Understanding how fipronil and s-methoprene kills fleas safely and stops fleas multiplying

When used together, they deliver a one-two punch: fipronil kills the grown-ups, and S-methoprene stops the babies from ever growing up.

How Fipronil kills adult fleas and ticks

Fipronil works by targeting the flea’s nervous system. It blocks certain receptors (called GABA receptors) that control the insect’s nerve signals. This disruption causes hyperexcitation and the flea’s nerves go into overdrive and eventually die.

The clever bit? Fipronil is selectively toxic. That means it affects insects way more than it does mammals. Your cat or dog won’t be harmed by the same mechanism because their nerve systems are different.

When you apply spot on flea treatment to the back of your pet’s neck, Fipronil spreads across the skin via the oil glands. It stays in the skin and hair follicles, continuing to zap fleas for weeks without needing to enter the bloodstream. Meaning after 48 hours it is water-resistant, just don’t get them wet before this! Click here to know how to correctly apply flea treatment.

How S-Methoprene stops fleas multiplying

S-Methoprene plays the long game. It mimics a hormone that fleas naturally produce — a hormone that tells young fleas to stay immature. When exposed to S-Methoprene, flea eggs won’t hatch properly and larvae won’t develop into adults.

It’s basically like tricking the baby fleas into thinking they’re not ready for adulthood — and as a result, they never reproduce. That means fewer new fleas, and the whole flea life cycle is brought to a grinding halt.

This is crucial because adult fleas are only about 5% of an infestation. The other 95% are eggs, larvae, and pupae hiding around your home. You can kill the adults, sure but if you don’t stop the cycle, they’ll just keep coming back.

Most pets tolerate these treatments really well. Occasionally, you might see minor skin irritation or a bit of scratching after application, but this usually clears up quickly. If anything your pet seems to be reacting to the flea treatment, it might actually be Flea Allergy Dermatitis.

What makes this combo so effective?

Fipronil acts fast to kill fleas that are already on your pet, giving them immediate relief. Meanwhile, S-Methoprene acts like a birth control for fleas, stopping new generations from developing. Together, they treat both the symptoms (itchy, biting fleas) and the source (hidden flea eggs in your environment).

And because the treatment lingers on your pet’s skin and coat, it continues to protect them long after application — usually around four weeks. That means fewer repeat treatments, less stress for your pet, and less hassle for you. Try the Itch Flea treatment for Cats and Itch Flea treatment for Dogs on subscription to get your pet’s tailored flea treatment straight to your door, every month, for free.

Dealing with fleas can feel like a losing battle but you’re most likely to end an infestation for good with treatments containing Fipronil and S-Methoprene, like Itch Flea treatment for Cats and Dogs. You're not just treating fleas, you're beating them. Here’s to a flea free, forever.