Serious side effects of Metronidazole are rare. Drinking alcoholic beverages while taking this medication may cause stomach pain. Metronidazole should be used with caution in patients with active or chronic severe peripheral and central nervous system diseases due to the risk of neurological aggravation. Flagyl possesses a wide spectrum of action in the attitude to anaerobic microorganisms. Antibiotics will not work for colds, flu, or other viral infections.