Quick answer

Probiotics are live microorganisms, mostly bacteria, that can benefit health when taken in adequate amounts. Many supplements hold 1 to 10 billion CFU per dose, but a bigger count is not better by itself: results belong to specific strains at tested doses. For healthy people there are no official recommendations, and the general digestive evidence is early.^1,2

Key facts

A probiotic's full name has three parts, genus, species and strain, and a result for one strain does not carry over to another.^2,3
Added bacteria show up in stool while you take them, but trials in healthy adults found no lasting change in the gut's bacteria.^4,5
Live counts can fall during storage, so a count stated through the expiration date tells you more than one stated at manufacture.²
Antibiotic and antifungal medicines might make some probiotics less effective.⁶
People who are seriously ill or whose immune system is weakened should ask a doctor before taking any probiotic.^2,3

What probiotics are

Probiotics are live microorganisms, such as bacteria and yeasts, that provide health benefits when taken in sufficient amounts.²

The most common probiotics are bacteria from the Lactobacillus and Bifidobacterium groups. Other bacteria are used too, and so are yeasts such as Saccharomyces boulardii.^1,3

Many of the microbes used in probiotic products are the same as, or close relatives of, microbes that already live in and on the body. They are sold in yogurt and other fermented foods, added to ordinary foods, and packed into supplements.^2,3

A probiotic is named by genus, species and strain. In Lacticaseibacillus rhamnosus GG, Lacticaseibacillus is the genus, rhamnosus the species and GG the strain.²

In 2020 the Lactobacillus genus was split up, so several familiar species now carry new genus names: Lactobacillus rhamnosus, for example, is now Lacticaseibacillus rhamnosus.¹

The strain is the part that matters most. Two products can both say Lactobacillus on the front and still contain organisms that behave differently, which is why a label that lists only species tells you less than one that names each strain.^1,3

How they work, and why the effect is temporary

Probiotics act mainly in the digestive tract. Some actions are shared by many strains: crowding out less helpful microbes, making short-chain fatty acids and making the colon more acidic. Others, such as making vitamins or acting on the immune system, belong to particular species or strains.¹

Swallowed bacteria are visitors rather than new residents. NIH describes any settling in the gut as temporary and different from one person to the next, and a trial that sampled the gut lining directly found the pattern varied by person, by gut region and by strain.^1,7

In a 2012 national survey, about 4 million US adults, 1.6 percent, had used probiotics or prebiotics in the past 30 days.³

What the evidence shows

There is one graded claim here, and it is general. Probiotic research mostly tests one named strain at one dose, and NIH says recommendations have to be specific to the strain. The claim below rests on an expert judgment about well-studied groups of bacteria, not on a trial of any one product.^1,8

Helps support digestive health by supplying live probiotic bacteria*^1,4,8

Early evidence

This rests on a general expert judgment, not on a trial of any one product. An international panel (ISAPP) concluded that well-studied Lactobacillus and Bifidobacterium species can be expected to support a healthy gut flora and digestive tract when a serving supplies at least 1 billion CFU. A review of 45 studies in healthy adults found some evidence for better stool consistency and bowel movements, but strains and doses varied, and the added bacteria were found only while people kept taking them.

Amount studied: At least 1 billion CFU per serving of well-studied Lactobacillus or Bifidobacterium species; trials of single strains used their own amounts

In seven randomized trials in healthy adults, probiotics did not change the diversity of the gut bacteria compared with placebo, and only one trial found any change in its overall make-up.⁵

The European Union's list of permitted health claims, which rests on reviews by its food safety authority, contains no general claim for probiotics at all.⁹

Research we do not turn into claims

Most probiotic trials enrolled people with a particular illness and tested one strain or blend for it. Those results belong to the strains tested and to the people studied, so this guide does not repeat them as reasons for healthy people to take a probiotic. Some trials looked at other outcomes this site does not cover.^1,3

If you want a probiotic for a specific health problem, NIH suggests asking a health care provider which product, what dose and for how long, since expert groups give advice only for named strains in named situations.^1,2

