We were using Front Line and Pongo got Fleas. We went to the Vet and the Vet told us that the Front Line has become less successful down here. She recommended the Comfortis and Pongo has not had anymore fleas. They did not have any bad side effects either, but she did tell us to give it to them with food. We will be staying with the Comfortis, I do not want to take a chance otherwise with Pongo. He is very allergic to fleas.