I have a 3 year old sheltie that has never had a problem. We have always used advantix 20 and under, but he gained a pound and my vet bumped us up to the 21-55 lb tube. I asked him if it was ok he assured me it was, so we used the whole first tube. That was a week ago now he hasn't had any side effects besides acting lethargic and just not himself. I called the vet today and the lady told me he was allergic to something in it. I'm just so worried about him now, I would just go crazy if something happened to him.. I'm just so upset about this now. I'm so afraid there is going to be some long term effect.