Flea collars are usually comprised of two things, organophosphate and carbamate insecticides. These are completely different from the safe chemicals used in spot-on treatments.
Frontline Plus contains Fipronil and Methoprene.
Advantage contains Imidacloprid and Benzyl Alcohol.
Program contains Lufenuron.Organophosphates are considered highly toxic to cats; while there are many products containing OPâ€™s labeled for use on dogs only; pet owners often do not read the label carefully and apply them to cats resulting in significant toxicity and death.
Also avoid using flea products which contain "organophosphates" because cats have very little tolerance for them. One type of organophosphate in particular which some cat flea products and many dog flea products contain is "permethrin". If the product contains a very small concentration of permethrin it should usually be safe as long as you follow the instructions, but that doesn't mean that your cat may have a lower tolerance for permethrin than other cats. For an example of the difference between dog and cat flea products: some flea products for dogs may contain as much as 40-60% permethrin, whereas cat flea products contain less than 0.1% permethrin. This example shows why even a small amount of dog flea treatments can be lethal to a cat. If you do decide to use a flea product which contains organophosphates please be sure to follow the instructions carefully and rinse it off of the cats fur very thoroughly so that the cat can't ingest the chemicals and become sick. (from www.pets-opedia.com
is an article on carbomate insecticides (it's long).