Nearly 4 million people in the United States take probiotics with the hope that this can boost their digestive health. Researchers, however, have found evidence that probiotics in food and supplements do nothing to up to half of the people who take them. They also discovered that probiotics can be potentially harmful.

In a new study published in the journal Cell on Sept. 6, Eran Elinav, from the Weizmann Institute of Science in Israel, and colleagues found that some people’s guts appeared resistant to the so-called good bacteria.

Elinav and colleagues involved 19 people taking probiotics consisting of 11 of the most common strains. They found that only eight of these individuals had notable colonization of good bacteria in their gut. Of these eight, three had significant colonization and five had mild colonization.

“If you take a probiotic that you buy at your local supermarket, you have no way of knowing whether it would pass from one end to the other or colonize your gut, where it may (or may not) induce health effects,” Elinav told Gizmodo.

In a second study also published in the journal Cell on Sept. 6, Elinav and colleagues involved 21 volunteers to find out if consuming probiotics can help with the recovery of natural gut bacteria after taking antibiotics.

They found that people who take probiotics after antibiotic treatment, a practice meant to restore healthy bacteria in the gut, are not necessarily better off. In fact, people who use probiotics took longer time to have their microbiome return to normal compared with those who did nothing at all.

Elinav said that the delay in restoring healthy bacteria in the gut may further increase a person’s odds for health problems that are linked to a chronically imbalanced microbiome such as allergies, obesity, and other inflammatory disorders.

He also said that the findings show that the habit of consuming probiotics after taking antibiotics may not be as harmless as previously believed.

“These results reveal a new potential adverse side effect of probiotic use with antibiotics that might even bring long-term consequences,” Elinav said in a statement published by Eurekaalert. “In contrast, replenishing the gut with one’s own microbes is a personalized mother-nature-designed treatment that led to a full reversal of the antibiotics’ effects.”

Both of the studies involved small number of participants. This means that further research is needed to confirm the findings.

Probiotic products available in the market include yogurts, supplements, and skin creams.

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