Get emergency medical help if you have any of these signs of an allergic reaction to metronidazole: hives; difficulty breathing; swelling of your face, lips, tongue, or throat. Call your doctor at once if you have any of these serious side effects:
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numbness or tingling in your hands or feet;
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white patches or sores inside your mouth or on your lips;
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pain or burning when you urinate;
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diarrhea that is watery or bloody;
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vision problems, pain behind your eyes;
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trouble concentrating, slurred speech, mood or behavior changes, tremors, muscle twitching, seizure (convulsions);
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fever, chills, muscle pain, confusion, headache, sore throat, neck stiffness, increased sensitivity to light, drowsiness, nausea and vomiting; or
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severe skin reaction -- fever, sore throat, swelling in your face or tongue, burning in your eyes, skin pain, followed by a red or purple skin rash that spreads (especially in the face or upper body) and causes blistering and peeling.
Less serious metronidazole side effects may include:
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stomach pain, diarrhea;
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dizziness, loss of balance;
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vaginal itching or discharge;
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dry mouth or unpleasant metallic taste;
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cough, sneezing, runny or stuffy nose; or
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swollen or sore tongue.