Probiotics: What They Can and Can't Do for Your Gut

The short answer: Probiotics are live microbes meant to benefit your health. The best evidence is that some probiotics may lower the risk of diarrhea while you take antibiotics. For most other uses, researchers still don't know which products help, and results from one strain don't carry over to others.

What probiotics are

Probiotics are live microorganisms intended to have health benefits when you eat them or apply them to the body. They are found in yogurt and other fermented foods, supplements and some beauty products. The most common types are Lactobacillus, Bifidobacterium and the yeast Saccharomyces boulardii ^[1]. Prebiotics are food components that feed helpful microbes, and products that combine both are called synbiotics ^[1].

Where the evidence is strongest

• During antibiotics: probiotics may lower the risk of antibiotic-associated diarrhea, mainly in young and middle-aged people. The quality of the studies was only moderate, so this is a tentative conclusion ^[1].
• C. difficile: it is moderately certain that probiotics can reduce the risk of C. difficile diarrhea in people taking antibiotics. The best strains, doses and length of use are not yet known ^[1].
• IBS: a 2018 review found possible benefits for overall symptoms and belly pain, but could not say which products work ^[1].

For most other uses, we still don't know which probiotics help and which don't ^[1].

Why the strain matters

Different probiotics can have different effects. A benefit found for one strain does not mean other strains or species will work the same way ^[1]. That is why it helps to look at the exact strains on the label.

Food first

• Yogurt and kefir with live and active cultures, plus fermented foods such as sauerkraut, kimchi and miso.
• Fiber-rich foods such as onions, garlic, bananas, oats and beans feed the helpful bacteria already in your gut.

Safety and who should be careful

• Probiotics have a long history of apparently safe use in healthy people, though few studies look closely at side effects ^[1].
• The risk of harmful effects is greater in people with severe illnesses or weakened immune systems. Serious infections have been reported in premature infants given probiotics ^[1].
• Some products have been found to contain microbes other than those on the label ^[1].
• If you are seriously ill, in the hospital, or have a weakened immune system (for example during chemotherapy), talk with your doctor before taking probiotics ^[1].