Cheristin is a topical flea control treatment developed just for cats. Cheristin acts rapidly to kill fleas in as little as 30 minutes and 98-100% fleas are gone in 12 hours. Cheristin has a simple and easy-to-use applicator which works for one full month. It is soft on cats and can be used on kittens over eight weeks of age. cats (8 weeks of age or older).