas, ticks, termites, mole crickets, thrips, rootworms, and weevils (#NPIC).

Fipronil may be found in turf products, seed treatments, pet care products, baits, and termite treatments (#NPIC). Fipronil is commonly used on rice crops, and for the treatment of rice seeds. It is also registered for use on commercial and residential turf. There are currently three tick and flea control products with fipronil for use directly on pets (#EPA).

Human Health Effects

Fipronil has a moderate acute toxicity to people and mammals. The oral LD50 is 97 mg/kg in rats and 95 mg/kg in mice. It has a slightly to moderately toxic when absorbed through the skin. The dermal LD50 in rabbits is 354 mg/kg. The dermal absorption rate is very slow. In a dermal toxicity study, researchers applied a radio-labeled fipronil solution to shaved rats. Less than 1% was absorbed after 24 hours (#NPIC).

In a study of chronic toxicity, where rats were fed fipronil daily for 52 weeks, researchers found changes in thyroid hormones, increased liver mass, and effects on the kidney. Seizures and seizure-related deaths were also noted. In a similar study, rats were fed fipronil daily for nearly two years. Benign and malignant tumors were observed in the thyroid gland. Due to this, the EPA has fipronil classified as a possible human carcinogen (#NPIC).

The inhalation of fipronil may cause convulsions and tremor (#NIOSH). Exposing the eyes can cause irritation (#PANNA).

Environmental Health Effects

In insects, fipronil disrupts the nervous system by blocking GABA gated chloride channels. This prevents the uptake of chloride ions, over stimulating neurons, causing paralysis and death (#CDPR).

Fipronil exhibits selectivity to insects by having a higher affinity for insect GABA receptors than mammalian GABA receptors. However, the primary biological metabolic product, fipronil-sulfone, is reportedly six times more effective in blocking GABA channels in vertebrates although it was a similar toxicity to the parent fipronil in mammals. In the environment, fipronil may degrade into fipronil-desulfinyl, a metabolite which is 9-10 times more active in mammals (#NPIC).

In mammals, fipronil has a moderate toxicity. It has a moderate to high toxicity to birds in acute exposures. The LD50 for Bobwhite quail is 11.3 mg/kg. Fipronil is very toxic to fish, and other freshwater orgamisms. Bluegill sunfish has an LC50 of 0.083 ppm. The fipronil-sulfone metabolite is even deadlier to aquatic organisms (#CDPR).

Because it is an insecticide, fipronil is highly toxic to honeybees and other non-target insects. Research found that fipronil killed 38.8-94.5% of beneficial predator insects when applied at rates suggested on the label. Prey consumption and reproduction also decreased (#NPIC).

Fipronil is only slightly mobile in soil, and is an unlikely water contaminant (#CDPR).


The general public can purchase fipronil in products such as lawn pellets, baits, and pet medications.

Precautionary Notes

Fipronil may degrade into products more or equally toxic. If handling fipronil, avoid exposed areas for a reasonable amount of time. Fipronil and fipronil-sulfone are highly toxic to aquatic organisms. Use caution near water sources. Fipronil may kill many beneficial insects, including honeybees and natural predators to pests.


California Department of Pesticide Regulation. Environmental Fate of Fipronil. (December 2001). [Accessed 8-8-10].

Environmental Protection Agency. Fipronil Use on Rice (Regen, Icon) and Pets (Frontline) HED Risk Assessment. (May 1998) [Accessed 8-8-10].

National Institute for Occupational Safety and Health. Fipronil. [Accessed 8-8-10].

Pesticide Action Network North America. Fipronil. [Accessed 8-8-10].

National Pesticide Information Center. Fipronil Technical Fact Sheet. [Accessed 8-8-10].

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