Capstar tablets contain nitenpyram, an ingredient that acts on the nervous system of adult fleas already living on a cat. Each dose is intended for cats and kittens that are at least four weeks old and weigh between 2 and 25 pounds. The tablet can be placed directly in the mouth or mixed into a small amount of food, and it begins acting within about half an hour of administration. Because nitenpyram addresses fleas currently on the pet rather than eggs or larvae in the surrounding environment, a cat may pick up new fleas and need another dose. An additional tablet can be given as often as once a day if reinfestation occurs. Each carton includes six tablets, with each tablet containing 11.4 milligrams of nitenpyram, and the product should be stored at room temperature.