Do not use dog meds on cats. There have been many cat deaths because people figure it is the same stuff. Honestly, I don't know what ingredient is toxic to the cats. I'm sure someone else will have a better idea. But, DON'T use dog meds on cats!
Many dog flea preventatives contain pyrethrin or permethrin which could be deadly to a cat. Cats should not even be exposed to a dog that has just been treated with one of these products. Even if they don't contain those chemicals, they should only be used on the animal they are labeled for.