I have the spray on Frontline, for use with horses. I just put some on a cat that turned up at the barn with fleas. The label says for use with dogs, cats, puppies and kittens. Don't know if it is the same stuff you get in the snap-open packets . . .
Frontline(and Advantage) are the ONLY products that are safe to be used on both. The dog version can be put on cats in the correct amount, and the cat version is safe for dogs. Again, in the correct amount in order to be effective, but that is the only difference between the 'cat' and 'dog' versions, as well as the diff sizes for dogs.