This is one vets opinion. Cat flea collars are just about useless. There are better and safer alternatives to flea control.
If you choose to use a flea collar, make sure it is labeled for cats only. It is very important not to use a dog flea collar on a cat!
IN my opinion, cat flea collars do little to kill fleas and some of the ingredients can be toxic to some cats. There are safer alternatives to flea collars available on the market for cats. For example, some of the new oral and topical medications can work very well and are safer. Examples of products include: Frontline®, Program® and Capstar®.
For more information on Cat Flea Control Problems, please read Flea Infestations in Cats