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 (extra label). Many drugs are commonly prescribed for off-label use in veterinary medicine. “Off label” means the drug is being used in a way that has not been reviewed or approved by the U.S. Food and Drug Administration (FDA) or Health Canada. In these instances, follow your veterinarian’s directions and cautions carefully, as they may be significantly different from those on the label.
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.
- Shake liquid forms well before use.
- Metronidazole should be given by mouth, with food.
- Metronidazole is very bitter, so do not 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.
Exposure to metronidazole may lead to adverse effects in pregnant humans, so it is essential to avoid accidental exposure. Wear gloves 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.
This medication should take effect within one to two hours. Effects may not be noticeable immediately, but gradual improvements are usually noticeable after a few days.
What if I miss giving my pet the medication?
If you miss a dose, give it as soon as you remember, and then continue with the regular dosing schedule. However, if it is almost time for the next dose, skip the missed dose and resume the regular dosing schedule. Do not give the pet two doses at once. It is very important to give the medication for the length of time your veterinarian has recommended.
Are there any potential side effects?
Metronidazole may cause the following side effects:
- Nausea, vomiting, diarrhea
- Regurgitation
- Decreased appetite
- Tiredness
- Drooling
Serious adverse reactions may occur. If any of the following signs develop, stop giving metronidazole and contact your veterinarian immediately:
- Lack of muscle control or coordination
- Tremors, seizures
- Weakness
- Vomiting
- Eye twitching
- Lack of appetite or yellowing of the skin, eyes, or gums (signs of liver toxicity)
- Scaling, hair loss, bruising, swelling, and bumps on the skin (rare; indicates a skin disorder in which blood vessels become inflamed in the skin)
Drug sensitivities can develop over time, with repeated exposure. Your pet may have no reaction after the first few doses but may develop a sensitivity later. It is important to watch for signs of an allergic reaction over the entire course of treatment.
Other side effects may occur. If you notice anything unusual, contact your veterinarian immediately. 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 with a known hypersensitivity or allergy to it or related drugs
- Pets that are pregnant or nursing
- Pets that are in a weakened condition or debilitated
USE WITH CAUTION in:
- Pets with certain liver disorders
- Pets that are on blood thinners
- Puppies or kittens
Are there any drug interactions I should be aware of?
Several medications should be used with caution when given with metronidazole, including:
- Certain chemotherapy drugs
- Certain gastro protectants (cimetidine)
- Cyclosporine
- Phenobarbital
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 before starting any new treatment.
Is any monitoring needed with this medication?
- Your veterinarian will let you know when and how they want to reassess your pet to ensure the medication is working and not causing side effects.
- At home, monitor for any side effects such as those noted above. If any occur, stop using the medication and contact your veterinarian.
How do I store metronidazole?
- Store metronidazole tablets, capsules, and oral solution in tightly sealed containers, protected from light, and at temperatures below 86°F (30°C).
- Store compounded forms of this medication according to the label. Some special liquid formulations should be stored in the refrigerator.
- Keep the container out of reach of children and pets. 
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. Be prepared to provide information about the product and your pet’s weight and symptoms.
