Has your dog been vomiting while on doxycycline? It is a common side effect. My pug was being treated with doxy X 4 weeks for anasplasmosis but had a terrible time with vomiting. The vet said doxycycline is usually quickly broken down and absorbed, so even with some vomiting she felt the medication should have gotten into her system. However, after several weeks symptoms returned, so we theorized that she actually did not get her full treatment dose. She is getting 2 more weeks prescribed and we'll see how she does.