Probiotics

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By Nikola · Precision Nutrition L1 · Updated 5 November 2026

Probiotics are live microorganisms, mostly bacteria and some yeasts, that give a health benefit when you take them in adequate amounts. That is the FAO/WHO definition, which an international expert panel confirmed in 2014. You find them in capsules and powders, and in some fermented foods such as yogurt with live cultures and kefir. The best evidence is for lowering the risk of diarrhoea caused by antibiotics, and even there it depends on the strain. Many supplements contain 1 to 10 billion colony-forming units (CFU) per dose, but more CFU does not automatically mean a better product.

What the research says

Effects are specific to the strain: a result for one strain does not carry over to another, even from the same species.

• Antibiotic-associated diarrhoea: moderate. Pooled data suggest certain species and strains might cut the risk by about half (51%). In a meta-analysis of 12 trials with 1,499 people, the strain LGG lowered the rate from 22.4% to 12.3%, but the effect was significant only in children. For the yeast Saccharomyces boulardii, 21 trials with 4,780 people showed a drop from 17.4% to 8.2% in adults and from 20.9% to 8.8% in children. Benefit was seen in adults aged 18 to 64, not in people over 65, and starting within 2 days of the first antibiotic dose worked better than starting later.
• C. difficile diarrhoea: moderate. A 2017 analysis of 31 studies found it is moderately certain that probiotics can reduce the risk of C. difficile diarrhoea in people taking antibiotics. The best types, doses and duration are not known.
• Irritable bowel syndrome (IBS): weak to moderate. A meta-analysis of 23 trials with 2,575 patients found a 21% lower risk that symptoms persist, but the studies were of low quality. The NHS says there is some evidence that probiotics may ease some IBS symptoms.
• Infectious diarrhoea in children: weak. A 2020 Cochrane review of 82 trials (12,127 participants) found 36% less diarrhoea lasting 48 hours or more, but the trials with the lowest risk of bias showed no difference. Two later trials in 1,729 infants and children found LGG no better than placebo.
• Eczema: none for treatment. A Cochrane review of 39 trials concluded that the evidence does not support probiotics for treating eczema. For prevention in infants, a 2018 meta-analysis reported risk falling from 34.7% to 28.5%, but results vary by strain and timing.
• Cholesterol, weight, inflammatory bowel disease: weak or unclear. Small cholesterol reductions need confirming, effects on weight were of questionable value, and a modest benefit is possible in ulcerative colitis but not shown in Crohn’s disease.

There is no EU-authorised health claim for probiotics as such. The one related claim in Regulation (EU) 432/2012 covers yogurt: live cultures in yogurt or fermented milk improve the digestion of the lactose in that product, as long as it contains at least 10⁸ CFU of the starter cultures Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus per gram.

Dose and forms

• CFU. Probiotics are counted in colony-forming units, the number of live cells. Many supplements give 1 to 10 billion CFU per dose, some 50 billion or more. Higher counts are not necessarily more effective.
• Strain, not brand. A probiotic is identified by genus, species and an alphanumeric strain code. Look for the strain that was tested for your purpose, at the dose used in the trials.
• Storage. Some products need the fridge, others keep at room temperature; follow the label.

Food sources

Fermented foods contain live microbes, but most have not been tested for a specific health effect, so they are not “probiotics” in the strict sense. Heat after fermentation kills the microbes.

Food	Live microbes?	What to know
Yogurt with live cultures	Yes	Made with L. bulgaricus and S. thermophilus; some products add Bifidobacterium or other probiotic strains
Kefir	Yes	Listed among probiotic foods by Harvard’s Nutrition Source
Sauerkraut, kimchi	Yes, if not heat-treated	Live cultures, but not typically proven probiotic strains
Miso, many cheeses, kombucha	Yes	Live cultures, but not typically proven probiotic strains
Sourdough bread, most shop pickles	No	Processed after fermentation, no live cultures when eaten

Source: NIH Office of Dietary Supplements, Probiotics fact sheet (updated 25 March 2025) and Harvard T.H. Chan School of Public Health, The Nutrition Source: The Microbiome, both checked 5 November 2026. Neither gives reliable counts of live microbes per 100 g, so the table shows type, not amount.

