Probiotics: what they are, when they really help and when they don't

Probiotics are everywhere: in pharmacies, in supermarkets, in yogurts. They are often presented as a help for everything, from the gut to mood. The evidence tells a more precise story: some strains work in well-defined situations, many uses are not proven, and in some frail people probiotics can be risky.

What probiotics are

The definition from the FAO and the World Health Organization (WHO), confirmed by international experts, is precise: probiotics are live microorganisms that, when taken in adequate amounts, confer a health benefit on the person taking them. So it is not enough for a bacterium to reach the gut alive: a benefit shown in studies in humans is needed.

A probiotic is identified by three names: genus, species and strain. For example, in Lacticaseibacillus rhamnosus GG the genus is Lacticaseibacillus, the species rhamnosus and the strain GG. The strain matters because a proven effect applies to that strain, at that dose and for that condition, and does not automatically extend to others.

Prebiotics, on the other hand, are not microorganisms: they are non-digestible substances that promote the growth of useful bacteria already present in the gut. Synbiotics contain both.

The new names of lactobacilli

In 2020 the old genus Lactobacillus, which had become too heterogeneous, was split into 25 genera. That is why the same bacterium can appear under two names on packs and in studies: Lactobacillus rhamnosus is now called Lacticaseibacillus rhamnosus, Lactobacillus paracasei is Lacticaseibacillus paracasei and Lactobacillus plantarum is Lactiplantibacillus plantarum. Among others, L. acidophilus and L. crispatus remain in the genus Lactobacillus; L. crispatus is the lactobacillus we discuss in connection with vaginal microbiota transplantation.

Fermented foods and probiotics are not the same thing

Fermented foods are made through the deliberate growth of microorganisms. Some still contain them alive when we eat them: yogurt, kefir, most cheeses, miso, tempeh and unheated fermented vegetables. Others do not, because they are cooked, pasteurized or filtered: bread, pasteurized fermented vegetables, soy sauce, vinegar, wine and most beers. Vegetables pickled in vinegar are not even fermented.

Even when it contains live microorganisms, a fermented food is not a probiotic for that reason. Traditional sauerkraut, for example, contains unidentified strains of Lactiplantibacillus plantarum in unknown amounts: it would become a "probiotic" only with a characterized strain, with a proven benefit, present in the right amount until the expiry date.

Yogurt with live cultures does, however, have a documented effect: its two typical bacteria, Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, improve lactose digestion and reduce symptoms in people who digest it poorly.

How probiotics work, and what they do not do

Laboratory studies have shown that probiotics compete with harmful germs for nutrients and for attachment to the intestinal wall, lower the local pH, produce antibacterial substances, stimulate mucus production and interact with the immune system of the mucosa. Everything we have listed is only a mechanism, not yet a benefit proven in patients.

Short-chain fatty acids are not nutrients "absorbed thanks to probiotics": they are formed in the colon when the microbiota ferments fiber and other dietary residues, and some probiotics can increase their production.

Two findings contradict the idea that probiotics "restore the microbiota". In healthy people, a review of seven randomized trials found no changes in the composition of the fecal microbiota compared with placebo. After a course of antibiotics, in a study in mice and volunteers, a multi-strain probiotic actually delayed the return of the original microbiota, whereas a transplant of the person's own stool, collected before treatment, restored it almost completely within a few days.

When there is evidence that probiotics work

Antibiotic-associated diarrhea

This is the use with the most consistent evidence. In children on antibiotics, a Cochrane review of 33 studies found diarrhea in 8% of those taking probiotics versus 19% of controls: one case is avoided for every 9 children treated, with moderate-certainty evidence. The benefit is clearer with higher amounts, and the most studied strains are L. rhamnosus and the yeast Saccharomyces boulardii. In adults and older people too, meta-analyses point to a moderate effect.

Prevention of Clostridioides difficile

In people on antibiotics, probiotics may reduce the risk of Clostridioides difficile diarrhea, but the absolute gain is small: according to the 2025 Cochrane review, one case is avoided for every 65 people treated, with low-certainty evidence, and safety was assessed in people who were not immunocompromised.

Guidelines do not agree. The one from the American Gastroenterological Association (AGA) suggests, with a conditional recommendation and low-quality evidence, certain specific strains or combinations, including S. boulardii; the benefit was seen mainly in people at high risk. The one from the American infectious disease societies (IDSA and SHEA) considers the data insufficient to recommend them outside clinical trials. When the infection keeps coming back, the recommended treatment is not a probiotic but fecal microbiota transplantation.

Premature babies

In premature and low-birth-weight babies, some strains reduce the risk of necrotizing enterocolitis, a serious inflammation of the intestine. American and European guidelines suggest them conditionally, only for specific strains and with quality-controlled products. It is a decision for the neonatal unit, not something to do at home.