How much people take

Labels count live microbes in CFU, colony-forming units. Supplements commonly hold 1 to 10 billion CFU per dose and some hold 50 billion or more, but NIH says the higher counts are not necessarily more effective. There is no official daily amount for healthy people.^1,2

Probiotic supplements come as capsules, powders, liquids and other forms, with a single strain or many strains.¹

When you compare products, look for the genus, species and strain of each organism, the CFU count, whether that count is promised through the expiration date, and the storage directions. The dosage guide explains the numbers, and the timing guide covers meals and storage.²

Probiotics in food

Yogurt, cheese, sourdough bread, pickles, apple cider vinegar, kombucha, miso, kimchi and sauerkraut are all made with microorganisms, but not all of them contain probiotics with proven health benefits.²

Live cultures are not the same thing as tested probiotic strains. A food can be full of living microbes that have never been studied for a health effect, and some fermented foods are processed so that nothing is left alive. The food guide covers yogurt labels and which fermented foods still carry live cultures.¹

Prebiotics are not probiotics: they are food components, mostly carbohydrates such as inulin, that gut microbes use as fuel. Products that combine the two are called synbiotics.^1,3

What's in ours

What’s in ours

Probiotic 40: Proprietary Blend of Probiotic Bacteria, 800 mg (40 Billion CFU) per serving, from the label

Probiotic 40 lists four named strains with 40 billion CFU for the blend, and Daily Probiotic lists 13 species with 20 billion CFU at the time of manufacture. Both counts are above the 1 billion CFU in the graded claim, but neither blend has been tested in a clinical trial, so that claim is general, not a result for these products. Our Apple Cider Vinegar capsules add one strain at 1 billion CFU at manufacture. NIH notes that more CFU is not necessarily better, so taking several of these together has no known advantage.^1,8

Side effects, medicines, and who should ask first

In healthy people, probiotics may cause gas. Side effects are usually minor digestive symptoms that go away on their own.^1,2

Rarely, probiotics have been linked to bacteria or fungi in the blood and to infections causing severe illness, mostly in people who were severely ill or had weakened immune systems.^1,3

Ask your pharmacist or doctor first

Antibiotics can kill helpful bacteria in the digestive tract, and NIH says antibiotic and antifungal medicines might make some probiotics less effective. Ask your pharmacist how to time a probiotic around your medicine, and keep taking the antibiotic exactly as prescribed.^2,6,10,11

Expected from how the two work, not shown in studies

NIH names antifungal medicines alongside antibiotics as able to make some probiotics less effective. If you take either, ask your pharmacist how to fit a probiotic around it.¹⁰

There are no official recommendations for probiotic use by healthy people, so there is no recommended amount and no set upper limit. NIH notes that products with higher CFU counts are not necessarily more effective.^1,2

Ask your doctor first if you have a weakened immune system, for example from chemotherapy, HIV, an organ transplant or long-term steroid treatment²
Ask first if you are seriously ill or in hospital³
See a doctor rather than starting a probiotic if you have fever, vomiting, bloody diarrhea or severe abdominal pain
Ask first if you are pregnant or nursing, or before giving a probiotic to a baby or child

Do not give a probiotic supplement to a premature baby. FDA warned in 2023 that preterm infants given probiotics in hospitals are at risk of severe, potentially fatal infection, and it has approved no probiotic as a drug for infants of any age.¹²

Questions people ask

What do probiotics actually do in the body?

They pass through the digestive tract, where some strains crowd out less helpful microbes, make short-chain fatty acids and lower the acidity of the colon. Other effects belong to particular strains. They are found in stool while you take them and fade after you stop.^1,4

Are probiotics different for men?

We found no research or recommendation specific to men. The same rules apply to everyone: choose named strains that were tested at a stated amount, and check that the count is promised through the expiration date.²

Do I need a probiotic if I eat yogurt?

Not necessarily, and there is no official recommendation either way. Yogurt with live and active cultures carries live bacteria, but its two starter cultures are not necessarily probiotic strains unless the label names an added one.^1,13

Can probiotics be harmful?

For most healthy people, side effects are minor and mostly gas. Rare but serious infections have been reported, mainly in people who were severely ill or had weakened immune systems, and FDA has warned against probiotics for preterm infants.^1,12

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Sources

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.