If your pup is having poop problems, this medicine might be just what he needs to feel better.
man lying on the couch with his dog sleeping on his lap
Credit: gollykim / Getty

Most dogs will experience diarrhea at some point in their lives, and many will be treated with a drug called metronidazole. Though commonly used and generally safe, this medication can still cause side effects and complications (like any drug can).

What Is Metronidazole Used for in Dogs?

Metronidazole is a prescription antibiotic and antiprotozoal drug that is effective against some bacterial and parasitic infections. Also known by the brand name Flagyl, this drug is commonly used by veterinarians to treat diarrhea in dogs. Metronidazole can kill anaerobic bacteria (that's bacteria that don't need oxygen to survive) and cross the blood-brain barrier to treat some infections in the central nervous system.

Metronidazole may be used in dogs to treat the following conditions:

  • Diarrhea
  • Inflammatory bowel disease (IBD)
  • Giardia
  • Oral and dental infections
  • Sepsis (full-body inflammatory response to infection)
  • Tear staining (medial canthus syndrome)

What Are Potential Metronidazole Side Effects in Dogs?

When given properly, metronidazole is considered generally safe for use in most dogs and is usually tolerated well. Common mild side effects include:

Though uncommon, metronidazole can adversely affect the central nervous system and liver.  Serious side effects are more likely to occur with high doses or long-term treatment. Contact your veterinarian immediately if your dog exhibits any signs of metronidazole toxicity:

  • Difficulty walking, weakness, or drunken gait
  • Nystagmus (abnormal, involuntary eye movements that look like the eyeballs are twitching from side-to-side or up and down)
  • Head tilt (involuntary)
  • Disorientation, confusion, or depression
  • Seizures or tremors
  • Rapid heart rate
  • Yellow appearance of eyes, skin, and/or mucous membranes (like the gums)

Metronidazole should be used with caution when dogs are taking cimetidine, phenobarbital, or warfarin. Humans on metronidazole should avoid alcoholic beverages as the interaction may cause nausea and vomiting. Fortunately, this is not a concern for our dogs unless it happens accidentally, so like always, keep alcoholic beverages out of your pets' reach.

Common Metronidazole for Dogs Dosage

Metronidazole is available orally as a pill or liquid suspension. Injectable forms are available for use in the vet's office. This drug has a very bitter taste, so liquids may be difficult to administer, even when flavored. It's best to give metronidazole with food to reduce the chance of gastrointestinal side effects.

The dosage of metronidazole typically ranges from 10-30 milligrams per kilogram given two to three times a day. Most dogs will need to be on this medication for at least five to seven days. Depending on the condition, some dogs will need treatment for a month or longer. Your veterinarian will determine the appropriate dosage and treatment course for your dog based on weight and what ailment is being treated. 

Can Dogs Overdose on Metronidazole?

Dogs can overdose on metronidazole if they get too much by mistake. As little as one extra accidental dose can cause toxicity in some cases. Metronidazole toxicity can occur even at recommended doses, especially if it's used long-term. In general, the toxic dose of metronidazole starts around 60 milligrams per kilogram. That's why it's so important to give this medication exactly as recommended by your vet.

Metronidazole overdose can damage the liver and central nervous system. Dogs tend to exhibit the serious side effects listed above.

If your dog gets too much of this drug, it's important to contact a veterinarian for advice immediately. Get in touch with your local vet or a pet emergency center, or call a pet poison control service like ASPCA Animal Poison Control at (888) 426-4435 or Pet Poison Helpline at (855) 764-7661. You may be advised to induce vomiting if the overdose happened recently. Most dogs will need follow-up treatment at a veterinary hospital.

What if My Dog Still Has Diarrhea After Taking Metronidazole?

Contact your vet if your dog still has diarrhea after finishing a course of metronidazole. Better yet, call your vet if the diarrhea is not improving (or getting worse) during metronidazole treatment. Your dog may need a longer course of medication or a different type of treatment.

Like humans, dogs have beneficial gut bacteria that aid in digestion and immune function. All antibiotics can harm beneficial gut bacteria, but metronidazole is believed to have a significant impact on the gut microbiome. This may be why some dogs on metronidazole still have diarrhea. If this is a concern for you and your dog, ask your vet about options. A good probiotic might help replace the beneficial bacteria in your dog's GI tract.