Metronidazole is an antibiotic commonly prescribed for treating bacterial and parasitic infections in dogs. The research documenting its toxicity dates back to the ‘80s and consistently shows the massive risks of using this drug. Regardless, veterinarians continue to prescribe it to ill dogs who are mostly coming into the clinic with digestive issues. The vets do inform dog owners about the risks, though usually not about the risks this drug poses to the dogs but the hazard it poses to them as owners, simply by administering it.
Your veterinarian will caution you to wear gloves when handling this medication, as simple contact with the skin can increase your chance of cancer. But we’re expected to give this to our dogs to … ingest? This article highlights the four horrifying, well-documented metronidazole toxicities and emphasizes the urgent need for better-informed veterinarians who reach for safer treatment options.
Metronidazole is mostly prescribed for digestive problems in dogs — giardia infections, helicobacter, colitis, diarrhea, vomiting, canine IBD (inflammatory bowel disease), constipation, etc. In addition to all this, veterinarians may also prescribe metronidazole for UTIs, dental infections, skin infections, endometritis, hepatic encephalopathy, bone & joint infections, respiratory tract infections, colorectal perioperative infections, central nervous system infections, sepsis, and other bacterial infections.
It’s interesting that a drug NOT approved by the FDA for veterinary use is so widely prescribed in veterinary clinics. In North America, metronidazole is sold under the brand name Flagyl and is routinely used in veterinary medicine off-label. In Europe and the UK, one commercially available product is on the market for dogs & cats, sold under the name Metrobactin. It’s approved for use on pets with bacterial infections of the gastrointestinal tract, urinary tract, mouth, and skin.
The package leaflet warns that metronidazole is not safe for dogs with liver diseases! It also warns against the use on dogs with a “hypersensitivity to the active substance” (that is, metronidazole) or to any of the excipients, which are not listed on the leaflet. How would somebody know that their dog is “hypersensitive” to metronidazole unless they’ve already had a bad experience? Under adverse reactions, Dechra (the manufacturer) lists vomiting, hepatotoxicity, neutropenia (low white blood cell count), and, simply, “neurologic signs.” Perhaps they’ve skipped the details of neurologic adverse effects on purpose … but we certainly won’t.
Metronidazole works by damaging bacterial and parasitic DNA, ultimately leading to their death. Its effect on the DNA doesn’t stop with bacteria and parasites. The European Agency for the Evaluation of Medicinal Products (EMA) reported in 1997 already that metronidazole has caused gene mutations in mammals, clearly stating that “recent studies in man show a genotoxic mechanism in vivo.” This toxicity was observed after a single therapeutic dose! Genotoxicity disrupts the proper functioning of the cells, leading to several permanent and life-threatening consequences.
The package insert for Metrobactin states: “Metronidazole has confirmed mutagenic and genotoxic properties in laboratory animals, as well as in humans.” These disclosures are crystal clear, in alignment with each other, and shared even by a company actively profiting from selling metronidazole.
Not everyone shares this level of transparency. Flagyl’s package insert includes a statement on genotoxicity that directly contradicts the EMA’s report. It says: “Although metronidazole has shown mutagenic activity in a number of in vitro assay systems, studies in mammals (in vivo) have failed to demonstrate a potential for genetic damage.” We reached out to Pfizer, the manufacturer of Flagyl, asking them to clarify this statement for us. Which specific studies are they referring to, and how come they diverge from other independent studies showing the opposite?
After skillfully evading our direct line of questioning for several emails, they finally outright refused to confront or comment on the EMA’s 1997 veterinary report confirming genotoxicity in mammals, saying: “It is not routine practice to compare labels between animal health and human medicines.” This is truly a bizarre response, not least because humans are mammals, and the EMA report specifically mentioned a human study! Pressed for details on the specific studies they’re referring to in their bold package statement, they told us that “authorizations are based on a robust and independent evaluation of the scientific data on quality, safety and efficacy,” yet again refusing to provide any details on this mysterious scientific data — even after being directly asked to disclose it multiple times. If those studies backed their claim, wouldn’t they be elated to share them?
