Metronidazole is given by mouth or injection and is used off-label to treat certain anaerobic bacterial and protozoal infections and gastrointestinal conditions in dogs, cats, and other animals. Give as directed. Side effects may include nausea, vomiting, diarrhea, regurgitation, decreased appetite, tiredness, and drooling. Do not use in pets that are allergic to it, are debilitated, or are pregnant or nursing. If a negative reaction occurs, contact your veterinarian.