Metronidazole is an antibiotic that is used to treat a wide variety of infections. It works by stopping the growth of certain bacteria and parasites.This antibiotic treats only certain bacterial and parasitic infections. It will not work for viral infections (such as common cold, flu). Using any antibiotic when it is not needed can cause it to not work for future infections.Metronidazole may also be used with other medications to treat certain stomach/intestinal ulcers caused by a bacteria (H. pylori).