THE PET PHARMACY By Wendy C. Brooks, DVM, DipABVP Educational Director, VeterinaryPartner.com

Brand Name: Flagyl

Available in 250 mg & 500 mg tablets

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 the treatment of colitis, which may or may not be caused by inflammatory bowel disease.

How this Medication Works

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.

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 it has 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, such as months. Side effects seen can relate to nausea or appetite loss, or they can be neurologic, such as staggering, head tilt to one side, dilated pupils, bizarre back and forth eye movements called nystagmus, and even seizures, which are seen 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 1 to 2 weeks after the drug is discontinued. This recovery period can be shortened to less than 48 hours by using a diazepam treatment protocol that was 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 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 , a 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 side effects.

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 our policy not to give dosing information over the Internet.

Date Published: 1/1/2001
Date Reviewed/Revised: 01/11/2010