I've used Sentinel and have for the last 2 years. I like that it has the heartworm and flea protection in one. Its also what my vet recommends. I just placed an order online for it for the first time so i can save money. I don;t know why i didn;t think of doing that before. All you have to do is tell the vet so that they fill out the fax prescription from when it comes in.