Metronidazole 101: What You Need to Know if Your Dog or Cat Needs Metronidazole

Metronidazole is a strong antibiotic and antibacterial medication commonly prescribed by veterinarians for anaerobic bacterial infections, protozoal (parasitic) infections, and non-specific inflammatory conditions of the stomach and large intestines in dogs and cats.

It is used to treat protozoal infections in dogs and cats including diarrhea-inducing agents such as giardiasis, entamoeba, trichomonas foetus (TTF), and balantidium, and to treat anaerobic bacterial infections.

Metronidazole has immune-modulating activity and may be prescribed to treat canine inflammatory bowel disease (IBD) and colitis caused by other antibiotics.

It may also be used to treat periodontal disease (especially in cats), Clostridium perfringens enterotoxemia, tetanus, E. coli, severe diarrhea, chronic diarrhea, diarrhea of undetermined cause, pancreatic insufficiency (with small-intestinal bacterial overgrowth), and complications of severe liver disease.

Topical metronidazole gel is used to treat skin infections, such as feline chin acne.

Your veterinarian may prescribe metronidazole under the brand names Flagyl®, Metizol®, Protostat®, or Metrogel®. It is generally given orally as a capsule, tablet, or liquid suspension. It is also available in topical gel or ointment forms.

An injectable form may be used in a hospital setting, and metronidazole benzoate is sometimes used in cats because it is more palatable.

Metronidazole is bactericidal; it kills bacterial microorganisms by disrupting their DNA and inhibiting repair enzymes in the cells that exist in anaerobic (unoxygenated) environments, resulting in the death of bacterial cells.

It is often used with other antibiotics when treating mixed-bacterial infections because it is compatible with many other antibiotics, including penicillin antibiotics, aminoglycosides, and some cephalosporins.

It also can penetrate the blood-brain barrier to treat central nervous system infections, is absorbed rapidly from the GI tract, metabolized by the liver, and excreted in the urine and the feces.

The most common side effect is gastrointestinal upset. Dogs and cats may also experience excessive salivation, gagging, regurgitation, pawing at the mouth, nausea, vomiting, and decreased appetite.

Less common side effects are diarrhea, depression, lethargy, weakness, neurologic signs, low white blood cell count, liver failure, blood in the urine, or dark urine due to pigment changes.

Neurologic signs may be seen after accidental overdose or, more commonly, with long-term moderate-to-high-dose therapy to treat difficult bacterial infections. Signs often begin seven to 12 days following the start of treatment.

Metronidazole is a short-acting medication. Adverse symptoms of the medication should subside within a day of stopping it. Newer studies do show alterations of the normal GI flora that can persist beyond two weeks after treatment.

• Metroninazole causes birth defects in laboratory animals, so it should be avoided in pregnant animals, especially in the first trimester. Some metronidazole is excreted in breast milk, and it should not be used in lactating animals.
• It should not be used in young puppies and kittens.
• It should be avoided or used with caution, at reduced doses, in animals with kidney or liver disease.
• Exposure to this medication may lead to adverse effects in pregnant humans, so gloves should be worn when handling the medication.
• DO NOT crush pills, as this creates powders that can become airborne, and inhaling the drug can lead to exposure.
• If your pet vomits after being given this metronidazole, wear gloves while cleaning up.

• Metronidazole is reported to elevate prothrombin time in animals on warfarin or other coumarin anticoagulants.
• Metronidazole may increase the blood concentrations of cyclosporine.
• Phenobarbital and phenytoin may increase the metabolism of metronidazole resulting in lower therapeutic levels.
• Supplements, flea medications, and chemotherapy drugs may also interact with metronidazole. Consult with your veterinarian about all the drugs and supplements your dog is taking.

Wedgewood provides medication options that help ensure accurate dosing, especially for hard to medicate pets. Click below for a complete list of Wedgewood’s dosing forms and strengths.

View all Metronidazole options

If you miss giving your pet a dose, give the next dose as soon as you remember or, if it is close to the next scheduled dose, return to the regular schedule. Do not give two doses (double dose) to catch up. If you are not sure what to do, call your veterinarian and follow their directions.

Symptoms of an overdose of metronidazole include gastrointestinal signs (anorexia, vomiting) and neurologic signs including depression, ataxia, disorientation, head tilt, tremors, bradycardia, rigidity, stiffness, and seizures.

Neurologic signs may occur due to acute overdose although they are more commonly seen in animals that are on long-term moderate or high doses (oral doses greater than 66 mg/kg/day). Signs of chronic toxicity often begin seven to 12 days following the start of treatment. After the drug is discontinued, it may be several days to two weeks before these neurologic signs begin to diminish.

Contact your veterinarian immediately if you suspect an overdose.

If you suspect your pet or another animal has accidentally overdosed or has eaten this medication inadvertently, immediately contact your veterinarian or the A.S.P.C.A.’s Animal Poison Control Center at 888-426-4435.

Remember to take your prescription container with you when you take your pet for treatment. If you or someone else has ingested this medication, call the National Capital Poison Center at 800-222-1222.

Will metronidazole cure my dog’s chronic diarrhea?

Metronidazole will not cure chronic diarrhea, but it will reduce the time it takes for acute diarrhea to resolve itself.

What other drugs will my pet need besides metronidazole?

In addition to prescribing metronidazole for your pet’s acute diarrhea, your veterinarian may prescribe other medications to lessen other symptoms that often come along with it.

Additional treatments include anti-nausea medications, proton pump inhibitors (to reduce acid production), motility inhibitors (to reduce cramping and the sense of urgency), bile acid sequestrants (to control bile acid malabsorption in chronic diarrhea cases), and vitamin B12 (cobalamin, which is often deficient in chronic diarrhea cases). Probiotics can be very beneficial to limit the negative effects of metronidazole on the GI flora.

Is metronidazole the best drug for TTF in cats?

Metronidazole is effective in treating TTF. Ronidazole, a nitroimidazole antibiotic/antiprotozoal from the same family as metronidazole, is also effective.

Can metronidazole actually make a dog’s diarrhea worse?

Possibly. Several studies have shown that metronidazole can make diarrhea worse by reducing populations of beneficial gut microbiome by altering the intestinal mucus, which helps to protect your dog's GI tract from inflammation and infection. Contact your veterinarian if your pet’s diarrhea worsens after taking metronidazole.

Why was my pet prescribed metronidazole for a tooth infection?

Oral and dental infections often affect the digestive tract in pets.

Why did my veterinarian prescribe metronidazole along with other IBD medications?

Metronidazole is often prescribed by veterinarians as adjunctive therapy to manage inflammatory bowel disease (IBD) by modulating the immune system and providing anti-inflammatory effects.

This article is meant to provide general and not medical advice. We strongly recommend that a veterinarian be consulted for the specific medical needs of your animal.

Pawlicy Advisor
American Kennel Club

Metronidazole for Companion Animals