A container of yogurt

For anyone concerned about gut health or their microbiome, popping a probiotic supplement has increasingly become the norm.

Consumers spent $2 billion on probiotic supplements in 2017—almost double the total in 2012, according to the Nutrition Business Journal. But while probiotics may be big business, consumers should know that the safety of these supplements is not guaranteed.

In fact, a study released today in the Annals of Internal Medicine concludes there are significant gaps in what we know about the safety of probiotics and similar products.

Here, what the researchers found, and what you should know if you use probiotics.

What Are Probiotics Supposed to Do?

Probiotics are microorganisms (the so-called “good bacteria”) that can help restore balance in the gut when “bad” bacteria become too plentiful.

More on Probiotics

The new study looked at prebiotics, too—which are seeing a similar rise in popularity among those concerned about gut health. These are substances that feed good bacteria, encouraging more of it to grow in your gut.

Both are naturally found in a wide variety of foods.

“Specific strains of probiotics have been shown to be beneficial for various health issues—helping to protect against antibiotic-related diarrhea, C. diff infection, IBD [inflammatory bowel disease], and helping maintain normal bowel function,” says Gail Cresci, Ph.D., R.D., staff in the department of pediatric gastroenterology at Cleveland Clinic Children’s, who was not involved in this study.

But not all products are equal, many are over-hyped, and different probiotics will interact with your specific microbiome in unique ways.

What the New Study Found

In the new study, researchers analyzed 384 randomized controlled trials (the gold standard of research studies) designed to test the efficacy of probiotics and prebiotics for various clinical conditions. What they found is that surprisingly few of them took a serious look at potential harm and adverse effects.

Of the 384 trials, only nine adequately reported key safety components and one-third of them gave no information on harms at all.

“The field of probiotic safety is characterized by a scarcity of studies in contrast with the long history of the use of these ingredients,” says Aida Bafeta, Ph.D., an epidemiologist at the University of Paris, Descartes-Sorbonne, and lead author of the review. “Several researchers have sounded the alarm that the current literature is not well equipped to answer questions on the safety of probiotics.” 

What Are the Potential Risks?

When you ingest probiotics—whether in a supplement or in food (they're found in many yogurt products, kombucha, kefir, and tempeh, for example)—you’re consuming live microorganisms. And those microorganisms, once introduced into your digestive system, do have the potential to cause infection.

For generally healthy people, the risk is low and consuming probiotics is unlikely to have severe negative effects.

But for other groups—such as people who are critically ill, people who are immune-compromised (with HIV or undergoing chemotherapy, for example), and premature babies—probiotics may not be so benign.

“Certain populations are more vulnerable to infection and may not be able to fight off an infection caused by a probiotic,” says Cresci.

There have been tragic cases reported of deaths resulting from infection in patients being given probiotics. “There is huge uncertainty about the safety of these interventions in the elderly, critically sick patients, including infants, and those who are immune compromised,” says Bafeta.  

Choosing a Probiotic

The two types of bacteria most commonly found in probiotic supplements (and also, the ones most studied) are from the Lactobacillus and Bifidodobacterium groups. “But within each species group there are hundreds of different strains, and each strain can act differently and may have different effects,” says Cresci.

In other words, just because there’s research showing that one particular strain of Lactobacillus may help prevent diarrhea doesn’t mean every strain of Lactobacillus will have the same benefit.

Whether you’re consuming your probiotics in food or a pill, it’s not always easy to know exactly what you’re getting.

“You want to look for a product that includes a specific strain designation on the label and contains about a billion live microorganisms,” says Cresci.

But because probiotic supplements—like all supplements—are not regulated by the Food and Drug Administration (FDA) in the same way that drugs are, there is still no guarantee you’re ingesting exactly what the label claims.

According to Cresci, you may get more viable bacteria from eating fermented foods (such as kimchi) than from popping a pill, and you may get other benefits as well.

“The bacteria in these foods secrete some beneficial byproducts, so when you eat a food that contains probiotics you will also be consuming those organic acids that are released from the bacteria,” she says.

Start with our past coverage on probiotic foods you can try, and foods that are good for gut health.

Proceed With Caution

“It’s important to note that this new review is not a statement about probiotic safety,” says Duffy MacKay, senior vice president, scientific and regulatory affairs, Council on Responsible Nutrition, an industry group. “It’s a call to action to the research community that they need to do a better job of reporting safety as part of clinical trials.”

And the study authors themselves echo that in their conclusion: “Many researchers in this area think that a detailed evaluation of potential harms is not necessary, however, the safety profile of an intervention should never be presumed. It should be rigorously evaluated and reported.”

So if you’re curious about probiotics, what should you do?

Remember that foods that are naturally probiotic may have benefits above and beyond what's found in supplements.

“There is no evidence that healthy people will get sick from probiotics,” says Patricia L. Hibberd, M.D., Ph.D., chair of the department of global health, Boston University School of Public Health.

But it's important to be cautious: “Always check with your doctor before using any probiotic product," says Hibberd, "so he or she can assess your risks and look at whether the probiotic could interact with any medications you’re already taking.”