The active ingredient effective against fleas, imidacloprid, spreads across your dog’s skin and coat within 12 hours. After this time, re-infesting fleas are affected within 3-5 minutes of contact with your dog, and killed within 1 hour. Unlike chewable tablets, the active ingredient is not absorbed into your dog’s bloodstream, so fleas are killed on contact without the need for a blood meal. As your dog naturally sheds their skin and fur, tiny imidacloprid crystals are also shed into the environment, killing flea larvae, and breaking the flea lifecycle.