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By Wendy C. Brooks, DVM, DipABVP
Educational Director,

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Metronidazole (Flagyl)

Brand Name: Flagyl

Available in 250 mg & 500 mg tablets, 750 mg extended release 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 central nervous system infections. 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.

A common use of this medication would be for colitis, which may or may not be caused by inflammatory bowel disease.

How this Medication Works

The normal tissues of our bodies depend on having 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.

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.

In anaerobic conditions, the metronidazole molecule changes so as to inhibit the DNA repair enzymes that normally would repair cells. This means death for anaerobic bacteria but no effect on aerobic tissues.

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.

Metronidazole is able to modify cell-mediated immunity so as to normalize excessive immune reactions, especially in the large intestine. We do not know how metronidazole is able to do this.

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 long time period (months). Side effects seen can relate to nausea or appetite loss or side effects can be neurologic (staggering, head tilt to one side, dilated pupils, and bizarre back and forth eye movements called nystagmus, and even seizures particularly in cats). Most cases of toxicity involve very high doses or chronic use (months) use of smaller doses. In cases of toxicity, recovery from neurologic signs typically takes one to two 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). The actual mechanism for the development of neurologic signs in toxicity is unknown as is the mechanism for recovery using diazepam.

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 administered 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.

Dosing of metronidazole should be modified in patients with liver disease.

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


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