In practice: a bowl of plain yogurt with live cultures or a glass of kefir, sauerkraut that has not been heat-treated, and a spoon of miso stirred into soup once it is off the heat are easy ways to eat live cultures. Many cheeses carry live cultures too, though not proven probiotic strains; our entries on sirene and feta cover their other nutrients. The yogurt bacterium L. bulgaricus was first isolated in 1905 by the Bulgarian researcher Stamen Grigorov, from homemade yogurt. For the foods that feed your gut bacteria, see our prebiotic foods list.

Side effects and interactions

• Usually mild. Probiotics are unlikely to cause harm in healthy people. The most common side effects are short-lived gut symptoms such as gas.
• Infections in vulnerable people. Most reported bloodstream infections linked to probiotics (bacteria or yeast in the blood) happened in people who were severely ill or had weak immune systems. At least 60 cases of fungal bloodstream infection linked to products with Saccharomyces yeast have been published since 1966. In one intensive care study, patients given LGG had a markedly higher risk of Lactobacillus in the blood.
• Medicines. The sources used here do not list specific drug interactions. Tell your doctor or pharmacist about any supplement you take.

Who should avoid it

• People with a weakened immune system or a serious illness, including hospital patients: talk to a doctor first. Guidance is to stick to strains and uses with proven benefit.
• Premature babies. In 2023 the US FDA warned that probiotics given to premature infants may cause severe, potentially fatal infections; one infant death and more than two dozen other adverse events were reported. Any use in newborns is a decision for the neonatal team, not a shop shelf.
• Anyone with a health condition or on long-term medication: the NHS advises speaking to a doctor before taking probiotic supplements.

How to choose a good product

• Full strain name on the label (genus, species, strain code), matching a strain studied for your purpose.
• CFU at the end of shelf life, not only at manufacture.
• Realistic expectations. The NHS notes that shop products can differ from those tested in trials, and it cannot be sure a product contains the bacteria listed, enough of them, or that they survive to reach the gut. Some products have been found to contain microbes not on the label.
• Independent testing. Seals from groups such as ConsumerLab.com, NSF International or US Pharmacopeia show a product was made properly and contains what the label says. They do not guarantee that it is safe or works.

We do not recommend specific products on this page.

FAQ

Do probiotics help after antibiotics?

For some strains, yes. Trials with Saccharomyces boulardii and LGG show a lower risk of antibiotic-associated diarrhoea, especially when started within 2 days of the first antibiotic dose. The benefit was not seen in people over 65.

Is yogurt enough, or do I need a supplement?

Yogurt with live cultures and kefir give you live microbes, but they are not the specific strains and doses tested in most trials. For general eating, fermented foods are a good habit; for a specific goal, such as antibiotic diarrhoea, the evidence is for particular strains in supplements.

Are probiotics safe to take every day?

For healthy people they are unlikely to cause harm, and side effects are usually mild, such as gas. People who are seriously ill, have a weak immune system, or are caring for a premature baby should speak to a doctor first.

Related entries and articles

• Kefir and Greek yogurt
• Sauerkraut and miso
• Sirene (Bulgarian white cheese)
• Other supplements: magnesium, ashwagandha, melatonin
• Gut health foods: what to eat for a healthy gut
• Prebiotic foods list
• Natural remedies for better digestion

Sources

1. NIH Office of Dietary Supplements: Probiotics, Fact Sheet for Health Professionals, updated 25 March 2025. ods.od.nih.gov
2. NCCIH: Probiotics: What You Need To Know. nccih.nih.gov
3. NHS: Probiotics, reviewed 24 November 2022. nhs.uk
4. Hill C, et al. (2014). Expert consensus document: The ISAPP consensus statement on the scope and appropriate use of the term probiotic. Nat Rev Gastroenterol Hepatol 11(8):506-514. PMID 24912386. europepmc.org
5. Harvard T.H. Chan School of Public Health, The Nutrition Source: The Microbiome. nutritionsource.hsph.harvard.edu
6. Commission Regulation (EU) No 432/2012 – permitted health claims. legislation.gov.uk
7. Petrova M. Bulgarian yogurt: an old tradition, alive and well. ISAPP blog. isappscience.org
8. NIH Office of Dietary Supplements: Dietary Supplements: What You Need to Know, updated 4 January 2023. ods.od.nih.gov

This page is for information only and is not medical or dietary advice.