Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066




(for veterinary information only)




250 mg & 500 mg TABLETS



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 a very effective anti-diarrhea medication. It is an effective antibiotic against certain protozoal infections, especially giardia.

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



Anaerobic Infection
The normal tissues of our bodies depend on the presence of 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" bacteria. 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.

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 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 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.



The most common side effects related to nausea and are generally minor.

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). Again, 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 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). The actual mechanism for the development of neurologic signs in toxicity is unknown as is the mechanism for recovery using diazepam.



In humans, 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.



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 administer 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 the cat as a condition called "Heinz body anemia" may result. This is not a problem for short term usage in cats and is not a problem at all for dogs.

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


Short version (to help us comply with "Lizzie's Law")

Page last reviewed: 1/1/2021