Pouchitis

In people who have had surgery for ulcerative colitis and in whom a pouch has been created from the last part of the small intestine, the same AGA suggests an eight-strain combination against pouchitis, the inflammation of the pouch, with very low-quality evidence. In Crohn's disease and ulcerative colitis the same guideline reserves probiotics for clinical trials.

Upper respiratory tract infections

A 2022 Cochrane review of 23 studies indicates that probiotics may reduce by about a quarter the number of people who catch a cold or other upper respiratory tract infections. The evidence is of low certainty.

When the evidence for probiotics is weak or against them

• Acute infectious diarrhea: according to the 2020 Cochrane review of 82 studies, probiotics probably make little or no difference to the number of people still ill after 48 hours. Guidelines disagree: the AGA advises against them in children with acute gastroenteritis in the United States and Canada, after two large negative trials in those countries, whereas the European society of paediatric gastroenterology (ESPGHAN) gives recommendations for some specific strains.
• Irritable bowel syndrome: in several studies some strains reduce bloating and flatulence. However, the studies are very heterogeneous, and the AGA reserves probiotics for clinical trials.
• Constipation: a 2014 meta-analysis found on average just over one extra bowel movement a week, especially with Bifidobacterium lactis, but with studies at high risk of bias. The 2023 American guideline on chronic constipation did not include them among the treatments it assessed.
• Recurrent urinary tract infections: no proven reduction in cystitis compared with placebo, in small, poor-quality studies.
• Eczema: probiotics probably do not improve the symptoms of atopic dermatitis, and according to the Cochrane review their use is not evidence-based.
• Asthma, autoimmune diseases and acne: we found no good-quality reviews showing a benefit.
• Mood and depression: a review of 47 meta-analyses indicates that probiotics might be associated with an improvement in depressive symptoms, but the evidence is heterogeneous and more studies are needed. They do not replace treatment.
• Blood pressure and cholesterol: systolic blood pressure falls on average by about 2 mmHg, too little to treat hypertension. LDL cholesterol falls by about 7 mg/dL and HDL does not change: probiotics do not replace treatment.

Safety: who should not take them without a doctor

In healthy people probiotics are generally well tolerated. The most common side effects are mild: bloating, flatulence, nausea, abdominal cramps.

The real risk concerns frail people. Bloodstream infections caused by lactobacilli have been described, in some cases linked to the very strain taken, as well as bloodstream infections caused by yeasts of the genus Saccharomyces, especially in people taking S. boulardii. The people most at risk are:

• people with very weak immune defenses or a serious illness;
• people admitted to intensive care;
• people with a central venous catheter or who are fed intravenously or through a tube;
• premature babies, for whom the neonatologist decides.

In these cases probiotics are used only on a doctor's advice, because most products are designed for healthy people. If you are pregnant or breastfeeding, have a chronic illness or take medicines, ask your doctor or pharmacist first.

Probiotic supplements in the United States and the United Kingdom: what the label guarantees

Probiotics are usually sold as foods or as dietary supplements. In the United States, the Food and Drug Administration (FDA) does not approve dietary supplements before they are marketed. Labels may carry claims about the structure or function of the body, which the FDA does not approve in advance: in that case the label must state that the FDA has not evaluated the claim and that the product is not intended to diagnose, treat, cure or prevent any disease.

In the United Kingdom, food supplements are regulated as foods: they are not medicines and are not intended to treat or prevent diseases. Official UK guidance counts the word "probiotic" itself as a health claim, and health claims must be authorised in the Great Britain register. In that register no claim on probiotics or on strains of Lactobacillus or Bifidobacterium is authorised: the only authorised claim concerns the live cultures in yogurt or fermented milk, which improve lactose digestion in people who have difficulty digesting it.

In practice, the word "probiotic" on a label does not mean a proven effect on a disease.

How to choose a probiotic, if your doctor recommends one

• Look on the label for the genus, species and strain code.
• Check that the strain has been studied for your problem: the benefit of one strain does not apply to another.
• Check the amount of each strain guaranteed until the expiry date and follow the storage conditions indicated.
• Choose reliable manufacturers: quality depends on the manufacturer, and most probiotics are not produced to pharmaceutical standards.

Frequently asked questions

Should I take a probiotic every time I take an antibiotic?

Not necessarily. Some strains reduce the risk of diarrhea, especially in children. If you have had antibiotic-associated diarrhea or a Clostridioides difficile infection in the past, talk to your doctor.

Is yogurt enough?

Yogurt with live cultures helps people who digest lactose poorly, but a fermented food is not the same as a probiotic studied for a specific purpose.

Does everyone need them to "boost their defenses"?

No. The guidelines we consulted do not recommend their regular use in healthy people, and in people who are well they do not change the microbiota in any proven way. When patients ask me for a probiotic "just to be safe", I explain that it is best to use one only when there is a specific reason and a strain studied for that reason.

Bibliography

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