I switched to this a few years ago when the fleas were unbearable, even with the other medication I was getting from the vet. Within a week of the first dose of Advantage my whole home settled right down and the cats and I were no longer getting any bites. My cats are 16, 18, and 24 pounds. (Yes, the vet and I have gone rounds and rounds with the big boy.) The 9 pounds and up vial is fine for them. Just one vial, and an upset cat for a few moments for putting stuff on their shoulders, and the fleas are gone.