By Rania Gollakner, BS, DVM, MPH

What is metronidazole?

Metronidazole (brand name Flagyl®) is an antibacterial and antiprotozoal agent used in the treatment of certain anaerobic bacterial and protozoal infections, such as those caused by Giardia and Trichomonas. It is often used to treat diarrhea and other intestinal problems.

The use of metronidazole to treat Giardia infections and most other infections in dogs, cats, small mammals, birds, reptiles, and other animals is off-label. Many drugs are commonly prescribed for off-label use in veterinary medicine. In these instances, follow your veterinarian’s directions and cautions very carefully.

How is metronidazole given?

Metronidazole is available as a capsule, tablet, and liquid suspension. It may also be compounded into a formulation (called metronidazole benzoate) that tastes less bitter and is easier to administer to cats. An injectable form is also available that your veterinarian will administer at your veterinary hospital.

"Metronidazole should be given by mouth with food."

Metronidazole should be given by mouth with food. Liquid forms must be shaken well before use. Metronidazole is very bitter, so take care not to crush these tablets as it will be difficult to administer to your pet. Follow the dosing instructions provided by your veterinarian. If you have difficulty administering the medication, contact your veterinary clinic for advice.

This medication should take effect within one to two hours, and while effects may not be visibly noticed immediately, gradual improvements are usually noticeable after a few days.

Exposure to this medication may lead to adverse effects in pregnant humans, so care must be taken to avoid accidental exposure. 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 giving this drug, wear gloves while cleaning up.

What if I miss giving my pet the medication?

If you miss a dose, give it when you remember, but if it is close to the time for the next dose, skip the dose you missed and give the medication at the next scheduled time and continue with the regular dosing schedule. Never give your pet two doses at once or give extra doses.

It is very important to give the medication for the length of time your veterinarian has recommended.

Are there any potential side effects?

Side effects of metronidazole may include nausea, vomiting, diarrhea, regurgitation, decreased appetite, tiredness, and drooling.

Metronidazole can cause neurological effects including lack of muscle control or coordination, tremors, seizures, vomiting, eye twitching, and weakness. These effects usually occur if metronidazole is given at high doses or for extended periods of time, but they can occur with lower doses as well. Contact your veterinarian immediately if you notice any of these signs with your pet.

Liver toxicity can occasionally occur. Signs of liver toxicity may include inappetence or yellowing of the skin, eyes, or gums. Contact your veterinarian if you notice these signs.

"Liver toxicity can occasionally occur."

Very rarely, metronidazole may cause a skin disorder called cutaneous vasculitis, in which blood vessels become inflamed in the skin. Signs of this skin disorder include scaling, hair loss, bruising, swelling, and bumps on the skin. Contact your veterinarian if you notice any of these signs.

This short-acting medication should stop working within 24 hours, although effects can be longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

Do not use metronidazole in pets that:

• have a known hypersensitivity or allergy to it or drugs related to it
• are pregnant or nursing
• are in a weakened condition or debilitated

Metronidazole should be used with caution in pets that:

• have certain liver disorders
• are puppies or kittens
• are on blood thinners

Are there any drug interactions I should be aware of?

Certain drugs may interact with metronidazole, including certain chemotherapy drugs, and gastro protectants (cimetidine), cyclosporine, phenobarbital, phenytoin, and warfarin. Your veterinarian will weigh the potential risks of metronidazole if your pet is taking these medications. It is important to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking.

"It is important to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking."

Is there any monitoring that needs to be done with this medication?

Monitor to ensure that the medication is working and monitor for adverse effects.

How do I store metronidazole?

Metronidazole tablets, capsules, and oral solution should be stored in a tightly sealed container, protected from light, and at temperatures below 30°C (86°F). Some special liquid formulations should be stored in the refrigerator. If your veterinarian has made a special formulation, follow the storage instructions provided with the product.

What should I do in case of an emergency?

If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility.

Related Articles