How Does Frontline for Cats Work?

Frontline is designed to kill fleas and ticks that feed on cats.
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When you use Frontline, fleas and ticks disappear. But how? The active ingredient in this flea and tick control medicine is fipronil, a pesticide used to kill ants, beetles, cockroaches, fleas, ticks, termites, weevils and other invertebrates. Frontline Plus adds methoprene, which kills insect eggs and larvae.

How Fipronil Works

Frontline kills fleas and ticks on contact. It works by disrupting the central nervous system in insects and invertebrates (ticks are arachnids). The chemical stops the neurons in the insect's brain from taking up chlorine, which causes an excess of neural activity that kills the parasite. Because mammals have different neurons than invertebrates, cats don't react in the same way to fipronil, making it safe to use on your kitty.

How Fipronil Provides Continuous Effects

When fipronil is applied to your cat's skin, it's stored in the oil glands beneath the skin surface. The chemical is then distributed continuously throughout the cat's skin and fur through the hair follicles. This is why you need to apply Frontline only once a month. If your cat digests fipronil -- for instance, after she bathes herself -- the fipronil will be stored in her fatty tissue, where it's broken down into smaller chemicals called metabolites. Your cat then passes the fipronil and its metabolites out of her system when she uses her litter box.

What Fipronil Doesn't Do

Fipronil kills fleas and ticks on your cat and any parasites that hop on after treatment, but does not kill flea and tick eggs or larvae. Because eggs and larvae do not have the same neural receptors as adult or pupal parasites, fipronil has no effect on them.

How Methoprene Works

Frontline Plus includes the insect growth regulator methoprene, which kills the eggs and larvae that fipronil misses.

Growth regulators such as methoprene stop insects from reaching the next stage of development, whether it be egg, larva, or pupa. They work by mimicking a hormone the insect needs to produce to mature. By preventing insect eggs and larvae from reaching the adult stage, the parasites cannot repopulate their host (your kitty).

Methoprene is ineffective on pupae and adult parasites, which is why it is paired with fipronil.

How Methoprene Affects Your Cat

Methoprene is considered nontoxic when ingested, so it shouldn't harm your cat when she bathes herself and ingests some of her medicine. It's not considered a skin irritant, which is why it's safe to use as a topical treatment. If ingested, methoprene is broken down by the digestive system and excreted through the cat's pee and poo.

According to the National Pesticide Information Center, trace amounts of methoprene can be stored in a cat's blood, liver, kidney and lungs, but the majority of the chemical passes through the system within five days of contact. That's why Frontline Plus needs to be reapplied monthly.

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