Major Side Effects You should check with your doctor immediately if any of these side effects occur when taking metronidazole. More common: Agitation back pain blindness blurred vision burning, numbness, tingling, or painful sensations in the hands or feet changes in speech patterns confusion convulsions decreased vision depression dizziness drowsiness eye pain fever hallucinations headache irritability lack of coordination nausea seizures shakiness and unsteady walk slurred speech stiff neck or back trouble speaking unsteadiness, trembling, or other problems with muscle control or coordination unusual tiredness or weakness vomiting weakness in the arms, hands, legs, or feet Less common: Black, tarry stools blood in the urine or stools body aches or pain chills clumsiness or unsteadiness difficulty with breathing ear congestion feeling of pelvic pressure frequent or painful urination loss of voice nasal congestion pinpoint red spots on the skin runny nose skin rash, hives, redness, or itching sneezing stomach and back pain (severe) unusual bleeding or bruising vaginal irritation, discharge, or dryness not present before taking the medicine Rare Bleeding gums bloating chest pain constipation cough dark-colored urine fast heartbeat indigestion loss of appetite painful or difficult urination pains in the stomach, side, or abdomen, possibly radiating to the back sore throat sores, ulcers, or white spots on the lips or in the mouth swollen glands yellow eyes or skin