Metronidazole for Acute Diarrhea in Dogs: An Evidence-based Treatment?

One of the most common medicines used by veterinarians is metronidazole. Though labeled as an antibiotic, like most drugs metronidazole has multiple effects. In addition to killing some bacteria, it can be used to treat the protozoal parasite Giardia, and it has a variety of anti-inflammatory and immunomodulatory effects. However, it is primarily used to treat diarrhea. 

Like many alternative therapies I write about, metronidazole is widely used on the basis of primarily anecdotal experience. Most vets will tell you they have given it to dogs with diarrhea and seen the diarrhea stop, convincing them that the drug is beneficial. However, while it is certainly a more plausible remedy than some (e.g. homeopathy), the exact mechanism by which metronidazole should resolve diarrhea isn’t clear. The impact of the drug on the microbial flora of the gastrointestinal tract or effects on immune function might explain any benefits, but this is pretty theoretical.

Despite the fact that most vets should know better, anecdotal clinical experience is pretty persuasive. Of course, we all realize that most cases of diarrhea in which a specific cause, such as a parasite or underlying disease, is not found are cases that are going to get better no matter what we do. The use of metronidazole is very often mostly to placate pet owners tired of dealing with diarrhea and unwilling to wait a week or so for it to go away on its own, which it most often will.

Ideally, we would use metronidazole and all our other therapies on the basis of reliable controlled research evidence, but we don’t live in an ideal world. Such evidence is unavailable for many veterinary therapies, so we make the best judgement we can based on the evidence we have. Since self-limiting diarrheas in otherwise healthy dogs is not a cutting-edge medical research topic, for decades we have had to take our best guess about this treatment without good evidence.

A couple of years ago, I was excited to see a study looking at metronidazole for acute diarrhea in dogs. I wrote about the study, and summarized the other evidence for and against the use of metronidazole in diarrhea cases, in a Veterinary Practice News (VPN) column. The study found no difference in the time to resolution of diarrhea symptoms between metronidazole (4.6 days) and placebo (4.8 days). Dogs given a probiotic got better a little faster (3.5 days), but the difference was not statistically significant. This study suggested that metronidazole has little effect on the kind of mild, self-limiting diarrhea it is most often used to treat. 

The finding for the probiotic was interesting, but also worth taking with a grain of salt. The overall evidence for probioticsin treating diarrhea is mixed, and it has proven much more difficult than proponents will admit to clearly prove they work. It is also interesting that the first two authors of this study are proponents of so-called “integrative medicine,” and there is a significant potential for bias here against pharmaceutical therapies and in favor of supposedly “natural” treatments like a probiotic.

After I wrote my VPN summary (but before it was published), another study of metronidazole for acute diarrhea in dogswas published. Since I only revisit recurring topics periodically, I hadn’t had a chance to look at it until recently. This study followed a broadly similar design, but with differences in the specific population of dogs studied and how they were included or excluded from the study groups. The results differed from the prior study in finding a marginally significant difference between the groups in the duration of diarrhea (metronidazole= 2.1 days, placebo= 3.6 days, p=0.04). Once again, the vast majority (about 89%) of dogs got better with no treatment, and the groups were quite small. 

Overall, this study is more supportive of a benefit, but this is still pretty weak evidence, with fewer than 20 dogs treated and both statistically and clinically marginal effects for a condition that almost always goes away by itself. Undoubtedly, many owners would be happy to have a treatment that cut short the duration of their dogs’ diarrhea by a day and a half, assuming it was affordable (which metronidazole is) and safe (which it seems to be, though there is not extensive evidence for this either). The authors, as far as I can find, have conventional, science-based views of medicine.

So where does that leave vets in making decisions about the use of metronidazole for diarrhea? My own view is that the evidence for benefits is weak and inconsistent, that it is a plausible treatment but one for which we have to specific mechanism of action we can confidently credit for the supposed benefits, and it is used predominantly because of anecdotal experience, not strong research evidence. The drug is also used, in this context, for treating a mild and self-limiting condition. It has no obvious risks of harm to patients with the doses and time it is typically given, but the evidence for safety is no stronger than the evidence for efficacy. 

As such, it shares a lot of characteristics with many alternative treatments. 

If vets feel obliged to give owners something to try and relieve the symptoms of acute diarrhea in otherwise healthy dogs without parasites or another proven cause of diarrhea, using metronidazole may be reasonable so long as the lack of good evidence is disclosed to clients. Using probiotics is probably just as reasonable, with little to distinguish the two approaches from an evidence-based medicine perspective. Not treating at all once appropriate evaluation is done to rule out potential causes or more serious problems is probably the most reasonable thing to do, but it is unlikely many vets will be able to justify this to owners upset about their dogs’ symptoms.

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