For simple diarrhea in dogs, metronidazole is usually the first thing your veterinarian recommends. Although metronidazole is an antibiotic by class, it works as an anti-inflammatory on the intestinal mucosa. As such, regardless of what is causing the inflammation that is resulting in diarrhea, metronidazole cools the flame. So, whether the diarrhea is due to stress, a diet change, dietary indiscretion (aka eating “dumb stuff”), or even a parasite or virus, metronidazole often helps, and research shows it can help quickly.
Metronidazole Dosage for Diarrhea
Typical metronidazole dosage for dogs for treatment of non-specific, simple diarrhea in dogs is 5 to 10 mg/kg (milligrams of metronidazole per kilogram of the dog’s weight) twice a day for five days. A kilogram equals 2.2 lbs., which means a 20-lb dog weighs about 9 kg. The dosage for a 20-lb dog is 45 to 90 milligrams of metronidazole twice a day. Using pounds, the dosage is 2½ to 5 mg per 1.1 lbs. of body weight. Metronidazole is a prescription drug dispensed as a 250 mg or 500 mg tablet. You need to know which you have.
For some dogs, a single dose is sufficient, especially when stress is the cause.
For chronic diarrhea, if metronidazole works and all other known causes of diarrhea have been ruled out, the condition is given the name “antibiotic-responsive diarrhea.” How it works is a mystery. That it works is a fact.
Metronidazole Treatments
In dogs, metronidazole is also used to treat:
- Biopsy-confirmed inflammatory bowel disease (IBD), although steroids also may be necessary
- Giardia, a protozoan intestinal parasite, although fenbendazole is the treatment of choice
- Anaerobic bacteria, which are mostly found in the gastrointestinal tract and underneath the gums (periodontal disease)
- Clostridial intestinal infections
- Certain liver and gall bladder infections
- Liver disease resulting in neurologic impairment (hepatic encephalopathy)
When metronidazole is used as an antibiotic, the dose range is typically 10 to 15 mg/kg twice a day.
Side Effects
Metronidazole toxicity is rare, but it can occur, usually with high doses given for an extended period. It shows up as sudden onset of severe, neurologic derangements, including:
- Seizures
- Tremors
- Stiffness
- Head tilt
- Incoordination (ataxia)
- Dilated pupils
- Confusion
- Nystagmus (rapid eyeball jerking back and forth)
With discontinuation of the drug, most dogs fully recover within a few days. Treatment for metronidazole toxicity is supportive care.
Additional, less scary but possible side effects of metronidazole include:
- Nausea
- Vomiting
- Diarrhea
- Loss of appetite
- Weakness
- Bloody urine
- Lethargy
- Weakness
- Neurologic derangements
One final note: With the increased importance of antibiotic stewardship to combat increasingly alarming rates of antibiotic resistance, your veterinarian may be reluctant to immediately prescribe metronidazole upon request for diarrhea. Trying a probiotic and bland diet may be suggested first for good reason.