Thanks for the information. I always note that any dog can develop any allergy to any medication at any time. It is possible, but not very likely. These type of allergies are uncommon and rare with these particular medications. The Vetmedin can cause some nervous system problems that cause uneasiness or anxiety, but I wouldn't suspect the itching unless there is a an allergic reaction. The allergic reaction would most likely cause a red rash on the skin or nasty looking sores. I would be more concerned about an allergy to something else in the environment, food or fleas. Fleas would be a big concern if most of the itching occurs in the rear end, especially if she is not on a flea preventative (even if you don't see fleas). A food allergy can develop to the same food they have been on for years as well. The itching is most likely causing the panting and agitation. In most cases, I would recommend 25mg of Benedryl, but I would be concerned about using it with her if she is already agitated or has heart problems. A bathe with an oatmeal based or other soothing shampoo may help relieve some of the itching tonight, until your vet can evaluate her in the morning. If she seems to get much worse tonight, I wouldn't hesitate having her evaluated by your nearest emergency vet. Please let me know if you have any other questions. Hope this helps.