Metronidazole for Dogs: Uses, Side Effects, and Alternatives

If your dog becomes sick with an infection or parasites, it’s good to know there are medications to help. In some cases, drugs developed for humans can be harmful to dogs, but metronidazole is a fairly safe and beneficial drug used for both people and dogs.

In This Article

• • What Is Metronidazole?
  • How Does Metronidazole Work?
  • How to Give Dogs Metronidazole
  • Side Effects in Dogs
  • Metronidazole Alternatives

Metronidazole is helpful for treating several health conditions in dogs, though it’s important to understand its usage and whether there are any side effects or alternative treatments.

What Is Metronidazole Used for in Dogs?

Veterinarians primarily prescribe metronidazole for dogs to treat diarrhea. However, Dr. Carly Fox, Senior Veterinarian in the Schwarzman Animal Medical Center’s Emergency & Critical Care Service, says new studies show that probiotics and dietary changes are equally effective treatments. For example, a paper in the journal Veterinary Sciences compared metronidazole treatment with synbiotic treatment (supplements containing both probiotic and prebiotic properties) in dogs with acute diarrhea. The researchers found that synbiotics might be a preferred treatment option as metronidazole “negatively impacts the microbiome without affecting clinical outcomes.”

However, metronidazole can treat other conditions, too, and when medically indicated, dogs can benefit greatly. These conditions include anaerobic bacterial infections, which are infections caused by bacteria that don’t require oxygen to grow. That could be anything from an abscessed tooth to a life-threatening illness.

Metronidazole for dogs is also used to treat protozoal infections, which are caused by single-celled organisms such as Giardia. Finally, the drug can be supportive in dogs with portosystemic shunts, which is a condition that causes abnormal blood flow to the liver.

How Metronidazole Works in Dogs

Metronidazole, sold under the brand name Flagyl, is in the nitroimidazole class of antibiotics. It kills anaerobic bacteria by entering their cell walls, disrupting their DNA, and causing cell death. The drug causes the DNA to lose its helical structure and break. At that point, the DNA can no longer function, and the bacteria die. It’s not fully understood how the medication kills protozoa, but again, it seems to be through disruption of their DNA.

How to Administer Metronidazole for Dogs

Although not approved for most uses in dogs by the U.S. Food and Drug Administration (FDA), metronidazole for dogs is safe to use off-label, meaning vets can still prescribe it. However, a liquid metronidazole for dogs, Ayradia, has recently been FDA-approved for the treatment of giardiasis.

Even though metronidazole can be used topically in humans, in veterinary medicine, it’s typically given orally as either a pill or a compounded liquid. Your vet or a pharmacist can create compounded medications, which are useful if your dog has trouble taking pills or needs a dosage strength that isn’t commercially available. In a hospital setting, your vet might also give your dog metronidazole intravenously.

Never start your dog on metronidazole without the direction of your veterinarian. When using the medication, follow all directions carefully. Do not stop the medication early or give extra doses.

Although Dr. Fox says there are no drug interactions to worry about, you should always talk to your veterinarian about any other medication or supplements your dog is taking, as well as your dog’s overall health. If your vet prescribes metronidazole for your dog’s diarrhea, ensure your dog has constant access to fresh water to prevent dehydration. Finally, pregnant dogs or dogs with an allergy to metronidazole or related medications shouldn’t take metronidazole. In addition, it should be used with caution in nursing dogs, puppies, and dogs with liver disorders.

Side Effects of Metronidazole for Dogs

The most common side effects of metronidazole in dogs are gastrointestinal. Dr. Fox explains that the drug can have serious consequences on your dog’s gut health. “These include profound negative effects on a dog’s gut microbiome. Flagyl has been shown to shift the microbial balance and cause a decrease in microbial diversity in the gut, which can last for prolonged periods of time.”

Side effects to watch for include:

• Nausea and vomiting
• Diarrhea
• Loss of appetite
• Lethargy and fatigue
• Drooling caused by excessive salivation

Dr. Fox warns that dogs can develop metronidazole toxicity, although it’s uncommon and usually only happens with chronic use of metronidazole at high doses or in dogs with liver dysfunction. It typically presents with nervous system issues such as ataxia (the inability to control voluntary muscle movements), lack of balance, eye twitching, or seizures. But thankfully, she says, “it’s usually reversible by discontinuing the medication and administering diazepam.”

If your dog exhibits any side effects, especially wobbliness or vomiting, stop using the medication at once and contact your veterinarian.

Alternative Medications to Consider

If your dog has diarrhea, Dr. Fox says, although metronidazole has been used in veterinary medicine for decades, alternative therapies are now recommended as first-line treatments because they’re equally or more effective. Some of these treatments involve probiotics, dietary changes, adding fiber to the diet, and other prescription medications.

However, she says that metronidazole is sometimes recommended, such as in dogs with Giardia infections or anaerobic infections/abscesses. “In many cases, metronidazole is the best antibiotic to use, and the benefits far outweigh potential risks. The choice to use metronidazole should be discussed with your veterinarian, but ultimately, you should follow their treatment recommendations.”