Probiotics: what the evidence says, and what it doesn't

Probiotics are one of the most popular supplements in the United States, and it is easy to see why. The idea is appealing: if gut microbes matter for health, why not just swallow some good ones? The honest answer from the research is more nuanced. Some specific probiotics help in some specific situations. Many common uses are not well supported. And for some people, probiotics may not be needed at all.

What counts as a probiotic?

An international expert panel defines probiotics as "live microorganisms that, when administered in adequate amounts, confer a health benefit on the host" ¹. Three parts of that definition matter. The microbes must be alive when you take them. They must be given in a dose that has been tested. And there must be evidence of a health benefit. A product that simply contains bacteria is not automatically a probiotic in this scientific sense.

Effects are also tied to the strain. "Lactobacillus" is a large group of bacteria. One strain may have been studied for a certain use, while a close relative has not. That is why results from one product cannot be assumed to apply to another.

What the gastroenterology guidelines say

In 2020, the American Gastroenterological Association (AGA) reviewed the evidence for probiotics across common digestive conditions ². Its key points, in plain words:

• Preventing C. difficile infection during antibiotics: for adults and children taking antibiotics, the AGA suggests that certain specific probiotic strains or combinations may be considered over no probiotic. This was a conditional recommendation based on low-quality evidence. The AGA also noted that people worried about cost or possible harm could reasonably choose not to take them.
• Treating C. difficile infection: the AGA recommends using probiotics only in clinical trials.
• Crohn's disease and ulcerative colitis: probiotics only in clinical trials. One exception is pouchitis, a condition after a specific surgery for ulcerative colitis, where an 8-strain combination may be considered.
• Irritable bowel syndrome (IBS): probiotics only in clinical trials.
• Acute diarrhea in children (in North America): the AGA suggests against probiotics.
• Preterm, low-birth-weight infants: certain combinations may help prevent a serious bowel condition called necrotizing enterocolitis. This is a hospital decision made by the baby's medical team.

The American College of Gastroenterology's IBS guideline also recommends against probiotics for overall IBS symptoms, because the studies are small, use many different strains, and give mixed results ³.

A probiotic is defined by strain and dose, not by the word on the front of the box.

Can probiotics help after antibiotics?

Many people take probiotics after a course of antibiotics to "restore" their gut. A 2018 study in Cell tested this idea in a small group of healthy adults ⁴. After antibiotics, people who took a multi-strain probiotic had a slower return of their own normal gut bacteria than people who took nothing. A third group, given back a sample of their own stored microbes, recovered fastest. This was one small study and it does not prove that probiotics are harmful. But it does question the common belief that probiotics always help the gut recover.

Are probiotics safe?

For most healthy people, probiotics are generally well tolerated, though some notice gas or bloating at first. Because they are sold as dietary supplements, they do not go through the same review as prescription drugs, and products can vary in what they actually contain. People with a weakened immune system, a central IV line, serious illness or certain heart valve problems should speak with their doctor before taking them, because rare but serious infections have been reported in these groups.

Questions to ask before you buy

1. What am I trying to treat or prevent? "General gut health" is not a condition the guidelines address.
2. Has this exact strain been studied for my problem? Look for the full strain name, not just the species.
3. What dose was used in the studies? And does the label promise that amount through the expiry date, not just at manufacture?
4. How will I know if it is working? Agree on a time limit, such as four weeks, and stop if there is no clear benefit.
5. Would food changes help more? For many people, more fiber and a wider variety of plants are a better first step.

The bottom line

Probiotics are not useless, but they are not a cure-all. The current gastroenterology guidelines support them only for a few specific uses, with specific strains. For most digestive symptoms, a clear diagnosis and a plan based on that diagnosis will do more than a supplement chosen off the shelf. If you are unsure, bring the product to your next appointment and ask.