Candy was previously on Revolution but we changed her to Sentinel because she's just 4 months, finished with her shots and would have to keep visiting the vet to change the dosage with Revolution whereas Sentinel stays the same. So I have seen with my own eyes two fleas on Candy. I have read up on Sentinel. It makes the female flea sterile, so it cannot reproduce; however, if your dog goes into a flea infested area (I live on a farm property), the dog can still contract fleas...bottom line is I need to supplement the Sentinel to get rid of these existing fleas. What does everyone else use?