I have covered the subject of probiotics many times in the ten years since the beginning of this blog. I last summarized the evidence in a 2017 post reviewing all of my previous articles as well as the results of a narrative review published that year. My conclusion at that time was:
- There are few studies, and those that have been done have significant limitations and often conflict.
- There is reasonable evidence for some clinical benefit in acute diarrhea associated with stress or antibiotic use.
- There is no high-quality, consistent evidence for most suggested uses of probiotics.
- The unregulated probiotic products on the market today are plagued with inaccurate labeling and poor quality control. This means that even if probiotics might be beneficial in some cases, it is unclear if the actual products available could achieve these benefits.
- There do not yet appear to be significant risks to probiotics, though the evidence for this safety also quite limited.
Just this month, a systematic review of probiotics for gastrointestinal (GI) disease in dogs has been published which critically evaluates and summarizes all of the research to date on this subject.
Jensen AP. Bjornvad CVR. Clinical effects of probiotics in prevention and treatment of gastrointestinal disease in dogs: A systematic review. J Vet Int Med. 2019;33:1849-1964.
Since the most common use of probiotics in veterinary medicine is for gastrointestinal disease, and dogs are the most common species treated (though many cats are treated with probiotics for gastrointestinal problems as well), this is a very relevant and useful review. Given the results of several studies showing that the most common treatment for diarrhea in dogs, the drug metronidazole, probably isn’t effective (1,2,3) probiotics are often seen as a viable alternative, so reliable evidence concerning their effects is needed. Systematic reviews are the most reliable form of evidence summary available, and they can give an accurate overview of the strength of evidence for particular, specific issues in medicine.
Unfortunately, the news is not good. The review found only 17 clinical studies relevant to whether or not probiotics are useful in canine GI disease. These studies were mostly small and often plagued by significant methodological limitations. The overall conclusions of this review were:
- the evidence points toward a limited and possibly clinically unimportant effect for prevention or treatment of acute gastrointestinal disease.
- for chronic gastrointestinal disease, dietary intervention remains the major key in treatment, whereas probiotic supplement seems not to add significant improvement
- this conclusion is based on a limited number of studies, with a wide methodological diversity, and mainly low sample sizes.
- there is a high risk that most of the studies evaluated in the current review were severely underpowered especially taking into consideration that baseline characteristics of study groups were generally very poorly documented
- 12 of the  studies reported industry involvement; with such a high degree of industry involvement, there is a risk of publication bias as the incentive to publish studies showing no effect of probiotics could be low
This doesn’t necessarily mean that probiotics aren’t useful or effective for GI disease in dogs. What it does mean is that there isn’t yet any good evidence supporting the use of probiotics for treating or preventing acute or chronic GI disease in dogs, and the evidence we do have is not very reliable. It is very frustrating to see so many research studies done in veterinary medicine that are too small or have other flaws that make it impossible to trust their findings. It is a waste of resources and ethically questionable to conduct studies that don’t do a reasonable job of answering the question they are designed to answer.
At this point, the best we can say about probiotics is that they might be useful but we don’t yet have real evidence to show this, individual products are often mislabeled and quality control is poor, and the risks appear to be low but these haven’t been effectively studied. Ten years since I first wrote about this subject, that is a disappointing level of evidence, and it is frustrating to still be guessing about this treatment.