Supplement · graded by use, three uses
Probiotics
Prevention of antibiotic-associated C. difficile diarrhea (at-risk hospitalized patients).
Grade C: human trials exist, but they are small, mixed or not yet repeated. This is not a finding that it does not work. How to read a grade →
Probiotics are live bacteria or yeasts taken as a pill or powder. Results depend on the exact strain. In hospital patients on antibiotics, a review of 38 studies found they cut a bad type of diarrhea, caused by a germ called C. difficile, about in half. For colds and for blood signs of inflammation in diabetes, the studies are smaller or tested only one strain. In very sick people or people with weak immune systems, probiotic germs have caused blood infections.
The C belongs to one of the three uses graded on this record. The other two carry their own letters.
As studied, and the trials
Esmaeilinezhad et al. (Cochrane Database of Systematic Reviews A review that searches for every study on a question using a method set in advance, then assesses them together., 2025; PMID PubMed ID: the permanent number the US National Library of Medicine gives a published paper. Each one links to that paper's record on PubMed. 40931979) pooled 38 randomized trials in 13,179 participants and found fewer cases of C. difficile-associated diarrhea when probiotics were given with antibiotics (RR 0.50), about one case prevented for every 65 people treated, and rated that evidence low certainty, downgraded for few events and publication bias. The earlier version, Goldenberg et al. (Cochrane Database of Systematic Reviews, 2017; PMID 29257353), had rated it moderate. The largest single trial, PLACIDE (Allen et al., Lancet, 2013; PMID 23932219), randomized 2,981 older inpatients to a lactobacilli and bifidobacteria preparation or placebo An inactive substitute made to look like the real treatment, so participants cannot tell which one they received. and found no significant reduction (12 versus 17 cases; RR 0.71, 95% CI 0.34 to 1.47). Smaller positive trials, Hickson et al. (BMJ, 2007; PMID 17604300) and Gao et al. (American Journal of Gastroenterology, 2010; PMID 20145608), each tested a different product. WHY C: probiotics are not one intervention, the trials that found a benefit tested different strains, and the largest trial found none, which is a replication The same finding reproduced by a separate research group. A result seen once can be chance or bias; a replicated one is harder to explain away. that failed. The pooled signal is real enough to record and too uncertain, and too spread across products, to grade higher. Benefit does not generalize to probiotic products as a class.
As studied, and the trials
Tabrizi et al. (Eur J Pharmacol, 2019; PMID 30959049): meta-analysis A study that pools the numbers from several trials into one combined estimate. of randomized trials of probiotic and synbiotic supplementation, reporting changes in inflammatory markers in patients with diabetes. Grade C for this use: the evidence is randomized and pooled, but every endpoint The outcome a trial measures to decide whether the treatment worked, such as heart attacks, blood pressure or a symptom score. is a surrogate marker A lab value or measurement used in place of the outcome that matters, such as LDL cholesterol standing in for heart attacks. Moving a surrogate does not always move the outcome., the trials pool probiotics with synbiotics rather than isolating either, and no trial reports a clinical outcome. This is a different and much weaker claim than the C. difficile prevention above, which is why the two are graded separately.
As studied, and the trials
Taverniti et al. (Microorganisms, 2026; PMID 42354894): 140 healthy adults randomized to CRL 1505 or placebo, double-blind Neither the participants nor the people assessing them know who received the real treatment until the trial ends., 122 analyzed; episodes defined by study clinicians. The primary endpoint The one outcome a trial names in advance as its main test. Results on other outcomes carry less weight. was the share of people with at least one, two or three episodes. Only the three-or-more threshold reached significance, and only at 16 weeks (12.7 percentage points fewer, p = 0.03); one or more and two or more did not. Total episodes were fewer on the probiotic (53 versus 78, p = 0.05). Three of the authors work for the company licensed to make the strain and five for the institute that owns it. WHY C: a single trial, from the strain's owner and licensee, positive on one of its three primary thresholds, that nobody has independently repeated. The result attaches to this one strain at this dose, not to probiotics as a class.
Read the grade that matches your goal. How EBL grades by use →
A letter describes the human evidence, not the product, the manufacturer, or the ingredient's worth.
See the full evidence record →Preventing C. difficile diarrhea: low-certainty pooled benefit, null largest trial. Inflammatory markers in diabetes also C.
As studied: dose, form, population and duration for every use
A grade describes the intervention that was tested. Where the product on the shelf differs from it, this says so.
