Metronidazole for Dogs and Cats

Medically Reviewed by Joshua Conrad, PharmD on October 09, 2024
Written by Kathleen Claussen, DVM
5 min read

Metronidazole (Ayradia, Flagyl) is an antibacterial and antiprotozoal medication. It is usually used to treat diarrhea and infections caused by a parasite called giardia.

The oral liquid formulation (Ayradia) is the only FDA-approved metronidazole product for use in animals.  It is approved for treatment of infections caused by Giardia duodenalis in dogs and puppies.  All other uses of metronidazole in dogs and all uses of metronidazole in other animals are “extra-label” or “off-label.” This means that there is evidence of safe and effective use but the FDA has not approved that use.

Metronidazole begins to work in 1 to 2 hours, with improvement usually seen in a few days.

Metronidazole is available by prescription from your veterinarian or from human pharmacies. It is available in various dosages and in tablet, capsule, and liquid forms.  Metronidazole is also available in an injectable form for use in the veterinary hospital. Contact your veterinary compounding pharmacy for alternative dosing options.

Metronidazole tablets have a very bitter taste that can make it difficult to give to some pets, especially if the tablets are crushed. A compounded form of metronidazole can be less bitter tasting.  Be careful using this compounded medication long-term in cats.

Metronidazole is given by mouth as a tablet, capsule, or liquid.  Metronidazole also comes in an injectable form that is used in the veterinary clinic. 

Metronidazole may be given with or without food. If stomach upset occurs when given on an empty stomach, try giving it with a small amount of food or a treat. If giving a liquid form of metronidazole, remember to mix or shake it well before giving. 

Always follow your veterinarian’s directions for giving medicines to animals. Remember to tell your veterinarian about any medicines, vitamins, supplements, or herbal therapies that you are giving your pet. Talk to your veterinarian before stopping any pet medicines.  

  • Metronidazole is an antibiotic that can be used to treat infections in the brain, spinal cord, and bone because of how well it can get to these parts of the body.
  • Metronidazole is useful to treat diarrhea and inflammation of the intestinal tract.
  • Metronidazole is effective in the treatment of giardia in animals.

You and your veterinarian should monitor your pet for improvement of their condition as well as for side effects of the metronidazole. Blood tests should be monitored if your pet has liver disease.

Do not use metronidazole in animals that are allergic to it or in animals that are pregnant or nursing.  Use metronidazole with caution in frail animals or animals with liver problems.

Side effects of metronidazole include:

  • Nausea
  • Vomiting
  • Diarrhea
  • Decreased appetite
  • Increased drooling
  • Tiredness

Dogs with mild side effects will usually improve quickly within a few days once the metronidazole has been stopped.

Metronidazole that is given at high doses or for long amounts of time may cause nerve or brain problems in dogs. Side effects include:

  • Body tremors
  • Muscle spasms
  • Difficulty walking
  • Seizures
  • Weakness
  • Vomiting
  • Eye twitching

If you are concerned that your dog is showing any of these side effects, stop the metronidazole right away and contact your veterinarian immediately. The toxicity may take several days to weeks to resolve after stopping the metronidazole, usually with supportive care and close monitoring.

If you suspect that your pet is having a serious side effect or may have been overdosed, call your veterinary clinic immediately. If it is outside of regular office hours, you can contact a local emergency veterinary hospital or an animal poison control center. 

You can reach the ASPCA Animal Poison Control Center at 888-426-4435 or the Pet Poison Helpline at 855-764-7661.

Using multiple medicines can sometimes change how your pet’s medicines work or increase your pet’s risk for serious side effects. Always tell your veterinarian about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements that your pet is using.

Make sure that you tell your veterinarian if your pet is using any of the following medications before using metronidazole:

Cyclosporine. Cyclosporine is used to treat inflammatory and immune-mediated diseases in animals. Metronidazole may increase the levels of cyclosporine in the pet, increasing the risk for cyclosporine-related side effects.

Fluorouracil (5-FU). 5-FU is a medicine used to treat cancer in dogs. If metronidazole is used with 5-FU, the blood levels of 5-FU may increase to toxic levels.

Phenobarbital. Phenobarbital is a medicine used to prevent seizures. Its use with metronidazole may cause the body to remove metronidazole from the body more quickly, causing the metronidazole to be less effective.

This may not be a complete list of medicines that can interact with metronidazole. Always check with your veterinarian.

  • Keep metronidazole out of the reach of children.
  • Wash your hands after giving metronidazole to your pet.
  • Do not handle metronidazole if you are allergic to it.  

Pregnant women should be extra careful when around metronidazole. They should avoid direct contact with the drug, avoid inhaling any dust if cutting the pills, and wear gloves when handling the medicine.

Since metronidazole can cause nerve and brain issues in animals at high doses, call your veterinarian or veterinary emergency clinic immediately if you think your pet has taken more metronidazole than prescribed.

If your pet missed a dose of this medicine, you can give that dose if it is close to the original dosing time. If it is closer to the next dosing time, skip the missed dose completely and give the next dose at the correct time. Continue giving the medicine at the correct times. Do not double the dose of the medicine.  

Metronidazole tablets and capsules should be protected from light and stored at room temperature.  Some liquid forms may need to be refrigerated.  Once tablets are cut into pieces, those pieces are good for three days. Compounded forms of metronidazole should be stored according to the compounding pharmacy’s recommendations.