Frequently Asked Questions
From Healthcare Practitioners

There’s no universal optimal number of strains or CFUs—what matters most is whether the specific probiotic strain or strain blend has been studied in a randomized placebo-controlled trial and shown to be effective for the condition you’re looking to address.

I’ve seen products with as little as 2 billion CFU work well, while others with 900 billion CFU/day also deliver great results. More isn’t necessarily better—efficacy depends on the strain(s) used and the supporting evidence.

Probiotic supplements can be formulated as either single-strain or multi-strain products. Some multi-strain formulations are carefully designed based on strain-specific research to ensure that the included strains work synergistically to provide health benefits.

However, not all combinations are backed by evidence, and a higher number of strains does not necessarily mean a more effective product. Strains can interact in ways that enhance, diminish, or even counteract each other’s effects. Because of this, selecting a probiotic with well-researched strains at clinically relevant doses is far more important than simply choosing one with the most strains.

The only universal recommendation for ensuring probiotic survival is to follow the dosing instructions provided by a trusted manufacturer.

Probiotic viability depends on factors such as strain resilience, encapsulation technology, and interactions with food and the gastrointestinal environment—all of which reputable manufacturers account for when formulating and establishing dosing guidelines. Because of this, adhering to label instructions remains the most reliable approach.

Regarding subtypes, there were too few studies evaluating subtype-specific effects to categorize probiotics this way. Additionally, many studies that attempted to account for subtype-specific effects had small sample sizes, making statistical analysis underpowered.

For this reason, we focused on specific symptoms rather than IBS subtypes. You can access symptom-specific probiotic research through:

• Answering quiz questions in the “Get Research Results” tab.
• Using direct symptom page links at the bottom of the website.
• Clicking: Compare Probiotics → Access Database → Symptom(s)

From there, you must specify the strength of effect (strong, moderate, weak) and select the relevant symptoms to review database results.

For information on commercially available probiotics that contain studied strains and doses, visit probioticfinder.org. To access our Top 5 Probiotic Summary Report, simply enter your email in the ‘What Are the Top Probiotics?’ box to the right. The report will be delivered to your inbox and available for download, including a breakdown of the most promising probiotics we’ve reviewed and a list of commercially available products featuring these strains.

Among the top probiotic picks, seven products appeared in at least one of the top five rankings within the database for the 10 core symptoms evaluated. Below are the page links for each of these seven probiotics, with the number of times they were featured across the core 10 symptoms noted in the right-hand column. 

To qualify as a top pick, a probiotic had to meet four key criteria:

1. An overall evidence quality weighted mean score >75%, as determined using the evidence quality indicators and scoring methods described here: IBS Probiotics Methodology.
2. A weighted mean effect size of at least 0.5, which reflects the magnitude of benefit compared to placebo. This threshold was set based on database findings, where approximately the top half of beneficial effect sizes were 0.5 or greater.
3. Clear dosing information available in the research studies to ensure consistency in use.
4. Commercial availability, meaning the product can be purchased by consumers. 

It’s important to recognize that studies assessed different ranges of symptom parameters—some broadly evaluating multiple symptoms, while others focused on a narrower scope. Because of this, some products may have demonstrated beneficial effects more frequently simply due to the breadth of the parameters measured, while others may have similar potential but were not as extensively studied in clinical trials.

Our Top 5 Probiotic Summary Report provides a deeper dive into some of the most promising probiotics, detailing their reported effects and listing commercially available products that feature these strains. To access the report, enter your email in the ‘What Are the Top Probiotics?’ box to the right. The report will be delivered to your inbox and available for download, including a breakdown of the most promising probiotics we’ve reviewed and a list of commercially available products featuring these strains.

You can reference ProbioticFinder.org to see which products feature studied probiotic strains. However, the database does not categorize probiotics by brand names. Instead, it organizes them based on the strain or strain-blend names used in research studies. This means that while some well-known brands may contain strains included in the database, they may not be listed under their commercial brand name.

In some cases, a probiotic’s research name aligns with a brand name, making it easier to find. Otherwise, the database lists:

• The formal strain or strain-blend names or common nicknames (e.g., LGG, De Simone formulation).
• A strain-level designation as a shorthand identifier (e.g., Lactobacillus acidophilus DDS-1 → DDS-1).
• For blends, each strain-level designation is listed with an “x” between them (e.g., the 3-strain blend of Bifidobacterium infantis M63, Bifidobacterium breve M16V, and Bifidobacterium longum BB536 → M63 x M16V x BB536).

