Dogs and Cats

Metronidazole (Flagyl) is used to treat protozoal infections in cats and dogs including Giardia, Entamoeba, Trichomonas, and Balantidium.
It is also used to treat anaerobic bacterial infections.
Metronidazole has immune modulating activity and may be prescribed to treat inflammatory bowel disease.
It may be used to treat colitis caused by other antibiotics, periodontal disease (especially in cats), Clostridium perfringens enterotoxemia, tetanus, diarrhea of undetermined cause, pancreatic insufficiency (with small intestinal bacterial overgrowth), and complications of severe liver disease.
Metronidazole may be used with corticosteroids in the treatment of inflammatory bowel disease, or gum disease (gingivitis/stomatitis) in cats.
Metronidazole is usually tolerated better if given with food and there are a wide variety of flavors and preparations made by compounding pharmacies in order to deal with the problems associated with the bitter taste. Because of the variety of uses for this drug, dose amount, frequency and duration of treatment vary widely.

Horses

Metronidazole is primarily used with other antibiotics to treat mixed bacterial infections in which anaerobic bacteria are present, for example, pleuropneumonia, peritonitis, and abdominal abscesses.
It is also used prophylactically after colic or other abdominal surgery when mixed bacterial infections are a risk.
Metronidazole is generally given orally although it is also absorbed rectally.
Rectal administration is occasionally used in the very sick patient when anorexia and weight loss are a problem.