In the past, I've used advantage & frontline for my dogs. About 3 years ago, I noticed that neither prevention was able to keep the fleas at bay. I don't know if the fleas built up an immunity or what. I began giving the dogs Comfortis, a once a month vet prescribed pill. No side effects . . . worked like a charm. I've been using it ever since, & my dogs continue to be flea free.