After analyzing over 40 probiotics across 69 trials, we’ve identified the most promising probiotics for managing the symptom of diarrhea in IBS. Our dataset (below) is ordered based on an automated scoring system, which we further refined using additional criteria (below) to select our top choices. Here are the results of our findings.

Our Top Choice(s)

1. Lactospore

In a small but promising study involving adults with IBS-D, Lactospore showed significant improvements in diarrhea symptoms. Over 90 days, participants experienced a 45.1% reduction in stool frequency and an improvement in stool consistency, shifting from loose stools to a normal form. (1) The effect size for reduced stool frequency was particularly strong, with a Cohen’s d of 2.93—a very substantial impact! Lactospore also demonstrated a strong effect size of 1.30 for decreased diarrhea symptoms overall. While the study was small, these compelling results earned Lactospore the #1 spot in our rankings. Further research with larger populations would help to validate these findings.

2. Unique IS-2

Bacillus coagulans Unique IS-2 was tested in both adults and children, showing impressive results in managing IBS symptoms, particularly for the symptom of urgency. In adults, urgency improved by 56%, with an effect size of 1.26. (2) In children, urgency improved by 71%, with an effect size of 1.10. (3) Although these studies included various IBS subtypes, the reduction of urgency—a symptom closely correlated with diarrhea—earned Unique IS-2 the #2 spot in our rankings.

3. Bio-Kult (Multi-strain)

The Bio-Kult multi-strain probiotic was studied in one of the largest IBS probiotic trials to date, involving 360 IBS-D patients. This probiotic led to a decrease in daily bowel movements, reducing the average from 6.1 to 2.5 per day. With an effect size of 0.92, Bio-Kult’s impact on diarrhea symptoms was substantial, earning it the #3 spot in our rankings. (4)

4. I.31 (Multi-strain)

In one of the clinical trials on the I.31 probiotic, participants with IBS-D and IBS-M, who were recruited during a diarrheal phase of their illness, experienced a dramatic improvement in stool consistency, shifting from diarrhea to more normal stool forms. (5) With an effect size of 3.3, I.31 had the highest impact on diarrhea symptoms among all the probiotics analyzed. However, its effect on global IBS symptoms was neutral, showing no significant overall improvement. Although the smaller sample size of the probiotic-only group in this study warrants further research, the exceptionally high effect size in these preliminary results earned I.31 the #4 spot in our rankings.

Our Criteria

The probiotics in our full database (below) are automatically ranked based on their 1) effect size for diarrhea relief, followed by 2) effect size for overall IBS symptoms (‘Global IBS Symptoms’), and finally by 3) the quality of evidence from the studies.

For this summary, we selected our top choices based on these rankings, along with the following additional criteria: 

• Evidence Quality Score: Greater than 75%, specifically for studies focused on diarrhea.  
  • For example, S. cerevisiae I-3856 was excluded because the study showing benefits for diarrhea had a lower quality score and conflicted with neutral results from three higher-quality studies.
• Effect Size: Probiotics had to demonstrate a moderate to high effect size (>0.5) for diarrhea-related symptoms.
• Clear Dosing Information: Only probiotics with clear and reliable dosing data were included.
• Commercial Availability: Probiotics needed to be commercially available to consumers.

No Direct Comparisons

It’s important to note that we do not have direct comparisons between these probiotics across identical groups or study methods. Therefore, we cannot definitively rank one probiotic as superior to another. Each strain has shown promise in managing diarrhea in IBS, but always consult your healthcare provider before starting any new probiotic or supplement.

*Population values represent the per protocol population size from studies specifically assessing a diarrhea-related parameter.
*The population reported for I.31 reflects the per protocol population of both the probiotic-only study arm and the placebo group.
*Effect sizes are reported as the weighted mean effect size across studies.

References

1. Majeed M, Nagabhushanam K, Natarajan S, et al. Bacillus coagulans MTCC 5856 supplementation in the management of diarrhea predominant irritable bowel syndrome: a double-blind randomized placebo-controlled pilot clinical study. Nutr J. 2015;15:21. doi:10.1186/s12937-016-0140-6.
2. Madempudi RS, Ahire JJ, Neelamraju J, et al. Randomized clinical trial: the effect of probiotic Bacillus coagulans Unique IS2 vs. placebo on the symptoms management of irritable bowel syndrome in adults. Sci Rep. 2019;9:12210. doi:10.1038/s41598-019-48554-x.
3. Sudha MR, Jayanthi N, Aasin M, et al. Efficacy of Bacillus coagulans Unique IS2 in the treatment of irritable bowel syndrome in children: a double-blind, randomised placebo-controlled study. Benef Microbes. 2018;9(4):563-572. doi:10.3920/BM2017.0129.
4. Ishaque SM, Khosruzzaman SM, Ahmed DS, Sah MP. A randomized placebo-controlled clinical trial of a multi-strain probiotic formulation (Bio-Kult®) in the management of diarrhea-predominant irritable bowel syndrome. BMC Gastroenterol. 2018;18(1):71. doi:10.1186/s12876-018-0788-9.
5. Barraza-Ortiz DA, Pérez-López N, Medina-López VM, et al. Combination of a probiotic and an antispasmodic increases quality of life and reduces symptoms in patients with irritable bowel syndrome: a pilot study. Dig Dis.2021;39(3):294-300. doi:10.1159/000510950.

When selecting our top probiotic picks, we ensure they are commercially available and backed by high-quality evidence, with an average score above 75% for the relevant symptom parameters. The probiotics must also demonstrate a moderate to high effect size (>0.5) for these symptoms. If there are more than five candidates, we limit our top picks to the best five based on the effect size for relevant symptoms and Global IBS symptoms, as well as evidence quality. This ensures our recommendations are practical and reliable.

When selecting our top probiotic picks, we ensure they are commercially available and backed by high-quality evidence, with an average score above 75% for the relevant symptom parameters. The probiotics must also demonstrate a moderate to high effect size (>0.5) for these symptoms. If there are more than five candidates, we limit our top picks to the best five based on the effect size for relevant symptoms and Global IBS symptoms, as well as evidence quality. This ensures our recommendations are practical and reliable.