Probiotics

Early ideas about fermented milk and gut bacteria

A historical review in the Journal of Clinical Gastroenterology states that, at the beginning of the 1900s, Elie Metchnikoff associated the long lives of rural Bulgarian people with regular consumption of fermented dairy products such as yogurt. He suggested that lactobacilli might counteract the putrefactive effects of gastrointestinal metabolism that contributed to illness and aging, and he regarded such bacteria as conducive to life, in contrast to antibiotics. A biographical review adds that his interest in ageing and possible intestinal auto-intoxication gave rise to the present interest in the gut microbiome and in probiotics for health and longevity.

Credit for the idea is disputed. A short historical article argues that Metchnikoff, who linked longevity among Caucasian peasants to a type of yoghurt, should not be regarded as the discoverer of probiotics, because a Polish physician, Józef Brudziński, described a study at least a dozen years earlier in which a bacterial suspension was used in infants with acute infectious diarrhoea. The two historical sources also name the population Metchnikoff observed differently, as Bulgarian rural people in one and Caucasian peasants in the other.

From a historical idea to a defined term

In October 2013 the International Scientific Association for Probiotics and Prebiotics (ISAPP) convened an expert panel to discuss the field. Its consensus statement notes that the FAO/WHO definition of a probiotic was then about thirteen years old and that FAO/WHO guidelines had followed about a year later. The panel reinforced the definition as relevant and sufficiently accommodating, and clarified inconsistencies between the two FAO/WHO documents. The sources reviewed for this page do not cover the later commercial development of probiotic products.

Working definition

The ISAPP consensus statement reaffirmed the FAO/WHO definition: live microorganisms which, when administered in adequate amounts, confer a health benefit on the host. The panel argued that more precise use of the term would help clinicians and consumers tell apart the many products on the market. The National Center for Complementary and Integrative Health (NCCIH) describes probiotics as live microorganisms intended to have health benefits when consumed or applied to the body, and names Lactobacillus and Bifidobacterium bacteria, and sometimes the yeast Saccharomyces boulardii, as common types.

Probiotics and fermented foods

A separate ISAPP consensus statement defines fermented foods and beverages as foods made through desired microbial growth and enzymatic conversions of food components, and states that fermented foods differ from probiotics. NCCIH notes that probiotics are found in yogurt and other fermented foods as well as in dietary supplements and beauty products. Fermented foods, and probiotic products sold as supplements, are therefore related to this page but are not the same subject; see the Machipedia pages Dietary Supplements, Dietary Approaches and Nutritional Therapy for those wider topics.

Forms in which probiotics are used

NCCIH lists yogurt and other fermented foods, dietary supplements and beauty products as places where probiotics are found. The trials summarized in Cochrane reviews used many different preparations. In the neonatal review the most common contained Bifidobacterium, Lactobacillus, Saccharomyces and Streptococcus species, alone or in combination, and the pediatric review of antibiotic-associated diarrhea also included Bacillus, Clostridium butyricum, Lactococcus and Leuconostoc cremoris preparations. Probiotic products sold as supplements are described more generally on the Dietary Supplements page.

Why the strain matters

The AGA guideline names particular strains or strain combinations in its suggestions rather than recommending probiotics as a class, and its recommendations apply only to those products. NCCIH states that for most conditions it remains unknown which probiotics may help, what amount may be needed and who is most likely to benefit. This page does not give strain, product or amount guidance, and results for one product should not be assumed for another.

Uses sorted by a government health agency

NCCIH sorts conditions by strength of evidence. It lists antibiotic-associated diarrhea including Clostridioides difficile infection, necrotizing enterocolitis and sepsis in premature infants, infant colic, ulcerative colitis remission and, as an add-on to treatment, periodontal disease as promising; irritable bowel syndrome, constipation, traveler's diarrhea, atopic dermatitis and several others as inconclusive or mixed; and Crohn's disease and prevention of urinary tract infection as showing no benefit in the studies reviewed. These are NCCIH's characterizations as of its 2019 page. The sources reviewed do not assess general wellness or immune-support claims.

Clinical questions addressed in guidelines

The AGA guideline posed eight questions about probiotics in gastrointestinal care: treatment of C. difficile infection, prevention of C. difficile infection in people on antibiotics, Crohn's disease, ulcerative colitis, pouchitis, irritable bowel syndrome, acute infectious gastroenteritis in children and necrotizing enterocolitis in preterm infants. The guideline names health care providers, dietitians and patients as its intended audience.

Antibiotic-associated and C. difficile-associated diarrhea

A 2019 Cochrane review of probiotics for preventing antibiotic-associated diarrhea in children included 33 trials with 6,352 participants. Diarrhea occurred in 8% of the probiotic groups and 19% of the control groups (risk ratio 0.45, 95% CI 0.36 to 0.56; moderate certainty). A subgroup analysis found a larger effect in trials using higher daily amounts, which the authors judged credible but called for confirmation in a large multicenter trial, and they described firm conclusions about other products as premature.

A 2025 Cochrane update on Clostridioides difficile-associated diarrhea in adults and children receiving antibiotics included 47 trials (15,260 participants). Across 38 trials the incidence was 1.6% with probiotics and 3.2% in controls (risk ratio 0.50, 95% CI 0.38 to 0.64; low certainty), and symptom-free detection of C. difficile in stool was not clearly reduced. The authors noted that 28 studies had an author affiliation with, or funding from, a probiotic company, and that large trials in people at low baseline risk are still needed.

