While most dogs tolerate amoxicillin well, some may experience adverse reactions. The most common side effect is digestive upset, which may manifest as diarrhea, vomiting, or appetite loss. These symptoms are usually mild and temporary but should be reported to your veterinarian if they persist or worsen. Dogs may develop an allergic reaction to amoxicillin, although it is relatively rare. Signs of an allergic reaction can include facial swelling, hives, itching, difficulty breathing, or collapse. If you notice any of these symptoms, seek immediate veterinary care. In some cases, prolonged use of antibiotics like amoxicillin can disrupt the normal balance of bacteria in the body, leading to an overgrowth of opportunistic pathogens. This can result in a secondary infection. Contact your veterinarian if you observe any new or worsening symptoms during or after treatment. Less commonly, dogs may experience side effects such as lethargy, drooling, coordination problems, or changes in urination patterns. If you notice any unusual or concerning symptoms, consult your veterinarian.