Metronidazole for Dogs and Cats

Stephanie Howe, DVM
By Stephanie Howe, DVM. Reviewed by Molly Price, DVM on Jan. 31, 2024

In This Article


PetMD’s medications content was written and reviewed by veterinary professionals to answer your most common questions about how medications function, their side effects, and what species they are prescribed for. This content shouldn’t take the place of advice by your vet. 

What Is Metronidazole for Dogs and Cats?

Metronidazole for dogs is an antibiotic and antiprotozoal medication that kills certain types of bacteria and protozoa parasites. In dogs and cats, it is most commonly used to treat infections of the digestive tract.

Metronidazole targets certain types of bacteria called anaerobic bacteria, which are bacteria that do not require oxygen to survive. It also treats protozoal parasites, such as giardia, especially when given in combination with other medications. Metronidazole can also be used to treat certain kinds of inflammation in the digestive tract.

Metronidazole is most frequently given as an oral medication and is also available as an injection administered in the hospital by a veterinarian. Your veterinarian will determine which combination is best for your pet, based on its circumstances and symptoms.

Metronidazole is FDA-approved for human use under the brand name Flagyl®. Metronidazole is currently not FDA-approved for use in animals. However, it is readily utilized in the veterinary field, and veterinarians can legally prescribe certain human drugs for use in animals in certain circumstances. This is called extra-label or off-label use because this use is not described on the drug label. Speak with your veterinarian about whether metronidazole is right for your pet.

In certain circumstances, your vet may recommend a compounded formulation of metronidazole. Compounded medications are prescribed if there’s a specific reason your pet’s health can’t be managed by an FDA-approved drug, such as if your pet has trouble taking pills in capsule form, the dosage strength is not commercially available, or the pet is allergic to an ingredient in the FDA-approved medication.

Compounded medications are not FDA-approved. They are created by either a veterinarian or a licensed pharmacist on an individual basis to best suit a patient’s particular needs. You can learn more about compounded medications here.

Metronidazole Considerations for Dogs and Cats

Metronidazole should not be used in animals that are sensitive to the medication or other medications categorized in the same antiprotozoal drug class, such as ronidazole. Metronidazole should be used with extreme caution in animals that are extremely ill, pregnant, nursing, or in animals with decreased liver function.

Always discuss your pet’s medical conditions and medications, including vitamins and supplements, with your veterinarian first before giving metronidazole, as there may be health risks to your pet if they are taking certain other medications. This medication should not be given to farm animals that are to be used for food, including egg-laying chickens and dairy animals.

How Metronidazole Works in Dogs and Cats

Metronidazole kills bacteria by disrupting their DNA. It is not fully understood how metronidazole and other medications in its 5-nitroimidazole drug class kill protozoa, but it is thought that they generate free radicals that disrupt the DNA of the protozoa.

Metronidazole is also used to treat inflammatory diseases in the digestive tract, and is thought to accomplish this by blocking certain inflammatory mechanisms.

Metronidazole Directions for Dogs and Cats

Follow the directions on the drug label or as provided by your veterinarian. The exact dose of metronidazole depends on your pet’s weight, as well as the type of bacteria or parasite present. Metronidazole should be given for the full treatment period prescribed by your veterinarian.

Metronidazole can be given with or without food, but giving it with food can decrease the risk of digestive upset.

Metronidazole should not be used in animals that are sensitive to it or other 5-nitroimidazole drugs like ronidazole.

This medication should not be given to breeding or pregnant dogs because it may cause birth defects, as the medication is secreted into breast milk.

Missed a Dose?

Speak with your veterinarian about what to do if you forget to give a dose of metronidazole.

Possible Side Effects of Metronidazole in Dogs and Cats

Metronidazole is generally well tolerated in dogs and cats. Side effects may include:

  • Vomiting or diarrhea

  • Fatigue (lethargy)

  • Weakness

  • Low white blood cell counts

  • Blood in the urine

  • Loss of appetite

  • Yellowing of gums, skin, or whites of the eyes

  • Neurologic symptoms including lack of coordination, dilated pupils, abnormal eye movements called nystagmus, or seizures

Neurologic side effects are more common in pets receiving higher doses or who are on this medication for long periods of time.

Human Side Effects

Metronidazole is also a prescription medication for humans, frequently with dosages different from those prescribed for your pet by a veterinarian. Due to possible side effects, humans should never use medicine dispensed for their pets and pets should not be given any medicine dispensed for a human’s use.

Pregnant women or individuals allergic to metronidazole should wear gloves when handling the medication and wash their hands after use.

If you accidentally ingest a pet medication, seek medical attention in person or call the national Poison Control Center hotline at 800-222-1222.


No specific monitoring is required for this medication, but your veterinarian may recommend routine testing depending on your pets' individual needs, other medications they may be prescribed, and/or the issue that initially caused your pet to be placed on this medication.

Call Your Vet If:

  • Severe side effects are seen (see above)

  • Your pet’s condition worsens or does not improve with treatment

  • You see or suspect an overdose.

  • Call your vet or pharmacist if you have additional questions or concerns about the use of metronidazole.

Metronidazole Overdose Information in Dogs and Cats

Overdoses of metronidazole in dogs can cause vomiting, decreased appetite, or life-threatening neurologic symptoms such as seizures. Overdoses in dogs often require emergency treatment to limit the absorption of the medication and minimize neurologic symptoms. If you suspect an overdose, immediately seek emergency veterinary care or contact an animal poison control center.

Consultation fees often apply.

Pet Poison Helpline (855) 764-7661

ASPCA Animal Poison Control (888) 426-4435

Metronidazole Storage for Dogs and Cats

Metronidazole should be stored at controlled temperatures from 59 F to 77 F. Always confirm storage requirements by reviewing the label.

Keep the container tightly closed to protect it from moisture and light.

Compounded medications should be stored according to the compounding pharmacy’s label.

Keep out of reach of children and pets.

Metronidazole FAQs

Is metronidazole for dogs an antibiotic?

Yes, metronidazole is an antibiotic commonly used in dogs to treat specific bacterial infections. Different antibiotics work against different types of bacteria, so your veterinarian may recommend this or another antibiotic for your pet depending on what kind of infection is suspected.

Can metronidazole cause dog constipation?

Constipation is not a typical side effect of metronidazole. The drug is often prescribed to dogs with diarrhea. Dogs with diarrhea may strain to defecate, which is easily confused with constipation. In addition, your dog’s diarrhea should improve after starting metronidazole. In the beginning, it is not uncommon to see fewer stools for a day or two after the diarrhea is resolved.

How long does it take for metronidazole to work in a dog with diarrhea?

Metronidazole should start taking effect within one to two hours; however, observable improvement in your pet’s symptoms may take a few days. For example, your dog’s diarrhea should begin to lessen within 1 to 2 days.

Is metronidazole safe for dogs?

Metronidazole can be used safely in dogs in very specific circumstances and under the direct supervision of your veterinarian. It is best to discuss your pet’s medications and medical conditions with your veterinarian before giving metronidazole, as there may be health risks to your pet if they are hypersensitive to it, if they have certain medical conditions, or if they are taking certain medications. Your veterinarian will determine whether this medication is right for your pet, based on their symptoms and circumstances.

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No vet writer or qualified reviewer has received any compensation from the manufacturer of the medication as part of creating this article. All content contained in this article is sourced from public sources or the manufacturer.

Stephanie Howe, DVM


Stephanie Howe, DVM


Dr. Stephanie Howe graduated from the University of Florida College of Veterinary Medicine in 2011, after receiving a Bachelor of Science...

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