What medication can be fatal for a dog and why?

Dogs respond well to many human medications. However -- toxicity is a function of dosage! You can easily overdose a dog on what would me a low dosage for a human. This applies to ALL medications in all species. The majority of canine deaths from ibuprofen, Aspirin, etc. are not the medication's lethality per se, but the amount given the animal, either in a loading dose or over time. That said, the following human meds may be administered to dogs by a Vet who will determine dosage for you: * Ibupren (careful here -- EASY to overdose). * Aspirin * Benydryl * Ampicillin * Amoxycillin * prednisone * D5W And others... However, I can't emphasize enough the care you have to take in correct dosing. Once more, get the Vet to tell you safe dosages BEFORE administering. Note that this is not true for cats.