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Metronidazole for dogs: Vet’s guide to dosage and side effects

Dog having his heart listened to by a vet
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Metronidazole is an antibiotic and antiprotozoal medication that is commonly used in dogs for multiple reasons, most frequently to treat diarrhea

Let’s take a closer look at what this drug is, when it should and shouldn’t be used, possible side effects, and more.

What is metronidazole for dogs?

Metronidazole, also known by its trade name Flagyl, is an antimicrobial medication that is effective against some anaerobic bacteria and protozoa. Metronidazole kills susceptible microorganisms by interfering with the structure of their DNA and causing subsequent cell death.

Metronidazole has the ability to penetrate many tissues in the body, including the central nervous system and abscesses. It is also thought to have anti-inflammatory effects in the gut. The drug is metabolized by the liver and excreted in urine and feces.

Because metronidazole has not been approved by the Food and Drug Administration (FDA) for use in veterinary medicine in the United States, it is prescribed “off-label” for dogs and cats.

When is metronidazole used in dogs?

Metronidazole can be used to treat several types of bacterial and protozoal infections, including:

  • Gastrointestinal tract infections, such as those with Clostridium difficile (“C. diff”) and Giardia
  • Abscesses or other collections of pus
  • Hepatic encephalopathy
  • Peritonitis
  • Dental infections
  • Meningitis
  • Genital tract infections

Metronidazole is often used in combination with other antibiotics to improve its spectrum of activity, which is especially useful in cases where multiple bacterial species are identified. 

In addition, it may be given after certain gastrointestinal surgeries to prevent infection. It can also be used as a radiosensitizer for dogs undergoing radiation therapy for solid tumors, meaning that it makes cancer cells easier to kill with radiotherapy and it makes smaller doses of radiation more effective.

Dosage of metronidazole for dogs

According to the NOAH Compendium of Data Sheets for Animal Medicines (opens in new tab), the recommended dose of metronidazole for dogs is 50 mg/kg/day (or about 22 mg/lb/day) for 5-7 days—this can be split into 25 mg/kg (or about 11 mg/lb) twice per day if needed. 

The appropriate dose may vary depending on the type and severity of infection, however. You should never give your dog metronidazole (or any medication!) without consulting your vet first.

Metronidazole comes in different oral formulations, including tablets, capsules, and liquids. It can also be administered via an injection at your local veterinary clinic. Many of the oral formulations taste bitter, so they are better tolerated when given with food. The medicine should be stored in a cool, dry, and safe place in its original packaging.

Oral metronidazole generally starts working within a couple hours of ingestion, but it may take a few days to start seeing improvements. If you feel that your dog is not improving, contact your veterinarian to see if your dog should be reassessed or if any changes need to be made to their treatment plan.

Sick dog lying on the couch

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Side effects of metronidazole in dogs

Metronidazole is generally safe in dogs, but few may experience side effects. The most common side effects include:

  • Nausea
  • Vomiting
  • Diarrhea
  • Inappetence
  • Regurgitation
  • Excessive drooling (due to the drug’s bitter taste)
  • Lethargy
  • Reddish brown urine

The most concerning side effect is neurotoxicity, which can present as tremors, incoordination, and seizures. This is typically seen when high doses are given for an extended period of time, and it can be life-threatening. According to the NOAH Compendium, neurological signs occur very rarely following the administration of metronidazole (i.e., less than 1 in 10,000 animals).

Other rare side effects of metronidazole include liver toxicity, which may manifest as reduced appetite and jaundice, and reversible bone marrow depression with a subsequent reduction in certain white blood cells.

If your dog experiences any side effects, you should contact your veterinarian for guidance as soon as possible. If you are concerned that your dog may have overdosed on metronidazole, veterinary attention should be sought immediately.

Factors to consider before giving metronidazole to dogs

There are several factors that must be considered before giving metronidazole to a dog:

Allergies / Pregnancy

Metronidazole should not be given to dogs with known allergies to the drug. In addition, metronidazole is not safe in pregnant or lactating animals. For these reasons, humans with allergies to metronidazole and pregnant/breastfeeding women should avoid handling the medication, or if this is unavoidable, they should wear gloves. Tablets should not be crushed, as this can create airborne powders that can be inhaled.

Kidney/ liver disease and seizure disorders

Because metronidazole is metabolized in the liver and excreted by the kidneys, it should be avoided or used with caution in dogs with liver or kidney disease. 

It should also be avoided in dogs with neutropenia (i.e., low neutrophils, a type of white blood cell) or seizure disorders where possible. Your veterinarian will weigh the pros and cons of treatment with metronidazole based on your pet’s medical history.

Other medications

Metronidazole is known to react with drugs such as cimetidine, cyclosporine, phenobarbital, phenytoin, warfarin, and certain chemotherapy drugs, so simultaneous use can reduce the effectiveness of one or both medications.

 It is extremely important to tell your vet about any medications you give your dog, whether it be over the counter (i.e., Pepto Bismol) or a prescription medication such as gabapentin, tramadol, or Apoquel, to ensure that treatment will be effective and to minimize the risk of side effects.

Vet examining dog

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Metronidazole for diarrhea in dogs

Metronidazole is most commonly used in veterinary medicine to treat diarrhea cases. In recent years, however, there has been increasing evidence that metronidazole is not as useful for gastrointestinal conditions as once thought. 

Most diarrhea cases resolve with only supportive treatment and do not need antibiotics at all. Some studies have even found that metronidazole may cause long-term changes to the gut microbiome in dogs, so it should really only be used in cases where it is definitely needed, such as when a culture and sensitivity has identified metronidazole as an effective antimicrobial or in the presence of sepsis.

Where can I buy metronidazole for dogs?

Metronidazole is a prescription drug, so it must be dispensed by your veterinarian or a pharmacy. Most veterinary clinics charge small fees for written prescriptions to take elsewhere, which should be considered before deciding where to fill your prescription.

Dr. Diana Hasler graduated with distinction from the University of Edinburgh Royal (Dick) School of Veterinary Studies in 2018. She has experience working as a small animal veterinarian in general practice, where she has treated many dogs, cats, rabbits, and rodents. She has also recently branched out into the field of medical communications, doing freelance work as a medical editor and writer. Dr. Hasler currently lives in Edinburgh where she enjoys spending time with her husband Gavin and playing with their feisty tabby cat Poppy.