Probiotics are live microorganisms, mostly bacteria and some yeasts, that can benefit health when taken in adequate amounts. They can play a supporting role in gut health, but the benefits depend on the exact strain, and the research is strongest for specific strains used in specific situations rather than for healthy people in general. Most healthy people tolerate probiotics well; people with weakened immune systems or serious illness should talk to a doctor first.
Key Takeaways
- Probiotics are defined as live microorganisms that, in adequate amounts, provide a health benefit.
- Effects are strain-specific: results for one Lactobacillus or Bifidobacterium strain cannot be assumed for another.
- The NIH says there are no formal recommendations for or against probiotics in healthy people.
- A higher CFU count is not necessarily better; look for the CFU guaranteed through the end of shelf life.
- Fermented foods and a fiber-rich diet are everyday ways to support a diverse gut microbiome.
- Side effects are usually mild, such as gas, but rare serious complications have been reported in people who are severely ill or have weakened immune systems.
Evidence grade: Moderate. Randomized trials and systematic reviews support specific strains in specific situations, such as during antibiotic use, but the NIH says there are no formal recommendations for or against probiotics in healthy people, and results for one strain do not carry over to others.
What Probiotics Are
An expert panel of the International Scientific Association for Probiotics and Prebiotics (ISAPP) defines probiotics as "live microorganisms that, when administered in adequate amounts, confer a health benefit on the host." The NIH Office of Dietary Supplements (ODS) notes that they are mainly bacteria but also include yeasts, and that they are found in some fermented foods such as yogurt, added to certain foods, and sold as supplements. Not every product labeled "probiotic" has proven health benefits.
Three related terms are easy to mix up:
- Prebiotics are food components, often types of fiber, that the microbes in your gut use as fuel.
- Synbiotics combine probiotics and prebiotics in one product.
- Postbiotics are preparations of non-living microorganisms or their parts.
Your Gut Microbiome in Brief
Your digestive tract is home to a large community of bacteria, archaea, viruses, fungi and other microbes, often called the gut microbiome. A widely cited 2016 analysis estimated that a reference adult carries about 38 trillion bacteria, roughly the same order as the number of human cells, weighing about 0.2 kg in total. That corrected the older, often-repeated claim that bacteria outnumber human cells ten to one.
Many of these microbes are helpful. The National Center for Complementary and Integrative Health (NCCIH) notes that some bacteria help digest food and produce vitamins. According to NCCIH, probiotics might help your body maintain a healthy microbial community or help it return to a healthy state after being disturbed, produce substances with useful effects, and influence your immune response. ODS adds that probiotics usually colonize the gut only temporarily, in patterns that differ from person to person depending on the microbes you already have.
Why the Strain Matters More Than the Label
Probiotics are identified by genus, species and an alphanumeric strain code, and ODS states that because effects can be specific to certain strains, recommendations for their use need to be strain-specific. NCCIH gives a plain example: if one kind of Lactobacillus helps in a particular situation, that does not mean another Lactobacillus, or any Bifidobacterium, will do the same.
This is why claims like "L. acidophilus supports digestion" or "probiotics boost immunity" are too broad. The useful question is whether the specific strain in a product has been studied, at that amount, for the outcome you care about.
| On the label | What it tells you | What to look for |
|---|---|---|
| Strain name | Genus, species and strain code, for example a letter-and-number designation after the species | Every strain named in full, so you can match it to research |
| CFU (colony-forming units) | The number of live microbes | CFU guaranteed through the end of shelf life, not only at manufacture |
| Serving size | How many capsules the CFU figure covers | Whether the number is per capsule or per multi-capsule serving |
| Storage | Room temperature or refrigeration | Directions you can follow so the microbes stay alive |
ODS notes that many supplements contain 1 to 10 billion CFU per dose and some 50 billion or more, but products with higher CFU counts are not necessarily more effective. Because microbes can die during storage, ODS and ISAPP advise choosing products that state the CFU at the end of shelf life.
What the Research Shows
Strains studied during antibiotic use
Antibiotics can disrupt the gut microbiome, and loose stools are a common side effect. This is one of the better-studied uses of probiotics. A 2019 Cochrane review of 33 trials in 6,352 children taking antibiotics found fewer cases of this side effect with probiotics, and in the trials using at least 5 billion CFU a day, 8% of children in probiotic groups were affected compared with 23% in control groups. The review rated this evidence moderate certainty and said the benefit of high-dose Lactobacillus rhamnosus or Saccharomyces boulardii still needs confirming in a large trial. ODS adds that benefits depend on the strain, the antibiotic and the person's age, and that reviews found reduced risk in children and adults aged 18 to 64 but not in adults 65 and older.
The picture is not entirely simple. In a small 2018 study in mice and people, taking a multi-strain probiotic after antibiotics delayed the return of the person's own gut microbes compared with no probiotic. If you are prescribed antibiotics, ask your doctor or pharmacist whether a specific probiotic makes sense for you.
Healthy people
ODS states that there are currently no formal recommendations for or against probiotics in healthy people, and NCCIH says much remains to be learned about whether they are helpful for various purposes. ODS also notes that more research is needed on probiotics and body weight. Claims that probiotics improve mood, weight or skin in everyone go beyond the evidence.
Food First: Fermented Foods and Fiber
Some fermented foods, such as yogurt, are sources of potentially helpful live microbes, although ODS notes that not every fermented food with live cultures contains probiotic organisms. If live cultures matter to you, check the label for them.
