According to Mahaney, amoxicillin is not recommended for dogs that have previously exhibited clinical signs of intolerance or an allergic reaction. He says that intolerance can include signs such as digestive upset (vomiting, diarrhea, or lack of appetite), lethargy, or changes in behavior. Signs of an allergic response can include digestive upset, as well as skin reactions such as redness, swelling, or hives. A potentially fatal type of allergic reaction, called anaphylaxis, is ...