Probiotics

What are probiotics?

The FAO/WHO definition is as follows: a probiotic is “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host.” The International Scientific Association for Probiotics and Prebiotics (ISAPP), at its 2014 expert meeting, still found this definition appropriate and stated that the term should be used more carefully to reduce confusion in the product market. ^[1]

This definition has two conditions: the microorganism must be alive and its benefit must be proven. A product having “probiotic” written on its label does not mean the second condition is met.

Why is the effect of a probiotic strain-specific?

Microorganisms are named by genus, species and strain. For example, in the recommendation texts of the American Gastroenterological Association (AGA) guideline, products are referred to by strain codes such as L. reuteri (DSM 17938 or ATCC 55730) and L. rhamnosus (ATCC 53103); the recommendation is not for all Lactobacillus species but for the strains or mixtures named. ^[6] Genus names are also changing: ISAPP’s fermented foods consensus states that Lactiplantibacillus plantarum was formerly named Lactobacillus plantarum. ^[9]

The safety review by Boyle and colleagues emphasizes the same point: the properties of probiotic species can be strain-specific, and the effect of one strain should not be generalized to other strains without confirmation by separate studies. ^[2] The Cochrane results below should therefore be read as an “average for all probiotics”: many different strains and mixtures are evaluated together.

What does CFU mean, is higher CFU better?

CFU (colony-forming unit; “kob” in Turkish sources) is a measure of the number of live microorganisms in a product. In the Cochrane review examining prevention of antibiotic-associated diarrhea in children, a prespecified subgroup analysis showed that doses of 5 billion CFU per day and above were more effective than lower doses (interaction P = 0.01); in the high-dose trials the diarrhea rate was 8% in the probiotic group and 23% in the control group. ^[3] This does not mean “higher CFU is better in every situation”: the result is valid for the products and strains in that review and for that situation; the authors of the review also state that the high-dose result needs to be confirmed by a large, well-designed multicenter trial. ^[3]

In which situations do probiotics work? Evidence summary

Results point in different directions by situation. Each row gives who was examined together with the limit of the evidence.

Diarrhea while using antibiotics

In children, Cochrane (2019) found the rate of antibiotic-associated diarrhea in 33 trials (6,352 participants) to be 8% in the probiotic group and 19% in the control group (RR 0.45; 95% CI 0.36–0.56); NNTB 9. The result stayed significant even in the analysis for missing data using the most unfavorable plausible assumptions (RR 0.61). The review states that no serious adverse event was attributed to probiotics, but that in observational reports outside the trials, serious events were reported in children with central catheters or suppressed immunity. ^[3]

The meta-analysis in adults also found an average risk reduction but reported that the effect was seen only in certain species and in trials where baseline diarrhea risk was not low; the numbers and limits are on the benefits page.

Clostridioides difficile

The 2025 Cochrane update includes 47 trials (15,260 participants). For C. difficile diarrhea, the event rate in 38 trials (13,179 participants) is 1.6% versus 3.2%; the evidence is of low certainty and the absolute difference is small (NNT 65). The review reports that in 28 trials the authors had a tie to a probiotic company or company support. The authors’ judgment that “short-term use is probably not associated with side effects” is for people who are not immunosuppressed and who are receiving systemic antibiotics. ^[4]

AGA distinguishes two separate situations. In those who already have C. difficile infection, it recommends probiotics only within a clinical trial. In adults and children receiving antibiotics, it gave a conditional, low-certainty recommendation in favor of the four named products (S. boulardii and three separate strain mixtures); those who are severely ill, or who value cost and the small risk, may find it reasonable not to use probiotics. ^[6]

Acute diarrhea in children: effect not shown

This is the section that reverses a commonly held belief. Cochrane’s 2020 update pooled 82 trials (12,127 participants; 11,526 children, 412 adults). In many trials the risk of bias was high or unclear and the results were highly variable. In trials at low risk for all bias indicators, there was no difference in the rate of diarrhea lasting 48 hours or longer (RR 1.00; 95% CI 0.91–1.09; 2 trials, 1,770 participants; moderate certainty); for the duration of diarrhea, the estimate was a shortening of 8.64 hours, but the interval ran from a shortening of 29.4 hours to a lengthening of 12.1 hours (6 trials; very low certainty). The authors therefore state that probiotics probably make little or no difference to the number of cases of diarrhea lasting 48 hours or longer and that it is uncertain whether they shorten the duration. ^[5]

Irritable bowel syndrome

In IBS, two sources are read in different directions: the American College of Gastroenterology’s (ACG) 2021 guideline gives a conditional recommendation against probiotics for global symptoms (evidence of very low certainty), while a 2014 meta-analysis found a mean reduction in symptoms but could not determine which strain was effective. The two sources and their limits are given side by side on the benefits page.

Other areas

Colic in breastfed infants, necrotizing enterocolitis in very preterm infants, allergy and eczema, upper respiratory tract infections, H. pylori treatment, weight and mood are each covered separately. AGA conditionally recommends specific strains and mixtures for preventing NEC in preterm infants with low birth weight; in Crohn’s disease and ulcerative colitis, it recommends use only within a clinical trial. ^[6]

Detailed guideProbiotic benefits: evidence for each claim→

Do probiotics strengthen immunity?

