How quickly does CAPACTION cat flea treatment work? Start killing your cat's fleas in the time it takes to watch one episode of your favorite sitcom. That's right - CAPACTION Oral Flea Treatment starts its job in thirty minutes, providing your cat with immediate relief. Almost all the adult fleas on your cat should be gone within four hours! CAPACTION is a pet flea medicine that can be used once a day if need be, although a single dose should do an effective job.