What is the best probiotic for a dog with diarrhea?

How the evidence ranks

The strain with the clearest win is not on the shelf

In a randomized placebo-controlled trial of 31 dogs with acute idiopathic diarrhea, Bifidobacterium animalis AHC7 at 2×10¹⁰ CFU a day resolved the diarrhea in 3.9 days against 6.6 on placebo (P<.01), and fewer of the probiotic dogs ended up on metronidazole (38.5% against 50%) (Kelley et al., 2009). The same strain was well tolerated for 12 weeks at a higher dose in 20 beagles (Kelley et al., 2010). No US product carries AHC7, so the best single trial in this list cannot be bought.

The other large trial belongs to a paste that combines Enterococcus faecium 4b1707 with kaolin: 148 dogs with acute uncomplicated diarrhea resolved in a median 32 hours against 47 on placebo (P=.008), with fewer needing extra treatment (3.5% against 14.8%) (Nixon et al., 2019). Kaolin is a drug ingredient, so the product is outside a supplements catalog; the trial cannot be credited to the probiotic alone.

What the US products have

FortiFlora, the single-strain SF68 sachet, has the most trials of any US dog product, and they need reading carefully. In shelter dogs with diarrhea treated with metronidazole, adding FortiFlora for seven days raised the share of days with normal stools from 46.9% to 65.6% (P=.0496), though the share normal by day 7 (68.8% against 37.5%) did not reach significance (Fenimore et al., 2017). Given on its own to 182 shelter dogs in a blinded placebo crossover, "statistical differences between groups of dogs were not detected," while the cats in the same study did benefit (Bybee et al., 2011). In 11 healthy dogs fed the strain for 16 days there were 0.6 fewer loose-stool days (Nybroe et al., 2022). So the dog evidence for SF68 is an add-on to treatment, not a standalone fix.

Visbiome Vet was tested in 25 dogs with acute hemorrhagic diarrhea: significant clinical recovery came by day 3 on the probiotic against day 4 on placebo, with lower Clostridium perfringens abundance at day 7 and fewer enterotoxin genes at day 21 (Ziese et al., 2018). Hemorrhagic diarrhea is a see-a-vet-now condition, so this supports the product alongside veterinary care, not instead of it; a later 18-dog trial found a multi-strain probiotic no different from fecal transplant in the same condition (Jugan et al., 2023).

Two trials of blends that are not sold under a US name still tell you what a probiotic can do: a five-strain blend cut the time to the last abnormal stool from 2.2 to 1.3 days in 36 dogs with acute gastroenteritis (Herstad et al., 2010), and three canine-origin lactobacilli normalized stools and lowered C. perfringens in 60 dogs (Gomez-Gallego et al., 2016). Against that, a 60-dog trial comparing a multi-strain probiotic, metronidazole and placebo found 3.5, 4.6 and 4.8 days to acceptable stool, a difference that was not significant (Shmalberg et al., 2019).

Pulling the randomized trials together, a 2022 systematic review of five of them concluded that probiotics "may improve clinical signs faster when compared to a placebo" and showed "no difference when compared directly to metronidazole" (Moore et al., 2022).

The consumer chews and powders

Most probiotic chews sold for dogs carry Bacillus coagulans, B. subtilis or blends whose strains are not named. Where the strain is B. coagulans GBI-30, 6086, the dog trials are in healthy animals: 5×10⁸ CFU a day produced firmer stools in 12 pointers (Wilson et al., 2025) with "minimal mitigation" of a diet change (Wilson et al., 2025), and 10 beagles showed no stool change at any dose (Acuff et al., 2021). No trial of B. subtilis DE111 in dogs was found. A label that does not name its strain cannot borrow any of these results.

Signs to stop comparing products and call the vet

Vomiting or diarrhea more than twice in 24 hours or with obvious illness, blood in the stool or black tarry stool, refusing water for a day, weakness or collapse, a swollen or painful belly with retching or drooling, or anything that might have been eaten: these are the emergencies the veterinary sources list, and the quiz stops on them. Diarrhea that has gone on for weeks without a vet visit is a reason to book one whatever you add to the bowl.