The way metronidazole damages the brain is not a secret in pharmacological circles, though it’s certainly an obscured fact in veterinary medicine. Metronidazole has consistently been confirmed to cause neurological damage by inducing seizures, tremors, and ataxia (lack of balance & coordination). This information has been known since the 1980s when a small study done between 1984-1987 confirmed metronidazole’s effect on the central nervous system in dogs.
Five dogs were included in the study, two of which suffered neurological dysfunction so severe they needed to be euthanized. It’s true that the dosage used in this study was on the high end of the spectrum, so it’s important that we also look at the studies that used lower doses, especially doses in the recommended range for use in dogs. According to AAVPT (American Academy of Veterinary Pharmacology and Therapeutics), “neurologic disturbances (ataxia, nystagmus, seizures, tremors, weakness) […] have been reported with doses as low as 30 mg/kg.” This is a lower dose than the recommended therapeutic dose, which is 50 mg/kg.
Metronidazole’s neurotoxicity doesn’t seem to be dose-dependent at all. At any dose, metronidazole poses a risk to our dogs! The EMA even wrote in their report that the highest dose without an adverse effect couldn’t be determined.
It’s unacceptable that someone could bring their dog into a vet clinic for chronic diarrhea only to find themselves with a neurologically damaged pet days later. One heartbreaking study reviewed medical records from four UK veterinary hospitals and found 26 confirmed cases of metronidazole-induced neurotoxicity. 54% of dogs were prescribed metronidazole for diarrhea. For 92% of dogs, the signs of neurotoxicity first appeared after ten days of treatment — most of the treatments continued for a full month regardless. The doses ranged from 26-112 mg/kg, once again confirming that neurotoxicity is NOT dose-dependent and occurs at lower doses too.
The International Agency for Research on Cancer (IARC) recognizes metronidazole as a Group 2 carcinogen. The basis for this classification is that the research they reviewed gave “inadequate evidence for carcinogenicity to humans” but “sufficient evidence for carcinogenicity to animals.” In the animal studies they looked at, metronidazole correlated with a high number of cancerous tumors in the lungs, liver, colon, mammary glands, testicles, and the pituitary gland. It also increased the chances of lymphoma in female animals.
Studies on rodents showed the development of mammary tumors and lymphoma at a 30 mg/kg dose. Metrobactin’s package insert openly discloses that “metronidazole is a confirmed carcinogen in laboratory animals.” It goes on to warn us against touching the pills without gloves and urges us to thoroughly wash our hands after handling this medication.
It’s natural to wonder how can a substance like this possibly be legal? Well, it’s not — but only for livestock. Using metronidazole to treat farm animals intended for human food is strictly prohibited. This means that a veterinarian legally cannot give this to a cow in the morning but can comfortably give it to a dog in the afternoon — and off-label at that!
The current mortality rate for cancer stands at 50%, meaning one in two dogs will lose their life as a result of it. How can veterinarians possibly justify using known carcinogenic drugs in their treatments? We urgently need more veterinarians to adopt a holistic approach in their practice and start pushing back against carcinogenic drugs, such as metronidazole!
In metronidazole’s case, reproductive toxicity is a culmination of its genotoxic and carcinogenic toxicities. We already mentioned some of the side effects relating directly to the reproductive organs, such as mammary and testicular tumors. In addition to that, metronidazole is highly dangerous for pregnant animals as it breaches the placenta and affects the organs of the fetus. It also enters the mother’s milk supply.
The EMA confirms long-term studies have shown reproductive toxicity and warns that metronidazole’s effects on the male reproductive organs could have implications on the animal’s fertility. Due to its genotoxic properties, metronidazole can cause gene mutations in a fetus, leading to birth defects. Pregnant women are warned against handling metronidazole tablets when their dog is undergoing treatment!
As metronidazole is predominantly prescribed for bacterial and parasitic issues within the digestive system (diarrhea, colitis, inflammatory bowel disease, constipation, vomiting), your first line of defense when noticing these symptoms will be to strengthen your dog’s GI tract. That will automatically fortify the immune system, helping your dog fight off possible infections more effectively. Here are some of the best ways to support the healing of your dog’s digestive tract safely and naturally:
- PROBIOTICS. They are essential for a rich and balanced microbiota in your dog’s gut. A quality probiotic supplement added to your dog’s daily meals can make the biggest difference for chronic diarrhea!
