Because of the adverse effects of nausea, vomiting, and neurological toxicity, metronidazole should be given carefully. The drug must be infused slowly to reduce these side effects. Nausea and vomiting are common side effects of metronidazole that can cause you to feel sick to your stomach and need to throw up. This is especially dangerous if you have any other medical conditions, such as heart disease or diabetes. Infusing metronidazole into a vein rather than taking it orally may help to prevent or reduce these side effects. Neurological side effects include irritability, depression, confusion, and dizziness. These symptoms can be similar to those caused by alcohol or other drugs. If you experience any unusual behavior, talk to your doctor before you start treatment with metronidazole. Metronidazole can cause serious side effects including anemia, changes in blood cell counts, impaired liver function, diarrhea, abdominal pain, constipation, headache, insomnia, anxiety, nervousness, tremors, difficulty breathing, and hallucinations. If you experience any of these symptoms while taking metronidazole, contact your doctor immediately.