The trial that would change this grade.
A positive trial of this shape, supported by further trials, would move this entry to B.
Why the current trials fall short
Pooled trials favor probiotics, but the largest single trial in older inpatients found no effect, and the products tested differ in strain and dose. A large trial of one product, with harm counted beside benefit, would show whether the pooled result holds.
Why people believe itTwo human sources; two published case reports of benefit; three reports of harm.
The case for probiotics is that antibiotics and modern diets thin out gut bacteria, and that swallowing live beneficial strains restores them. Pooled trials find fewer cases of antibiotic-associated diarrhea and C. difficile infection, and some find lower inflammation markers. They are among the most widely used supplements in the world.
What supporters point to, and what each source actually shows. None of it is enough to move this letter higher; the trial described above under Evidence wanted is what would.
- Human trial, blood or body markerPooling 18 trials in 1,337 people with diabetes, probiotics lowered the inflammation markers CRP and TNF alpha. Blood markers, with very large differences between trials. PMID 30959049
- Human trial, blood or body markerIn 36 healthy adults, 10 weeks of a diet high in fermented foods such as yogurt and kimchi raised gut bacterial diversity and lowered inflammation markers. Fermented foods, not a probiotic capsule, and markers, not a health outcome. PMID 34256014
A case report describes what happened to one or a few patients. It cannot show the treatment caused the change, and reports of success are published more often than reports of failure.
- Two cancer patients with relapsing or chronic diarrhea after antibiotics recovered after probiotics were added, one alongside a short antibiotic course. Two patients, with no comparison. PMID 15284590
- An 82-year-old woman with severe colitis after antibiotics improved after bowel washouts followed by a live E. coli Nissle 1917 probiotic. Two treatments given together, in one patient. PMID 9817997
- An otherwise healthy patient taking a probiotic developed an infection of a heart valve with Lactobacillus, the probiotic organism, after a cut in the gums. PMID 33088710
- A man in his 60s with ulcerative colitis and an artificial heart valve, taking a Lactobacillus probiotic daily, developed a bloodstream and heart valve infection with the probiotic strain that spread to his brain. PMID 35764338
- A patient with lymphoma undergoing a stem cell transplant developed sepsis from Lactobacillus acidophilus, a common probiotic organism. No abstract is indexed; the title reports the infection. PMID 22890287
Searched PubMed, October 2026: 70 case report record(s) returned. 59 describe infection of the blood, heart valves or organs caused by the probiotic organism itself, mostly in premature infants, critically ill or immunocompromised patients, and people with central lines or damaged heart valves; three are listed. Six describe recovery from diarrhea or colitis with probiotics given alongside other treatment; two are listed. The other five are laboratory, genetic and review papers.
How this letter has changed · latest: October 2026 · Added
Taverniti et al. (Microorganisms, 2026; PMID 42354894) randomized 140 healthy adults to Lacticaseibacillus rhamnosus CRL 1505 at 1 billion CFU a day or placebo for 12 weeks. Fewer people on the probiotic had three or more respiratory infections by week 16, while the one-or-more and two-or-more thresholds of the same primary endpoint were not significant. A single positive trial nobody has independently repeated is C under the written definition, and the authors include the strain's owner and licensee. The use is graded for this strain only; the headline stays C. Applied the written grade definition at the editor's request of October 2026 (Arnold Sandlow, D.C.).PMID 42354894
The use-level A rested on one pooled review and did not cite the largest trial. PLACIDE (Allen et al., Lancet, 2013; PMID 23932219), 2,981 older inpatients, found no significant reduction in C. difficile diarrhea. The 2025 Cochrane update (Esmaeilinezhad et al.; PMID 40931979) still finds fewer cases overall, about one prevented for every 65 people treated, and rates that evidence low certainty. The positive trials tested different products, so the benefit does not attach to probiotics as a class. The record's headline grade was already C and does not change.PMID 23932219 PMID 40931979
Probiotics is not one intervention. Prevention of antibiotic-associated Clostridioides difficile diarrhea holds Grade A on pooled randomized evidence. Inflammatory markers in type 2 diabetes is C on surrogate endpoints. The headline letter is now C, the grade of the general-purpose use, so that an A earned on one narrow clinical endpoint is not read as a verdict on the category.
