After application, the active ingredient imidacloprid spreads across the skin and coat, killing existing fleas within 12 hours. After this time, re-infesting fleas are affected by the active ingredient in 3-5 minutes and killed within 1 hour, on contact and without the need for a blood meal. Imidacloprid is shed into the environment as your pet sheds the skin and fur, killing flea larvae within 20 minutes. This breaks the flea lifecycle.