Acute infectious diarrhea

A 2020 Cochrane review of probiotics for treating acute infectious diarrhoea included 82 trials with 12,127 participants, mostly children. Results varied widely between trials and publication bias was evident. In the main analysis restricted to studies at low risk of bias, no difference was found in diarrhoea lasting 48 hours or longer (risk ratio 1.00, 95% CI 0.91 to 1.09; moderate certainty), and the authors were uncertain whether probiotics shorten the illness (very low certainty).

Preterm infants and necrotizing enterocolitis

A 2023 Cochrane review in very preterm or very low birth weight infants included 60 trials with 11,156 infants. Probiotics may reduce necrotizing enterocolitis (risk ratio 0.54, 95% CI 0.46 to 0.65; low certainty) and probably reduce all-cause mortality slightly (risk ratio 0.77, 95% CI 0.66 to 0.90; moderate certainty). In extremely preterm or extremely low birth weight infants, few data existed and probiotics may have little or no effect on necrotizing enterocolitis (risk ratio 0.92, 95% CI 0.69 to 1.22; low certainty). Formulations varied between trials and the authors called for further large, high-quality trials.

Inflammatory bowel disease and pouchitis

A 2024 overview of 22 systematic reviews with an updated meta-analysis of 45 randomized trials reported an odds ratio for clinical remission of 2.00 (95% CI 1.28 to 3.11) in ulcerative colitis and 1.61 (95% CI 0.21 to 12.50) in Crohn's disease, the latter interval spanning no effect. Probiotics were associated with lower odds of recurrence in relapsing pouchitis but not with better endoscopic outcomes, and no protective effect against recurrence was found in Crohn's disease. The authors rated overall certainty as low. The AGA took a more cautious position, described below.

What the American Gastroenterological Association recommended

The AGA published its guideline in 2020 using the GRADE approach. For C. difficile infection, Crohn's disease, ulcerative colitis and irritable bowel syndrome it recommended probiotics only in the context of a clinical trial, which its summary table labels as no recommendation owing to a knowledge gap. For children with acute infectious gastroenteritis it suggested against probiotics (conditional recommendation, moderate certainty), noting that two high-quality studies in the United States and Canada showed no benefit.

The guideline made strain-specific suggestions in three areas: prevention of C. difficile infection in adults and children on antibiotics, pouchitis (one named eight-strain combination, conditional, very low certainty), and preterm, low-birth-weight infants. Its comment on C. difficile prevention says people who place high value on avoiding potential harms, particularly those with severe illness, or on avoiding cost may reasonably decline. These suggestions apply to the named products, not to probiotics in general.

What the sources reviewed do not cover

Guidance from the World Gastroenterology Organisation was not located among free-access sources for this page, and the sources reviewed do not report a randomized comparison of probiotics with other approaches used for irritable bowel syndrome. Other approaches have their own Machipedia pages, including Low-FODMAP Diet and Gut-Directed Hypnotherapy, but the sources here do not compare them with probiotics. The AGA guideline was due for review about three years after 2020 and later updates were not examined.

Infections and other described risks

A review in Clinical Infectious Diseases lists risks described in case reports, trials and experimental models: systemic infections, harmful metabolic activity, excessive immune stimulation in susceptible people, gene transfer and gastrointestinal side effects. It cites a 2011 report from the Agency for Healthcare Research and Quality as concluding that the literature was not well equipped to answer safety questions with confidence, and calls for research on how often and how severely adverse events occur. A systematic review in people with cancer found five case reports of probiotic-related bacteraemia, fungaemia or positive blood cultures, and described adverse-event reporting as variable and inconsistent.

NCCIH states that risk is higher in people with severe illness or weakened immune systems, and that the US Food and Drug Administration has warned health care providers that premature infants given probiotics can develop severe or fatal infections.

Children, preterm infants and trial adverse events

The pediatric Cochrane review found no serious adverse events attributable to probiotics in 24 trials, but notes that observational studies outside its scope reported serious events in severely debilitated or immunocompromised children, including those with central venous catheters. A neonatology review estimates probiotic-induced sepsis probably occurs in under 0.5% of treated preterm infants, and states that product contamination exists but appears manageable with quality control. The 2025 Cochrane update found adverse events were not clearly more frequent with probiotics, with abdominal cramping, nausea and soft stools commonly reported in both groups, but its authors limit that conclusion to people who are not immunocompromised.

Product quality and labeling

NCCIH reports that some products have been found to contain microorganisms not listed on the label. A laboratory study of 26 dietary supplements containing viable bacteria sold in Slovenian pharmacies found that three contained insufficient numbers of viable bacteria, that 83 of 111 labelled species were confirmed by PCR on DNA from the product, and that two labelled strains were mislabelled by both methods. The study covers a small set of products from one country and does not give a rate for other markets.

Suitability and who advises

NCCIH advises talking with a health care provider before using a probiotic supplement, especially for people with health problems, and states that probiotics should not be used to delay seeing a provider about a health problem. People with severe illness, weakened immune systems, central lines or preterm infants are the groups the sources identify for particular caution. The sources reviewed identify no probiotic-specific practitioner register or training standard. Related Machipedia pages include Dietetics and Nutritional Therapy, which describe professions that sometimes advise on diet and supplements.