In a 17-week Stanford study of 36 healthy adults, a diet high in fermented foods such as yogurt, kefir, kombucha and kimchi steadily increased gut microbiome diversity and lowered several inflammatory markers, while a high-fiber diet changed other measures. It was a small study, but it supports a food-first approach.
Fiber feeds your gut microbes. The Dietary Guidelines for Americans list dietary fiber as a nutrient of public health concern because most people do not eat enough. Vegetables, fruit, beans, whole grains, nuts and seeds are the main sources; our guides to high-fiber diets and digestive wellness and which fruit is highest in fiber have practical ideas. Add fiber gradually and drink enough fluids.
How to Choose a Probiotic Supplement
- Start with your reason. If it is a specific situation, such as a course of antibiotics, ask your doctor which strain and amount have been studied for it.
- Check that every strain is named in full on the Supplement Facts panel.
- Look for CFU through the best-by date, and do not assume more is better.
- Follow the storage directions on the bottle.
- Give it a fair trial and stop if you notice no benefit or have side effects that do not settle.
If you decide to try a capsule, OPA Probiotic-40 is our daily probiotic. Each two-capsule serving lists 40 billion CFU at manufacture and at least 20 billion CFU through the best-by date from a 4-strain blend, and a 60-capsule bottle is a 30-day supply. The product page does not list the four strain names, so check the Supplement Facts panel on the bottle and compare those strains with the research above. Our OPA Probiotic-40 guide covers the label in more detail.
Digestive comfort also depends on the rest of your diet. If heavy meals leave you uncomfortable, our article on digestive enzymes explains how they differ from probiotics.
Safety and Interactions
- Usual side effects. ODS says probiotics are unlikely to cause harm in healthy people and side effects are usually minor, mainly short-lived digestive symptoms such as gas.
- Weakened immune system or serious illness. Probiotics have been linked to rare cases of bacteremia, fungemia and other serious infections, mostly in people who were severely ill or immunocompromised. If you take medicines that suppress the immune system, are undergoing treatment that weakens immunity, have a central venous catheter, or are seriously ill or in hospital, talk to your doctor before taking any probiotic.
- Premature infants. NCCIH reports severe or fatal infections in premature infants given probiotics, and the FDA has warned health care providers about this risk. Never give a probiotic to an infant without a pediatrician's advice.
- Pregnancy, breastfeeding and children. Ask your doctor before starting a probiotic.
- Product quality. NCCIH notes that some probiotic products have been found to contain microorganisms other than those listed on the label, and that few studies have looked at safety in detail.
- Do not delay care. NCCIH advises against using probiotics as a reason to postpone seeing your health care provider. Ongoing digestive symptoms, blood in the stool or unexplained weight loss need medical evaluation.
Frequently Asked Questions
What are probiotics?
Probiotics are live microorganisms, mostly bacteria and some yeasts, that provide a health benefit when taken in adequate amounts. They are found in some fermented foods and in dietary supplements, and each is identified by genus, species and strain.
Do probiotics help with bloating and gas?
Some strains have been studied for digestive comfort, but results depend on the strain and the person. Gas and bloating can also be mild side effects in the first days of taking a probiotic, so give it time and stop if symptoms do not settle.
Is a higher CFU count better?
Not necessarily. The NIH says products with higher CFU counts are not necessarily more effective. What matters more is whether the specific strain was studied, and whether the CFU is guaranteed through the end of shelf life.
Should I take a probiotic while on antibiotics?
Reviews have found that certain strains, started within 2 days of the first antibiotic dose, can reduce the chance of antibiotic-related loose stools in some groups. The best strain and timing depend on your situation, so ask your doctor or pharmacist.
Are probiotic foods as good as supplements?
They are different. Fermented foods with live cultures provide a variety of microbes along with nutrients, and a small study found a fermented-food diet increased microbiome diversity. Supplements provide specific strains at stated amounts, which matters if you are following research on a particular strain.
Who should not take probiotics without medical advice?
People with weakened immune systems, serious illness, a central venous catheter or a hospital stay, as well as premature infants. Pregnant or breastfeeding women and parents considering a probiotic for a child should also ask a doctor first.
Sources
- NIH Office of Dietary Supplements. Probiotics: Fact Sheet for Health Professionals. Updated 2025.
- National Center for Complementary and Integrative Health. Probiotics: What You Need To Know. NIH.
- Hill C, Guarner F, Reid G, et al. Expert consensus document. The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nature Reviews Gastroenterology & Hepatology. 2014.
- Sender R, Fuchs S, Milo R. Revised Estimates for the Number of Human and Bacteria Cells in the Body. PLoS Biology. 2016.
- Guo Q, Goldenberg JZ, Humphrey C, El Dib R, Johnston BC. Probiotics for the prevention of pediatric antibiotic-associated diarrhea. Cochrane Database of Systematic Reviews. 2019.
- Suez J, Zmora N, Zilberman-Schapira G, et al. Post-Antibiotic Gut Mucosal Microbiome Reconstitution Is Impaired by Probiotics and Improved by Autologous FMT. Cell. 2018.
- Wastyk HC, Fragiadakis GK, Perelman D, et al. Gut-microbiota-targeted diets modulate human immune status. Cell. 2021.
- U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2020-2025. 2020.
Disclosure: OPA Nutrition sells supplements, including products mentioned in this article. This content is for education only and is not medical advice. Statements about dietary supplements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease. Talk to your healthcare provider before using any supplement, especially if you take medication or have a medical condition.
Written and reviewed by Michael Gonzales, Founder, OPA Nutrition · Last reviewed: · How OPA researches and writes
