Do Probiotics Work for Dogs? What the Studies Found

The evidence is mixed, and much of the research in dogs is small.

Some trials found a small benefit. Others found no difference. Pooled together, the evidence in dogs with gut problems looks limited.

What a probiotic is

An international expert panel reaffirmed the long-standing definition of a probiotic: live microorganisms that provide a health benefit to the host when given in adequate amounts (Hill et al., 2014, Nature Reviews Gastroenterology & Hepatology).

That is a consensus on terminology, with no dogs involved. Whether a given product meets it in a given dog is a research question.

What the reviews found

A systematic review of 17 studies in dogs published before April 2017, most of them small, concluded that the data point toward a very limited and possibly clinically unimportant effect in sudden gut upset, and that in long-running problems probiotics seemed not to add significant improvement beyond diet changes (Jensen and Bjørnvad, 2019, Journal of Veterinary Internal Medicine).

A 2023 meta-analysis of randomized trials in dogs with sudden diarrhea grouped prebiotics, probiotics and synbiotics (probiotic and prebiotic combined) together. It found no clinically significant shortening of the episode and rated the certainty of that evidence very low to moderate (Scahill et al., 2023, Veterinary Journal).

A systematic review gathers the studies that meet set criteria, and a meta-analysis goes a step further and combines their numbers.

What shifts the gut microbiome

Diet and antibiotics can both reshape the gut microbiome in dogs. The papers below looked at the microbiome itself, not at how a probiotic performed.

In a diet trial of 64 Labrador retrievers and beagles at one pet care center, the assigned diet changed the gut microbiome in a large, repeatable way (Coelho et al., 2018, Microbiome). But a review says such changes pale beside those seen in dogs with gut disease (Pilla and Suchodolski, 2020, Frontiers in Veterinary Science). Some authors of the diet trial are affiliated with a pet food company, and that company made the diets.

A metronidazole study in 24 healthy dogs found changes in gut bacteria still present weeks after the drug was stopped (Pilla et al., 2020, Journal of Veterinary Internal Medicine). A prescribed antibiotic course is a decision for the prescribing veterinarian.

None of these papers tested whether a dog's diet or antibiotic history changes how well a probiotic works.

What the individual trials found

In 36 dogs with sudden, self-limiting diarrhea, a placebo-controlled trial found the last abnormal stool came sooner with a probiotic combination than with placebo, 1.3 days on average versus 2.2 (Herstad et al., 2010, Journal of Small Animal Practice). In 60 dogs with sudden diarrhea, a probiotic, an antibiotic and a placebo did not differ significantly in time to formed stool. The authors called the trial underpowered and reported that its probiotic capsules lost potency in storage (Shmalberg et al., 2019, Frontiers in Veterinary Science).

Shelter results are no clearer. A placebo-controlled trial of 773 healthy dogs entering a UK shelter found diarrhea on a smaller share of days with a synbiotic than with placebo, 2.0% versus 3.2% (Rose et al., 2017, Journal of Veterinary Internal Medicine). Its capsules came from a commercial probiotics maker, and its first author lists a probiotics company as an affiliation. A shelter trial that also enrolled cats detected no difference in its 182 dogs, where diarrhea was uncommon (Bybee et al., 2011, Journal of Veterinary Internal Medicine).

None of these trials tells an owner how long to wait before judging a product. A gut problem that lingers needs a veterinary exam.

What a label audit found

A study published in 2011 tested 25 commercial probiotics marketed for animals and found that only 4 of the 15 making specific claims about live organisms met their label claim. The authors urged veterinarians and owners to demand evidence of quality control and efficacy (Weese and Martin, 2011, Canadian Veterinary Journal).

That test is more than a decade old and cannot grade any product sold today. It does show that a label alone was not a reliable guide. A review of the evidence in dogs adds that strain-specific effects have not been evaluated sufficiently (Schmitz and Suchodolski, 2016, Veterinary Medicine and Science), so it is worth asking, with your vet's help, which strains a product contains and whether they have been studied in dogs.

When to skip the aisle and call the vet

If your dog has blood in the stool, keeps vomiting, is losing weight, stops eating, or has diarrhea that lingers or keeps coming back, call your veterinarian. A supplement does not replace an exam, and waiting on one to work while a real problem develops costs the dog time.

Where your veterinarian fits in

Whether a probiotic makes sense at all is a vet question, and your vet will want to know your dog's diet, history and medications, and what the stool has actually been doing for the past month. Write that last part down before the appointment. Owners remember impressions, and vets can work with a dated list.

Two of the reviews above, both about dogs, are free to read in full and worth bringing along: the systematic review by Jensen and Bjørnvad and the review by Schmitz and Suchodolski.

Where to go from here

We read the joint-supplement research the same way in senior dog joint care.