Every change to a letter on this site is dated and published. The full evidence log →
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EBL earns no commission on any supplement it grades. A purchase made through the dispensary link below pays EBL nothing, the practitioner discount goes to you in full. The dispensary is Fullscript, a practitioner dispensary, and many of the professional-grade brands in it are sold mainly through practitioners rather than in health food stores. Grades are assigned by published rubric before any commercial relationship exists and are never influenced by one. Full disclosure.
Highly strain-specific. Lactobacillus acidophilus + Bifidobacterium longum studied most. 10–50 billion CFU common.
Strain specificity matters. A specific eight-strain mixture (inflammatory bowel disease), Lactobacillus rhamnosus GG (diarrhea) and Akkermansia muciniphila (metabolic health) have the strongest specific evidence.
In the cited antibiotic trials the probiotic was given once a day (Allen et al., Lancet, 2013; PMID 23932219), twice a day (Hickson et al., BMJ, 2007; PMID 17604300), or as one or two capsules a day (Gao et al., Am J Gastroenterol, 2010; PMID 20145608), alongside antibiotic treatment. The summaries do not state whether doses were taken with meals.
Checked against the published summaries; the full papers could not be read. What the cited trials did, not a schedule. Every supplement, by trial timing →
Severe immunocompromise, critical illness, central venous catheters and short-bowel syndrome. Bacteremia and fungemia from probiotic organisms are documented in those groups, which is why use in these settings calls for clinician oversight. Active SIBO may worsen with some strains.
Antibiotics reduce or abolish the effect of a live organism, which is a timing problem rather than a hazard. Immunosuppressants: the concern is the bacteremia risk above rather than an interaction. No prominent pharmacological interaction otherwise.
The antibiotic pairing is better evidenced than the timing framing suggests: trials give both together and still measure benefit. The intuition that an antibiotic kills a live organism is sound, but the trial evidence complicates it. Randomized placebo-controlled trials of probiotics for antibiotic-associated diarrhea give the probiotic during the antibiotic course, and meta-analyses of them find a reduction in diarrhea. So co-administration does not abolish the effect in the setting where it has actually been measured. What is not established is an optimal separation interval; no trial has compared spacing schedules head to head. Liao et al. (Journal of Clinical Gastroenterology, 2021;55(6):469-480; PMID 33234881): meta-analysis of randomized placebo-controlled trials of probiotics for preventing antibiotic-associated diarrhea in adults, with subgroup analyses by population and strain.
The immunosuppression concern is an infection risk with a documented case literature, not a pharmacological interaction. The page already says this is not an interaction, and that is correct: nothing about a probiotic changes the level or action of an immunosuppressant drug. The risk is that a live organism given to someone who cannot contain it becomes an infection. A systematic review identified 117 cases of Saccharomyces fungemia and attributes the rise to clinical use of Saccharomyces boulardii, and an opinion review of probiotics in immunosuppression finds the efficacy evidence strongest after liver transplantation and in cirrhosis while flagging safety as unresolved elsewhere. Case counts are not rates, and the denominator is unknown. Vinayagamoorthy et al. (Medical Mycology, 2023;61(2):myad014; PMID 36806741): systematic review of Saccharomyces fungemia identifying 117 cases and their underlying risk factors, treatment and outcome. Stadlbauer (Beneficial Microbes, 2015;6(6):823-8; PMID 26287986): review of the efficacy and safety of probiotics in immunosuppressed patients.
Gut symptoms, bowel regularity, inflammatory markers, microbiome testing (optional)
Primary evidenceFour papers, each linked to its PubMed record.
Tabrizi et al. (Eur J Pharmacol, 2019; PMID 30959049): meta-analysis of randomized trials of probiotic and synbiotic supplementation on inflammatory markers in diabetes. This is the evidence the grade rests on, and its endpoints are surrogate markers. IMPORTANT SCOPE NOTE: Wastyk et al. (Cell, 2021; PMID 34256014) randomized healthy adults to a high-FERMENTED-FOOD diet or a high-fiber diet, and found the fermented-food arm increased microbiome diversity and lowered inflammatory markers. It tested FOOD, not a probiotic supplement, and is carried here as mechanism and context rather than as evidence for a supplement. Hill et al. (2014; PMID 24912386) is the ISAPP consensus definition, not a trial. Grade C on surrogate endpoints. Preventing antibiotic-associated C. difficile diarrhea is graded separately, also at C. Upper respiratory infections are graded separately at C on one strain-specific trial, Taverniti et al. (Microorganisms, 2026; PMID 42354894).
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