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As consumer interest grows in probiotics and other supplements that claim to regulate gut microbes, experts are posing a critical question: Are they safe?

Probiotics are increasingly popular, from Greek yogurt and kombucha to pills chock-full of bacteria in the supplement section of the grocery store. But a new analysis published Monday in Annals of Internal Medicine finds that many studies of probiotics and similar products fail to adequately report on safety and adverse events. And without that information, the authors say, it’s impossible to broadly conclude whether the products are safe.


“There seems to be an unbridled enthusiasm for the [microbiome,]” said Dr. Pieter Cohen, a physician who studies dietary supplements at Harvard Medical School and wasn’t involved in the new research. “But a lot of these studies would not be considered robust, and they don’t measure the downsides.”

The study looked at how often those downsides were studied in 384 randomized, controlled trials of probiotics, prebiotics, and synbiotics. Probiotics contain live microorganisms that are supposed to give some sort of health benefit to the user. Prebiotics are supposed to nourish the growth of beneficial bacteria in the gut. Synbiotics are a combination of the two.

The study’s authors found that 37 percent of trials didn’t report safety results and 28 percent didn’t report harms-related data. And even among the studies that did mention adverse events among study participants, 16 percent didn’t use adequate metrics and 37 percent used only generic statements, such as “the treatment was well-tolerated.” Some reports have linked probiotics to possible side effects such as gastrointestinal problems.


Reporting of serious adverse events was even less common: 80 percent of the trials didn’t report the number of serious problems that cropped up during the study. And almost none of the studies included a definition of adverse events or serious adverse events.

“We can’t say if the probiotics are safe or not, because we don’t have the data,” said Aida Bafeta, one of the study’s authors and an epidemiologist at the Institute of Social and Preventive Medicine in Lausanne.

Megan Thielking/STAT Source: Harms reporting in randomized controlled trials of interventions aimed at modifying microbiota, Annals of Internal Medicine 2018.

It’s not the first time experts have raised a red flag about the safety of such supplements sold directly to consumers. A 2011 report by the federal Agency for Healthcare Research and Quality found that the scientific literature is “not well equipped to answer questions on the safety of probiotics in intervention studies with confidence.”

Government regulations don’t address the safety of the probiotics shoppers can buy at the grocery or drugstore, supplement shops, or online. If a probiotic is intended to diagnose, treat, or prevent a disease, the Food and Drug Administration regulates it like a drug and a biological product and the manufacturer has to test it in humans and apply for FDA approval. But many probiotics don’t make such health claims and can be sold with little or no regulation. The new report shows safety questions loom over that over-the-counter market.

“The market ran ahead of the science,” said Linda Duffy, who oversees probiotic and microbiome research projects at the National Center for Complementary and Integrative Health, part of the National Institutes of Health.

Experts say there are other safety concerns the study couldn’t capture: Products tested in clinical trials might be a higher quality than what’s sold in stores. Cohen said some commercial products might contain additional strains that consumers aren’t aware of or more bacteria than the amount listed on the label. Others might not contain the microorganism listed on the label at all.

“Do I think most of the products out there in the consumer marketplace have this quality assurance stamp of approval? No, I do not,” said Duffy.

Her advice to consumers: Read the label carefully, look for the studies, and remember the limitations of the research that exists.

“We don’t have a definitive amount of data yet that says we can absolutely conclude that there is any probiotic strain that we can absolutely give a health claim to at this point,” she said.

Cohen said it’s critical to shop only from companies that have evaluated safety, quality, and efficacy of a product and that include both the strain and the amount of that strain on the label. And, he added, it’s important to talk to a physician about whether taking probiotics and similar products will be helpful for a particular condition. Probiotics are typically marketed toward a healthy population as a dietary supplement, but the actual indications for their use are much more narrow.

“We have this narrative that live bacteria used in food production can only be beneficial for the human body,” Cohen said. “But just because something is useful in making bread does not mean that it necessarily has a health benefit.”

  • People feel that in our era, we should be able to take supplements and do better. This is a cognitive bias.

    The microbiome is and exciting area of inquiry, but the science on it is primitive, and far from complete. Until that science is further developed, using pro-biotics could cause more harm than good in some people.

    We must still submit to the fates, despite our apparent Promethean advances.

  • I have chronic gastritis and probiotics and enzymes supplements have benefited me greatly! I have found so much relief from those two than rx drugs from my dr. I went out and bought them because I had to wait long periods of time between dr visits so these helped while being treated. Everyone is different and this article focuses on the unknown results as most likely negative results.

  • The problem I have with probiotics is they’re all different. Is it acidophilus? Is it bifidobacterium? What strain and does it matter? There is no general theory of probiotics that explains what they should be and what they should be used for. There is some work on a type of intestinal worm that seems beneficial in treating inflammatory bowel disease, if you can call that a probiotic.

    • Of course the probiotics are all different. And yes, we really do not know very much about the microbiome except that it is extremely complicated. But the point is that people do get substantial relief from taking active, quality, probiotics. Doesn’t help the average populace who has to try different ones to see what works for them.

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