This structure ensures that probiotics are classified based on their clinical relevance rather than being tied to a single brand. Many strains appear in multiple products, and listing them under only one brand could create bias or misrepresent their broader availability.

For easy navigation, direct links to specific probiotic strain and strain-blend pages are available at the bottom of the home page.

The definition of probiotics stems from a 2014 consensus statement from the International Scientific Association for Probiotics and Prebiotics (ISAPP), which describes probiotics as:

“Live microorganisms that, when administered in adequate amounts, confer a health benefit on the host.”

Alongside this definition, the ISAPP consensus paper outlines additional criteria for microorganisms to be considered probiotics, including:

• ✔ They must be a defined entity
• ✔ They must be delivered at an effective dose

• ✔ They must be safe for their intended use

What does it mean for a probiotic to be a “defined” entity?

A probiotic product should transparently disclose the levels of microbes present, ideally identifying them to the genus, species, and strain level.

In some instances, species-level identification may suffice—for example, yogurt cultures known to metabolize lactose (a possible health benefit for those intolerant to lactose). However, strain-level identification provides greater specificity and ensures:

• Product consistency
• Marketing transparency
• Facilitates reproducible research on health benefits

Additionally, human studies utilizing defined probiotics and doses are essential to substantiate specific health benefit claims.

Someone seeking a targeted health effect from a probiotic would likely benefit from a clinically studied probiotic supplement rather than relying on fermented foods with live and active cultures. While fermented foods have been broadly associated with some health benefits, they contain non-defined microbial consortia, meaning their microbial composition and CFU count can vary significantly from batch to batch, region of production, and fermentation conditions. This variability makes it impossible to guarantee specific strains, doses, or health benefits for someone looking for a precise, evidence-based outcome.

Essentially, probiotics offer precision, while fermented foods contribute to overall dietary diversity. Fermented foods can be a valuable part of a healthy diet but should not be relied upon for targeted health effects unless they contain well-defined, clinically relevant probiotic strains at sufficient doses to address a specific health concern.

At this time, the site focuses exclusively on probiotic supplements that are not delivered in a functional food or beverage matrix. While some products containing defined microbial strains studied in IBS populations—delivered through a food or beverage matrix—may be added to the database in the future, they are not currently included.

Traditional fermented foods with live and active cultures do not meet the technical definition of a probiotic if their microbial composition is undefined. Due to natural variability in strains and doses—shaped by batch differences, production region, and fermentation conditions—these foods cannot be consistently classified. As a result, they are not included in the database and cannot be categorized based on probiotic content.

Currently, the database does not include a section on dietary fiber and its role in supporting probiotics, but I completely agree that fiber intake is helpful—and often overlooked—when discussing probiotics.

In general, I support promoting a varied, fiber-rich diet, especially since different probiotics contain varied carbohydrate-metabolizing genes. This means that some strains may be better stimulated by specific fermentable carbohydrates than others.

For now, a broad yet practical approach is to focus on the 3 D’s:

• Diversity – Incorporate a wide variety of plant-based foods to supply a broad range of fermentable carbohydrates and phytonutrients.
• Dose – Aim for ≥14g of dietary fiber per 1,000 kcal per day to meet recommended intake levels.
• Diligence – Consistency is key for maximizing benefits from both endogenous and supplemental microbes.

That said, fiber and IBS can be a tricky combination (thank goodness for Fodzyme! 😄). If there’s enough interest, we could explore carbohydrate metabolism genes in some of the top-performing probiotics to develop more targeted food recommendations that might better support specific probiotic strains.

 If you’d find this resource helpful, let us know here!

This was a key consideration in designing the database, given that placebo responses in IBS populations tend to be particularly strong, with many interventions failing to outperform placebo. To ensure reliable comparisons, we only included placebo-controlled trials in our database.

Additionally, our ranking system is based on effect size measurements (explained here), which provide a statistical measure of the magnitude of benefit relative to placebo. A p-value indicates whether an observed difference is statistically significant, meaning it is unlikely to be due to random variation. However, it does not quantify the size of the effect or its clinical importance. Effect sizes, on the other hand, measure the magnitude of improvement relative to placebo, allowing for a more meaningful comparison of probiotic efficacy.

