Unfortunately, there are rare individuals that can not tolerate any of the topical spot - on products, and other flea control options such as sprays or dips may be needed. (vin.com)
There are other types of flea control options, of course, but this happens to be one of the most effective. (tinpaw.com)
Talk to your vet about flea control options, then be sure to treat all the cats in you house: If one has fleas, they probably all do. (pets.webmd.com)