- ENZYMES. Necessary for the health and longevity of the cells, digestive enzymes play an important role in repairing compromised parts of the digestive system that have been impacted by a parasite or bacteria.
- QUALITY DIET. Don’t underestimate the healing properties of a species-appropriate, nutritionally-diverse diet! Small changes go a long way when improving the nutritional profile of your dog's meals.
- FASTING. In the early stages of digestive disturbances, implementing a fast can be very helpful. Your dog’s upset digestive system will benefit from the reset as you work on finding the underlying cause for the disturbance.
- FIGHT THE PARASITES. If you suspect your dog has a parasite, immediate action is needed. Order blood, stool, and urine tests with your holistic veterinarian to get a clear picture of what is going on. Drugs like metronidazole are often prescribed on a guesstimate, without confirmation of true parasite presence. Giardia is one of the most common parasites, infecting around 30% of dogs. Find natural remedies for giardia and other parasites in this article!
- ENRICHMENT. Restoring the balance within your dog’s gut is a process that takes time and patience. Taking care of your dog’s physical and mental enrichment is important every day, but even more so when the immune system is weakened. Did you know that the canine brain and gut share a direct connection? Keep your dog's brain occupied and don't skip the daily walks!
- ASK FOR HELP. Chronic diarrhea often resolves itself once the homeostasis in the gut is restored. That said, some cases of chronic diarrhea need urgent veterinary care. The telltale signs are refusing to eat, refusing to drink, apathy, lack of energy, refusing to make eye contact, shaking, and excessive sleeping. These signs need to be taken very seriously — if you notice them in your dog, call a holistic veterinarian immediately.
There's a substantial amount of independent research consistently proving four main toxicities of metronidazole, with the first official reports dating as far back as the 80's and 90's. Not only has this product not been pulled from the market as a result of these reports, but it's been continuously pushed onto the market in spite of them. Metrobactin, the European metronidazole drug approved for veterinary use, hit the market in 2016 — that’s 19 years after the EMA revealed metronidazole’s multiple toxicities in both animals and humans.
We cannot rely on regulatory authorities to guard the market from toxic drugs. We cannot trust them to block medications with horrifying adverse effects that are worse than the original condition. Most painfully, we also cannot unconditionally trust veterinarians anymore. Whether they are ill-informed or simply manipulated by the drug manufacturing companies, the fact of the matter is that dog owners cannot rely solely on veterinary opinion.
Even worse, we’re at a point where it’s unsafe to rely on veterinary consultation alone. We are our dogs’ caretakers and protectors … but within the pharmacological hegemony, we must also be their fact-checkers and expertly informed medical decision-makers.
Laurentie, Sylviane. “Neurotoxicity of metronidazole, an antibiotic and antiparasitic medication, in dogs: risks even at low doses.” Veterinary Pharmacovigilance, 03/2021.
Dow, S. W. LeCouteur, R. A. Poss, M. L. Beadleston, D. “Central nervous system toxicosis associated with metronidazole treatment of dogs: five cases (1984-1987).” PubMed, 01/08/1989.
Tauro, A. Beltran, E. Cherubini, G. B. Coelho, A. T. Wessmann, A. Driver, C. J. Rusbridge C. J. “Metronidazole-induced neurotoxicity in 26 dogs.” Wiley, 26/11/2018.
Committee For Veterinary Medicinal Products. “Metronidazole.” EMA, 07/1997.
“Metronidazole.” The United States Pharmacopeial Convention, 2007.
“Metrobactin.” Dechra, 2021.
“Metronidazole (Group 2B).” National Library of Medicine.
“Flagyl.” FDA, 08/2003.
Luna’s passion for learning about canine psychology and behavior began when she adopted a severely reactive puppy from a local shelter. She is now a big advocate for positive reinforcement and compassionate training. As a writer, she strives to spotlight the topics that fly under the radar and be the voice for all who cannot speak for themselves.