By structuring our ranking system around effect sizes, we ensure that the probiotics highlighted in the database have demonstrated clinically relevant benefits beyond placebo effects, rather than simply achieving statistical significance.

It’s true that some of the most evidence-supported probiotic strains and blends aren’t commonly found in stores and can be more challenging to find. This was a major reason why IBSprobiotics.org was created. Many of the most promising probiotics come from smaller manufacturers that may lack the budget for widespread retail distribution. As a result, clinicians’ recommendations are often shaped by what’s easily accessible rather than by what has the strongest clinical evidence.

For now, many of these lesser-known probiotics are primarily available online—either through direct company websites or platforms like Amazon. Supporting these companies directly through their company websites is the best way to help expand their reach, increasing the likelihood that their products will eventually become more widely available in retail stores.

Like all probiotics, spore-forming probiotics exhibit strain-specific effects, meaning some strains may demonstrate more profound benefits than others. Some spore-forming microbes in the database were supported by methodologically sound studies, while others had flaws that led to their exclusion.

You can read more about the spore-forming microbes that were assessed here:
Unique-IS-2 | LactoSpore | GanedenBC | LBSC

Many of you have asked how often the site will be updated with new research, and as a fellow clinician, I completely understand how important it is for a resource like ibsprobiotics.org to be routinely updated.

The plan is to review and publish updates annually, incorporating new studies, probiotics, and analyses into the database. Beyond that, I also hope to refine and expand the tool gradually based on clinician feedback, ensuring it evolves to include the most valuable features and insights.

Of course, keeping the site updated and improving its functionality takes both time and financial resources. The best way to support ongoing updates is to subscribe and share this resource with others who might find it useful. As the site gains more traction, I’ll be able to expand its scope, integrate more research, and develop even better tools to support clinicians.

Your input and engagement are essential in shaping the future of this project, and I truly appreciate your support. If you have any suggestions or ideas, please don’t hesitate to reach out.

Additionally, if you have research on probiotics in IBS that you’d like to submit for review, I welcome submissions of randomized, placebo-controlled trials here. 

I’ve received many questions about how IBSprobiotics.org compares to other valuable probiotic resources like the Probiotic Chart by AE Probio and Probiotic Advisor. Each of these tools plays an important role, and while they take different approaches and have varying scopes, they all share the same goal—helping clinicians identify the best probiotic options for their patients.

As I highlight some of the distinguishing features of IBSprobiotics.org, it’s important to recognize that these tools don’t have to be used in isolation. Cross-referencing multiple resources can add value when making clinical decisions. Understanding the key features of each tool allows you to know which resource to turn to for specific types of probiotic guidance.

Broad Scope vs. IBS-Specific Deep Dive

• Probiotic Advisor and AE Probio Chart are excellent tools for broad probiotic guidance, covering probiotics across a wide range of conditions—from gastrointestinal health to immune function, mental health, metabolic health, and more.
• IBSprobiotics.org, on the other hand, is narrowly focused on IBS, providing a deep dive into probiotic efficacy for IBS symptoms with structured evidence grading and independent statistical analysis. 

What Makes IBSprobiotics.org Unique?

While other tools are fantastic for listing probiotics by strain, product, or food source, IBSprobiotics.org takes an additional step by applying rigorous statistical validation and effect size comparisons to help determine which probiotics have shown the most promise for specific IBS symptoms.

• Independent Statistical Analysis & Evidence Grading

1. 1. • IBSprobiotics.org independently validates IBS-related studies through statistical analysis to assess the reliability and strength of the findings. A structured evidence grading system ensures that only high-quality, rigorously conducted RCTs are included, while weaker studies are either filtered out or carry less weight to prevent misleading conclusions.

• Symptom-Specific Ranking

1. 1. • Because IBS symptoms vary significantly (e.g., bloating, stool consistency, urgency), IBSprobiotics.org ranks probiotics based on specific IBS symptoms. This targeted approach helps clinicians and patients identify the most relevant probiotics for their individual symptom needs.

