Diarrhea can be caused by many factors. The common ones are:
- Specific bacteria infection
- Virus infection
- Allergy-style response (including mast cell issues)
The typical results is ‘bio-forming’ your guts to be more friendly to the cause. This can result in bacteria shifts across multiple taxonomies which may persist after the root cause has been eliminated.
Excluding antibiotic induced diarrhea
“However, the quantity of Enterococcus and Bifidobacterium groups were significantly higher in healthy controls than in diarrheal group (p = 0.000076 and 0.001, respectively).”
- So lots of Bifidobacteria probiotics and
- Enterococcus faecalis probiotics (linked to list). There are lots of studies on its use in veterinary practise.
“the effectiveness of zinc at a dose of 20 mg was higher than that of probiotics”
Antibiotic associated diarrhea (AAD)
- AUTHORS׳ CONCLUSIONS: Moderate quality evidence suggests a protective effect of probiotics in preventing AAD (in children). 
- Based on high-dose probiotics, the number needed to treat (NNT) to prevent one case of diarrhea is seven (NNT 7; 95% CI 6 to 10). However, a GRADE analysis indicated that the overall quality of the evidence for the primary endpoint (incidence of diarrhea) was low  – that is 1 person in 7 showed benefit.
While Bacillus lecheniformis significantly reduced the incidence of AAD in patients older than 65 relative to no treatment, L. acidophilus, L. casei Shirota, S. cerevisiae (boulardii) lyo, mixture of L. acidophilus and B. bifidum, and mixture of L. acidophilus CUL60, CUL21, B. bifidum CUL20 and B. lactis CUL34 were not effective in reducing AAD in this population…
a mixture of L. rhamnosus R011 and L. helveticus R052 did not demonstrate efficacy in preventing AAD in adults of all ages
Probiotics did not have a significant effect on incidence of gastrointestinal disorders when compared with placebo only;17 on incidence of gastric pain when compared with no treatment only[18 on incidence of cramping, nausea, fever, soft stools, flatulence, and taste disturbance when compared with placebo or usual care;10 and on incidence of undisclosed adverse events when compared with placebo or no treatment.Probiotics for Antibiotic-Associated Diarrhea and Clostridium difficile Infection: A Review of Clinical Effectiveness 2018
This is usually caused by bacterial infection. See this paper on treatment.
These results are contrary to my expectation, i.e. urban-myth about the effectiveness of probiotics. Zinc, Enterococcus faecalis probiotics , and high dosages bifidobacterium appears to be best course.