“Immunity” is a general claim; the measurable outcome is the frequency of infection. Cochrane’s 2022 update included 23 trials (6,950 participants; children aged 1 month–11 years, adults with a mean age of 37.3, older people with a mean age of 84.6) in the meta-analysis. The risk of having at least one upper respiratory tract infection was lower with probiotics (RR 0.76; 95% CI 0.67–0.87; 16 trials; low certainty); for three or more infections RR 0.59 (4 trials; moderate certainty); an episode of infection was a mean of 1.22 days shorter (low certainty). Most trials used one or two strains and gave 10⁹ or 10¹¹ CFU per day for more than three months. Trials in which the proportion receiving influenza vaccination exceeded 50% were excluded. ^[7] These findings do not support the conclusion that probiotics “strengthen immunity”; they show a small-to-moderate reduction in the number of infections under some conditions.

How is the information on the label read?

When reading a label, the information that relates to the evidence needs to be separated from the rest. The strain code (for example, genus, species and then a letter-number code) matters, because guideline recommendations and trial results are tied to that strain; the species name alone (such as “Lactobacillus”) does not carry a trial result. ^{[6] [2]} The CFU on the label is tied in Türkiye’s health claim condition to the threshold of “at least 1.0×10⁶ cfu/g” (“kob/g” in the Turkish text); this threshold is not proof of benefit but a product condition sought for the claim to be used. ^[11]

Should healthy people take probiotics?

A review evaluating the evidence on probiotic use in healthy people (Merenstein et al., 2024) concluded that specific probiotics could be considered by some people for certain indications — for example, supporting gut function while using antibiotics or reducing the risk of respiratory tract infection. The same review found no sufficient level of evidence to support unconditional, population-wide recommendations for the other preventive outcomes examined; mental, dental and immune health were outside the scope of this review. ^[12] In other words, continuous probiotic use by a healthy person “for gut flora” does not meet a proven need.

Are probiotics safe? Who should be careful?

According to Boyle and colleagues, the general safety record of probiotics is very good; however, they should be used with care in groups such as premature infants and those with immune deficiency. ^[2] In a case series examining the records of five university hospitals in Finland (2009–2018), at least 20 (43%) of 46 patients with Saccharomyces fungemia were using an S. boulardii probiotic; the odds ratio for bacteremia or candidemia compared with the control group was 14 (95% CI 4–44). This design does not show causality, but the association is strong; the authors state that the use of these products in patients whose gut integrity may be impaired should be evaluated carefully. ^[8] The Cochrane C. difficile review concludes that short-term probiotic use is unlikely to be associated with side effects in people receiving systemic antibiotics who are not immunosuppressed. ^[4]

Probiotics in Türkiye: a food supplement or a medicine?

In Türkiye, most probiotic products are food supplements; production and placing on the market are overseen by the Ministry of Agriculture and Forestry. The text in the TİTCK (Turkish Medicines and Medical Devices Agency) health claim list is as follows: a food supplement is not a medicine, cannot be used for preventing or treating diseases, and there is no food supplement approved by the Ministry of Health. ^[11]

The same list contains the “Probiotic microorganisms” claim with this text: “This product contains probiotic microorganisms. Probiotic microorganisms help regulate the digestive system and support the immune system.” The condition for the claim is that the product contain at least 1.0×10⁶ cfu/g (“kob/g” in the Turkish text) of live probiotic microorganisms. ^[11] The TİTCK re-evaluated these claims with its Office Approval (Makam Olur) dated September 27, 2024. According to the announcement, it was decided that the probiotic claim be changed to “Probiotic microorganisms contribute to balancing the gut microbiota” and that the condition be at least 1.0×10⁸ CFU/g of at least one species in the daily intake; the prebiotic component claim, on the other hand, was suspended on the grounds that there was no sufficient scientific evidence. In the same institution’s current full list, a note “has been suspended” appears next to both claims, with information that the evaluation is continuing; until the decision is finalized, existing products using these claims may use the wording only on their labels. ^{[10] [11]}

In the Ministry of Agriculture and Forestry’s Restricted Substances List for Food Supplements (last updated 21/09/2026), the “Probiotic microorganism” row gives a range of at least 1×10⁶ and at most 1×10¹⁰ cfu/day for both ages 4–10 and ages 11 and over. The row names the strains evaluated and found suitable under the application procedure and requires the strain to be declared on the label with its current taxonomic name and strain code. ^[13] This range is a product condition; it does not show that a given strain benefits a given condition.

Are yogurt, kefir and pickles probiotics?

Fermented foods can contain live cultures, but according to ISAPP a fermented food and a probiotic are not the same thing: for a food to be counted as a probiotic, the strain must be defined, benefit must have been shown in a human study and a sufficient number of live organisms must be present until the end of the product’s shelf life. In heat-treated fermented vegetables, no live microorganisms remain. ^[9]

Detailed guideWhich foods contain probiotics? Fermented foods table→

Prebiotics and timing

Prebiotics are not live microorganisms but substrates selectively used by microorganisms; they are covered on a separate page. There is a separate guide on how probiotics relate to meals, antibiotics and duration.

Detailed guideWhat are prebiotics? Foods, evidence and limits→ Detailed guideWhen are probiotics taken? Meals, antibiotics and duration→

This guide does not recommend personal products or doses. Use in severe illness, immune suppression, with a catheter, in pregnancy or in infants is assessed together with the physician.