• Effect Size Comparisons: The Closest Approximation to Head-to-Head RCTs

1. 1. • One of the biggest challenges in probiotic research is that most studies don’t compare different probiotic strains head-to-head. This makes direct comparisons difficult. While not a perfect substitute for head-to-head RCTs, effect size analysis provides the closest approximation possible, offering a standardized way to compare the relative effectiveness of different probiotics for different IBS symptoms. 

• Dose Optimization Based on All Available Evidence

1. • IBSprobiotics.org integrates findings from all studies on a given strain for a specific symptom to determine:
     • The overall trend of the evidence (positive, neutral, or negative).
     • The magnitude of the effect compared to placebo.
     • The most effective dose, especially in cases where studies report different dosages.
   • This provides a data-driven approach to probiotic dosing. 

Why All of these Resources Are Valuable

Both Probiotic Advisor and the AE Probio Chart are excellent resources for clinicians and individuals looking for broad probiotic guidance across various health conditions. For those seeking highly targeted recommendations, IBSprobiotics.org provides a deep, data-driven approach specifically tailored to IBS symptom management.

I hear you! Seeing variations in recommendations across different resources can feel confusing and overwhelming—especially in an already complex topic like probiotics.

Know that much of these differences often comes down to how each resource evaluates the evidence, including:

• Different evidence grading structures (how studies are weighted and interpreted).
• Varying study inclusion criteria (which studies are considered to inform recommendations).
• Differences in product availability by country (some products may not be accessible in all regions).

IBSprobiotics.org took a particularly narrow approach, applying strict inclusion criteria to ensure only the most rigorously tested probiotics are considered. In some cases, this means that certain products didn’t make the cut—not necessarily because they lack potential, but because they didn’t meet the specified thresholds for inclusion.

Cross-referencing multiple resources can be helpful in both directions—whether you’re feeling limited and need to expand options or overwhelmed and need a more refined, symptom-specific focus.

While individual microbiota responses to probiotics can vary, the systematic reviews I’ve encountered on this topic largely suggest that probiotics do not have a significant or lasting impact on overall microbiota structure in most cases. Some strains may temporarily increase in abundance while they’re being taken, but they are typically cleared within days to weeks after stopping. Probiotic effects seem to be more functional than compositional—modulating gut microbiota metabolism, immune signaling, and stabilizing microbial networks rather than dramatically altering which bacteria are present over the long term.

In the scenario described above, if infections were confirmed through clinical diagnostic tests after probiotic use, that would certainly be concerning and should be reported to the appropriate channels (e.g., FDA MedWatch, local health departments, etc.) to rule out product contamination. However, I haven’t seen research demonstrating a consistent pattern of probiotics leading to infections requiring antibiotics in otherwise healthy, non-immunocompromised individuals. That said, there have been rare cases where probiotics were linked to opportunistic infections, though these have almost exclusively occurred in severely ill or immunocompromised patients.

On the other hand, if these shifts were identified via direct-to-consumer microbiome testing, I’d interpret the results with caution. These tests remain investigational and provide only a snapshot of a highly dynamic gut ecosystem influenced by many external factors. The presence of certain microbes alone doesn’t necessarily indicate harm, as many microbes function as pathobionts—harmless under normal conditions but potentially pathogenic in certain contexts. Since microbiome sequencing has yet to be sufficiently clinically validated for guiding treatment decisions, or proven superior to standard empiric probiotic trials, clinical outcomes remain the best tool for assessing probiotic effects for now.

The studies evaluated for the site were conducted all over the globe, though some regions were better represented than others. Here’s an approximate regional breakdown of the studies included:

• Europe: 35.0%
• East Asia: 25.0%
• North America: 15.0%
• South Asia: 15.0%
• Middle East & North Africa: 7.0%
• Africa: 3.0%

A key limitation in this area of research is that most clinical trials don’t analyze probiotic effects in the context of genetic background, diet, or environmental factors, even though these factors are known to shape the gut microbiome. Because of this, we can’t say with certainty how well findings from one population might apply to another.

That said, one of the most consistent findings across IBS probiotic trials—regardless of study location—is that there are always both responders and non-responders. This suggests that individual variation may play a bigger role in probiotic effectiveness than broad population differences. From a practical standpoint, the best way to determine whether a probiotic is beneficial is to try it—if there’s no meaningful symptom improvement, discontinuing makes sense.