Problems with Frontline Spot-On for Dogs

Updated November 21, 2016

Frontline is a common flea preventive for dog owners, which is prescribed by a veterinarian. However, there are many problems associated with Frontline of which pet owners should be aware. The active ingredient in Frontline is an insecticide called fipronil, which can cause adverse reactions in many dogs or have long-term effects. The reactions tend to mild but, in extreme cases, can lead to seizures and have led to increased scrutiny of spot on flea treatment, which includes Frontline, by the U.S. Environmental Protection Agency.


According to Merial, which produces Frontline, Frontline Plus is the #1 veterinarian-recommended flea and tick protection. Merial claims that Frontline Plus kills 100% of fleas within 12 hours and continues to work for 30 days. It is safe for puppies and kittens as young as eight weeks and is waterproof. The active ingredient, fipronil, is stored in the oil glands under your dog's skin and is continuously distributed into the skin and hair through the hair follicles. It works by attacking the fleas' nervous systems, causing paralysis and death.


The World Health Organization classifies fipronil as a Class II moderately hazardous toxin when applied orally or inhaled. Fipronil can potentially disrupt the central nervous system, which can lead to uncontrolled nervous system activity and, in severe cases, death. Symptoms of fipronil poisoning include excitability, lack of coordination and tremors. In the mildest cases, it can simply irritate your pet's skin, causing rashes and itchiness.

Warning to Humans

Because Frontline is applied topically to a dog's skin, humans actually come in contact with the chemical when petting their dogs. A study by K. A Jennings et al. published in "Veterinary and Human Toxicology" tested the levels of exposure by petting dogs after 24 hours and at weekly intervals with white gloves. While there was no residue transmitted to the gloves after five weeks, there were significant amounts after 24 hours, enough for Jennings et al. to conclude that repeated exposure, such as monthly application, could pose serious health risks for humans as well (Reference 3). Fipronil is classified as a Group C (possible human) carcinogen.

Warnings for Long-Term Use

Merial claims that fipronil cannot interfere with a dog's central nervous system, as it does with the fleas, because it isn't absorbed into the bloodstream from the skin, making it perfectly safe. However, tests done on rats cast doubt on these claims. Thus, organs that could be affected by repeated exposure include the liver, kidney and thyroid. If you are concerned about your pet, contact your veterinarian for a blood panel.

Taking Action

The U.S. Environmental Protection Agency has stepped up evaluation of flea and tick spot on products, such as Frontline, due to a number of complaints about adverse reactions. If your dog has such a reaction, visit and use the Ask a Question function to contact it. Include the name and EPA registration number of the product, active ingredients of the product if known, breed and age of animal, length of time between application and reaction, description of reaction, date reaction occurred, city and state where it occurred and your contact information.

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