It depends on which topical it is. Frontline is interchangeable. It's actually the same ingredients just different dosages based on the size of the pet. I have heard that fleas in many areas have built up a resistance to Frontline and it doesn't work well. We used to have a chart somewhere here that shows how much to use on a cat. Some people with rescues or several outdoor cats would get the Frontline for large dogs and measure it out for their kitties because it was much cheaper that way.Please be careful doing this. Seen somewhere using flea control for dogs can kill your cats. Not sure about which product. Been using revolution because my young cat always lick it off somehow, if she doesnt reach it, she uses her paw. And revolution is safe orally and still works. Revolution for cats btw.