Pills containing nitenpyram (Capstar) start to work almost immediately, within 30 minutes. The nitenpyram leaves the cat's body in approximately 24 hours and with that the protection from fleas also disappears. It is recommended to give the tablet regularly for ongoing relief from fleas. Pills containing Spinosad (Comfortis) also start to work quickly and kill fleas for approximately one month. Be sure to read the label and the instructions for administering before purchase.