Long-Term Metronidazole Benzoate Use in Cats

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If your cat suffers from chronic diarrhea, your vet might prescribe metronidazole benzoate, marketed under the brand name Flagyl. Kitty would take this drug long-term only for diarrhea; for other conditions that metronidazole benzoate treats, the medication would probably be given short-term.

Metronidazole Benzoate

The prescription synthetic antibiotic metronidazole benzoate not only has the capability of destroying bacteria, it can also get rid of protozoa. It effectively rids cats of anaerobic infections, the type that don't require oxygen to flourish. It has useful properties that many other antibiotics can't claim: it penetrates bone so it can be used to treat dental disease. It also penetrates the blood/brain barrier, the membrane protecting the brain from foreign substances, which enables it to treat infections of the central nervous system. It's only approved for people, not pets, by the U.S. Food and Drug Administration, but veterinarians may use the drug off-label.


Your vet might prescribe metronidazole benzoate for a short while after diagnosing your cat with giardiasis, a protozoal parasite infection that causes diarrhea. According to the Companion Animal Parasite Council, the drug is the most commonly used "off-label" therapy, but it's efficacy level is between only 50 percent and 60 percent. If your cat suffers from colitis, a vet might prescribe metronidazole benzoate for long-term use. Short-term, it's also prescribed for clearing up other diarrhea-causing conditions.

Long-Term Side Effects

If given for short periods, as it might be to combat giardias in your cat, metronidazole benzoate has few side effects. However, if you're giving it to your cat long-term or in high doses, that's another story. Many side effects of long-term use are neurological in nature. These include seizures, disorientation, staggering, head tilting, rapid back-and-forth eye movements and constantly dilated pupils. If your cat exhibits these behaviors, call your vet. She'll likely tell you to stop giving your cat the medication, and you'll probably have to wait it out. It can take up to two weeks for the cat to return to normal, and there's a chance he could succumb to the neurological issues caused by the medication. Your vet will advise you about ways to keep Kitty safe during this ordeal.


Don't give metronidazole benzoate to pregnant or lactating cats, or those with liver disease. If Kitty suffers from epilepsy and also takes phenobarbital for seizure control, metronidazole benzoate will lose effectiveness in combination with that drug. If your cat receives cimetidine, better known as Tagamet, for gastrointestinal ailments, any potential side effects of metronidazole benzoate might increase.

Always check with your veterinarian before changing your pet’s diet, medication, or physical activity routines. This information is not a substitute for a vet’s opinion.

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