How Frontline Works

Frontline, marketed by Merial, effectively keeps fleas, ticks and lice off dogs and cats, when used as directed. It's safe to use on pregnant and nursing canines and felines, and on kittens or puppies older than 8 weeks. How does Frontline work? It doesn't enter the bloodstream, which lowers the risks of side effects. Instead, the product is released continually over the animal's body for a month after application.

Frontline contains two active ingredients. These consist of 9.8 percent fipronil and 8.8 percent S-methoprene, with the other 81.4 percent of ingredients inert. Generally, pets become flea-free within 24 hours after application, and remain so for the next 30 days. Wait 24 hours after application before bathing your pet or allowing him to go swimming or otherwise get wet. While dogs or cats might experience some mild irritation at the application site, other side effects are relatively rare. In some parts of the country, fleas are becoming resistant to Frontline. Ask your vet if that's an issue in your region. If so, she can recommend another flea and tick preventive.

Frontline is available either from your veterinarian or over the counter, with versions specifically designed for cats and dogs. Frontline is manufactured in topical or spray forms. Weigh your pet before purchasing Frontline, since the amount necessary for efficacy is based on weight. Wear gloves when applying either type of Frontline. If using the topical preventive, snip off the tip of the applicator and empty the contents onto the skin between your pet's shoulder blades — that way, he can't lick it. If using the spray, "ruffle" the animal's coat and spray thoroughly until damp. To protect the face and head, spray the product onto your glove and gently rub it in. Wash your hands thoroughly after using either form of Frontline.

Once applied, fipronil collects in your pet's hair follicles and skin oils, continuously releasing for the 30-day period and protecting the animal against parasites. Fipronil acts on the insect's nervous system, so that the creature can't relax muscles. Eventually, it becomes completely paralyzed and expires. If your pet is flea-infested, expect to see many fleas on his topcoat within hours after application — rather than on his skin, where they bite. That indicates the Frontline is eradicating the infestation.

S-methoprene works like an insect hormone. It's formally known as an "insect growth regulator." It's the element in Frontline that prevents fleas and ticks from reproducing on your pet, by interfering with normal reproduction. While it's used in various pesticides, you've probably unknowingly consumed it in food products. That's because S-methoprene is used on crops and livestock, including corn, cereals, peanuts and cattle. The National Pesticide Information Center reports that S-methoprene appears to be noncarcinogenic and is excreted quickly from the body when consumed.