If your dog is suffering from diarrhea, inflammatory bowel disease, tooth or gum disease, or parasites, your vet may prescribe a drug called Metronidazole (Flagyl). In this blog, we explore the uses, dosage, and side effects of Metronidazole for dogs.
Questions to Ask If Your Veterinarian Prescribes Metronidazole
While metronidazole has been shown to be an appropriate choice for some conditions, in other cases, it may not be as effective as many veterinarians assume. We recommend that you talk with your veterinarian about whether metronidazole will be beneficial for your dog. Here are some questions to ask:
In many cases of dog diarrhea, the answer may be no. Though some health issues, such as life-threatening bacterial infections, do require antibiotics, other conditions may resolve with supportive care alone.
For example, dogs with bloody diarrhea (hemorrhagic gastroenteritis) are often given antibiotics, but according to veterinary consensus guidelines, antibiotics are appropriate for this condition only when sepsis is present. Researchers have found that as long as sepsis is not involved, even hemorrhagic gastroenteritis responds just as well to supportive care as to antibiotics.
In fact, several studies have shown that metronidazole can actually make diarrhea worse, not only by reducing populations of “good” anaerobic bacteria but also by altering the intestinal mucus.
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:
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.
Metronidazole is available orally as a pill or liquid suspension. Injectable forms are available for use in the vets office. This drug has a very bitter taste, so liquids may be difficult to administer, even when flavored. Its 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.
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 its used long-term. In general, the toxic dose of metronidazole starts around 60 milligrams per kilogram. Thats why its 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, its 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.
When Is Metronidazole the Right Choice?
Metronidazole is an extremely useful antibiotic and antiprotozoal medication that’s been around since the 1950s. In both human and veterinary medicine, it’s used to treat infections caused by anaerobic bacteria (bacteria that don’t need oxygen to survive). Since it kills the bacteria rather than simply halting their growth, metronidazole has the potential to work faster and more efficiently than other antibiotics (such as clindamycin) that also target anaerobic bacteria.
Metronidazole is effective against Bacteroides fragilis, for example, which is a normal bacterial member of the gut and mouth microbiomes but can also be a culprit in wound infection, abscesses, chest infection (often associated with pneumonia), and liver infection. Metronidazole is also used to treat infections of the gallbladder and bile ducts, and because it can enter bone, it’s useful for infections involving the jaw and mouth.
Metronidazole also works well against Clostridioides—a group of bacteria that includes Clostridium difficile (C. diff), which is a well-known cause of diarrhea in humans and animals.
In both cats and dogs, metronidazole may be used to treat cases of gastritis caused by Helicobacter bacteria (such as H. pylori) in the stomach. It’s also a valuable weapon against sepsis, a life-threatening immune system response that damages tissues and organs. When it’s used for Helicobacter therapy and for sepsis, metronidazole is usually combined with other antibiotics in order to cover a broader range of bacteria.
In the past, metronidazole worked well against Giardia, a protozoan parasite that causes diarrhea in dogs. Over time, however, that organism has developed a resistance to metronidazole, so this medication is no longer effective by itself against Giardia. (We’ll have more to say about the problem of antimicrobial resistance a little later in this article.)
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