Our vet recommends using Advantage. She said that the cheaper flea drops are not good for a cat, and they build up in the cat's liver. She also said that they don't last as long. They must have been invented for northern, yankee, cats.
Advantage is expensive, so to cut the cost, I purchased the drops for large cats. I put half a tube on each cat, thereby making it the correct dose.
Of course, I wrote "half a tube per cat" on the box, just in case another family member isn't as thrifty as I am.