Merial, the manufacturer of the flea-and-tick treatment Frontline, says the product is safe to use on dogs. It does note, however, that any pesticide may sometimes cause an adverse reaction and advises that if signs of these appear, owners should seek advice from a veterinarian. However, in 2009, The New York Times reported that the Environmental Protection Agency (EPA) was investigating claims of more serious side effects amid suggestions that the existence of counterfeit products may be to blame.
Frontline can cause skin irritation. The manufacturer's application instructions for Frontline advise owners to wash off any spilt fluid promptly, if it comes into contact with their skin, to prevent redness and irritation.
Effects of Ingestion
Headaches, nausea, sweating and stomach pains can result from the accidental swallowing of Fipronil--the active ingredient in Frontline---according to the National Pesticide Information Center. Seek medical advice if someone swallows the product.
Seizure if Swallowed
In extreme cases where Fipronil is swallowed, seizure may result. Seek medical advice.
The National Pesticide Information Center reports that Fipronil, the active ingredient of Frontline, is classified as a possible human carcinogen, indicating that there are concerns that repeated exposure to the substance may put people at greater risk of developing cancer.
Skin Irritation in Dogs
Dogs may have irritation of the skin at the place where Frontline was applied. Merial describes this as a "rare" and "temporary" side effect, but advises owners to seek veterinary advice if the irritation is persistent or becomes more severe.
Hair Loss in Dogs
According to The New York Times, there have been "a large number" of anecdotal reports of dogs losing hair after Frontline was applied.
Tremors in Dogs
The New York Times again notes "a large number" of anecdotal reports of dogs experiencing tremors after receiving a dose of Frontline.
Seizures and Death of Dogs
The EPA reported in a 2009 press release that some owners had claimed their dogs suffered seizures after Frontline was applied and that some dogs later died, although the agency could not confirm if these claims were accurate. In the case of any adverse reaction to the medication, the agency advises owners to immediately bathe the pet, using a mild soap and rinsing after with large quantities of water.