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What are probiotics and how might they help?

Probiotics are beneficial bacteria or yeasts that can be given to dogs that may support a balanced gut environment. They are usually used short term during tummy upsets, and can be provided longer term for dogs with recurring sensitivities (or ‘tricky’ systems). Their effects are likely condition and strain specific (Shmalberg et al. 2019) and thankfully the research into these little gut soldiers is significant and ongoing.

Acute diarrhoea: what does the research show?

Interestingly, in a university-run randomised controlled trial with ‘uncomplicated’ acute diarrhoea, the time taken to return to acceptable stools was 3.5 days on the probiotics, 4.6 on metronidazole (prescription drug), and 4.8 on a placebo. The research team suggest there was no significant difference (Shmalberg et al. 2019) between these times, but it does highlight that not all cases require prescription medication, and that supportive care: including diet and hydration, can often be sufficient for mild, uncomplicated upsets.

Chronic tummy issues and IBD: where probiotics look more promising

The evidence seems more clear cut in dogs with chronic tummy issues and IBD (Inflammatory Bowel Disease), when given VSL#3: a high strength, brand name multi-strain probiotic formula containing a mix of 8 different live bacterial strains, mostly Lactobacillus, Bifidobacterium, and Streptococcus thermophilus. The dogs in this trial showed reduced clinical scores and improved tissue immune markers compared with a prescription drug combo (Rossi et al. 2014). This suggests some dogs with chronic conditions may show improvements when probiotics are used, although larger controlled trials are needed.

Choosing a probiotic: strain first, marketing last

Look for canine-researched strains and a clear total CFU at end of shelf life. Make sure you adhere to the storage instructions, too. Whilst the research doesn’t provide exact evidence on the method of delivery, powders often mix more easily with meals for fussy eaters, and don’t contain additional additives, fillers or binders used to create chews - which can sometimes be difficult for sensitive tummies. It’s worth noting that probiotics effects are strain specific and condition dependent (Shmalberg et al. 2019), so any ‘off the shelf’ probiotics might not garner the intended result. 

Products should outline expected use cases such as “digestive support” and provide clear serving guidance, along with batch numbers for traceability. 

Specific research: Bacillus subtilis C-3102 & Bacillus velezensis C-3102

In healthy dogs under study conditions, Bacillus subtilis C-3102 was associated with firmer stools, reduced smelly by-products, and even help dogs get a bit more out of their food by improving nutrient digestibility (Scauf et al. 2019). In a more complex, double-blind, placebo-controlled study, there was also evidence that B. subtilis C-3102 improved stool quality in dogs with chronic diarrhoea. 

Whilst Bacillus velezensis C-3102 is newer on the dog scene, there is evidence that dogs receiving chews with this specific probiotic reported fewer scooting and licking episodes. Researchers propose it may influence stool form and fermentation activity (short-chain fatty acids), which are associated with a balanced gut environment (Rossi et al. 2025).

When probiotics are not enough

Red flags to watch out for include blood in your dog’s stool, repeated vomiting, lethargy, severe dehydration, or any one of these (even if mild) in very young or elderly dogs, and for all dogs if persistent for more than 48-72 hours.

For mild tummy upsets, hydration, small frequent meals, and a canine-specific probiotic can be considered. Rossi et al (2014) note that larger scale studies are warranted to evaluate clinical benefits and therefore expectations must be set accordingly, but the research so far is clearly worthy of consideration.