Metronidazole (Flagyl) can cause side effects. Common side effects from metronidazole use include nausea, headache, anorexia, diarrhea, abdominal cramping and vomiting. People who have a known hypersensitivity or allergy to metronidazole (Flagyl) or other nitroimidazole derivatives should not use the medication. People with liver and kidney problems should not use metronidazole unless directed to by their doctor. Metronidazole (Flagyl) and alcohol should not be used together. Questions and concerns about adverse effects associated with metronidazole use should be discussed with one’s medical provider and pharmacist.