Probiotics: What They Do, Who Needs Them and How to Choose

Probiotics are one of the fastest-growing supplements in the US, promising everything from better digestion to clearer skin and a stronger immune system. The science behind them is real but much narrower than the marketing. Benefits depend on the specific strain, the dose and the condition, and a probiotic that helps one problem may do nothing for another. This guide explains what probiotics are, what the evidence actually supports, and how to choose a product if you need one.

For more on supporting your gut through diet, see our guides to high-fiber foods and fiber supplements.

What are probiotics?

An international expert panel defines probiotics as “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host.” That definition has three important parts: the microbes must be alive when you take them, you need enough of them, and there must be evidence of a benefit. A food that simply contains bacteria isn’t automatically a probiotic.

Most probiotics are bacteria from the Lactobacillus and Bifidobacterium groups, which are naturally found in the gut and in fermented foods. Saccharomyces boulardii is a yeast. Probiotics are different from:

• Prebiotics: fibers such as inulin that feed the beneficial bacteria already in your gut.
• Postbiotics: inactivated microbes or their byproducts, a newer and less-studied category.

Why strains matter

Bacteria are named by genus, species and strain, like a family name, first name and ID number. For example, in Lacticaseibacillus rhamnosus GG, “GG” is the strain. (Many Lactobacillus species were renamed in 2020, so you may see both old and new names on labels.)

Two strains of the same species can behave very differently, so research on one strain doesn’t apply to another. That’s why “contains Lactobacillus acidophilus” tells you little, while a named, studied strain tells you you’re buying something with evidence behind it. Good products list the full strain name on the label.

What the evidence supports

Based on Cochrane reviews, the 2020 American Gastroenterological Association guideline and the trials listed in the sources.

Probiotic foods vs supplements

Fermented foods are made with live microbes, and many still contain them when you eat them. They’re not always “probiotics” in the strict sense, because their strains and amounts usually aren’t tested for a specific benefit, but they’re a practical way to add live microbes to your diet.

• Yogurt and kefir: look for “live and active cultures” on the label. Kefir usually contains more types of microbes than yogurt.
• Sauerkraut and kimchi: choose refrigerated, unpasteurized versions; shelf-stable jars are heat-treated and the microbes are dead.
• Miso, tempeh and kombucha: also fermented, though cooking miso in boiling soup kills its microbes.

Limited evidence In a 2021 Stanford study, healthy adults who ate about six servings of fermented foods a day for 10 weeks had a more diverse gut microbiome and lower levels of several inflammation markers, while a high-fiber group didn’t see the same changes. It was a small study, but it suggests fermented foods are worth including. Fiber still matters, because it feeds the microbes you already have; see our list of fruits high in fiber.

How to choose a probiotic

1. Start with your goal: pick a product with strains studied for that specific use, such as L. rhamnosus GG or S. boulardii for antibiotics.
2. Check the strain names: the label should list genus, species and strain for each organism.
3. Read the CFU count: CFU means colony-forming units, the number of live microbes. Look for the amount “at the end of shelf life” or “through expiration”, not “at time of manufacture”, since microbes die over time.
4. Don’t chase huge numbers: more CFUs or more strains aren’t necessarily better. Match the dose used in studies.
5. Check storage: some products need refrigeration. Follow the label and keep them away from heat and moisture.
6. Look for testing: probiotics are sold as dietary supplements, which the FDA doesn’t approve before sale. Third-party seals and companies that publish their research are a good sign. Our guide to reading a supplement label explains more.

How to take probiotics

• With antibiotics: start on the first day of your antibiotic course and continue for 1 to 2 weeks after it ends. Take bacterial probiotics at least 2 hours apart from the antibiotic dose. S. boulardii is a yeast, so antibiotics don’t kill it, but antifungal medicines can.
• With or without food: follow the label. Many bacterial strains survive stomach acid better when taken with a meal.
• Give it time: try a product consistently for about 4 weeks. If you notice no difference, it probably isn’t helping you, and there’s little point continuing.
• Expect the effect to stop: most probiotic strains don’t permanently settle in your gut. Benefits usually fade within weeks of stopping.

Side effects and safety

For healthy people, probiotics are generally safe. Mild gas and bloating are common in the first few days and usually settle down.

If you have ongoing digestive symptoms such as diarrhea, constipation, pain or blood in your stool, see a doctor rather than trying probiotics on your own.

Probiotics FAQ