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Metronidazole (Flagyl)
Revised: January 11, 2023
Published: January 01, 2001

(For veterinary information only)

The size of the tablet/medication is NOT an indication of a proper dose. Never administer any drug without your veterinarian's input. Serious side effects or death can occur if you use drugs on your pet without your veterinarian's advice. 

It is our policy not to give dosing information over the internet.

Brand Name: Flagyl

Available in 250 mg & 500 mg tablets, 375 mg capsules

Uses of this Medication

Metronidazole is an antibiotic especially effective against anaerobic infections (infections that grow without the presence of oxygen.) Unlike many antibiotics, metronidazole is able to penetrate the blood-brain barrier and treat infections in the brain and spinal cord. It also penetrates bone, making it especially useful in oral/dental infections. In addition, it has anti-inflammatory properties in the large intestine and is an effective anti-diarrhea medication. It is an effective antibiotic against certain protozoal infections, especially Giardia.

In short, metronidazole is used to treat diarrhea, especially of large intestinal origin, anaerobic infections, and against certain single-celled intestinal parasites.

How this Medication Works and How is it Used

Anaerobic Infection
The normal tissues of our bodies depend on oxygen for survival and, indeed, we have an elaborate circulatory system to see that oxygen is transported throughout our bodies. In other words, our cells live aerobically (in the presence of oxygen). Abscessed tissues and tumors with poor central circulation do not receive blood supply and oxygen. Only bacteria that can live without oxygen can survive in these areas. These bacteria are called anaerobic bacteria.  In anaerobic conditions, the metronidazole molecule changes to inhibit the DNA repair enzymes that normally would repair cells. This means death for anaerobic bacteria but no effect on aerobic tissues.

Radiotherapy for Cancer
Metronidazole is also used in radiotherapy for cancer as this DNA effect can sensitize anaerobic tumor tissues to radiation, making a smaller dose of radiation more effective.

Inflammatory Diarrhea
Metronidazole can modify cell-mediated immunity to normalize excessive immune reactions, especially in the large intestine. We do not know how metronidazole is able to do this, but metronidazole is widely used as a first-line medication for diarrhea. Its effectiveness may relate to its anti-inflammatory properties or to its effect on anaerobic bacteria, or a combination of both.

Metronidazole is usually given twice daily. If a dose is accidentally skipped, do not double up on the next dose but simply give the forgotten dose when it is remembered and time subsequent doses accordingly. Metronidazole is best given with food and stored at room temperature away from light.

Metronidazole is famous for its bitter taste. It can be compounded into an oral liquid but even with flavoring, it is difficult to disguise the bad taste.

Side Effects

Side effects are not commonly seen with this medication unless the patient is taking especially high doses (general anti-diarrheal doses should not be a problem) or moderate doses for a lengthy time period (months). Side effects seen can relate to nausea or appetite loss, or can be neurologic: staggering, head tilt to one side, dilated pupils, bizarre back-and-forth eye movements called nystagmus; and even seizures, particularly in cats. Again, most cases of toxicity involve really high doses or chronic use (months) use of smaller doses.

In cases of toxicity, recovery from neurologic signs typically takes 1-2 weeks after the drug is discontinued. This recovery period can be shortened to less than 48 hours by using a diazepam treatment protocol published in the Journal of Veterinary Internal Medicine (May/June 2003). We do not know why neurologic signs develop from toxicity, nor do we understand why diazepam helps.

Interactions with other Medications

In people, concurrent use of metronidazole with alcoholic beverages induces marked nausea. This should obviously not be a problem for pets.

If metronidazole is used concurrently with phenobarbital (medication for epilepsy), metronidazole may not be as effective. If metronidazole is used concurrently with cimetidine (Tagamet HB®), there is a slight increased risk for metronidazole use to yield the side effects mentioned above.

Concurrent use of metronidazole and the immunomodulator cyclosporine may lead to increases in cyclosporine blood levels and may thereby increase the side effects potential of cyclosporine.

Concerns and Cautions

Metronidazole is best given with food.

Metronidazole can cause birth defects if given to a pregnant patient. It should never be used during pregnancy.

It is not unusual for compounding pharmacies to formulate oral liquid preparations of metronidazole as these are often easier to give than tablets. The benzoate form of metronidazole is often used as it has a less bitter taste than regular metronidazole. The benzoate form is not recommended for long-term use in cats because a condition called Heinz body anemia may result. This is not a problem for short-term use in cats and is not a problem at all for dogs.

The dosing of metronidazole should be modified in patients with liver disease.

For storage, keep away from light and store at room temperature. Do not refrigerate.

If you miss a dose, do not double up on the next dose. Simply give the medication when it is remembered or pick it up with the next dose, allowing at least the proper interval between doses according to the label instructions.

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