A new report has found little evidence that probiotics have an effect on healthy adults, suggesting that the largest demographic buying them may be doing so for no reason.

Probiotics are live bacteria that, as the National Institutes of Health explains it, are “similar to beneficial microorganisms found in the gut.” Available in certain foods like yogurt, they are also sold in supplement form under brand names like Culturelle.

While they are widely believed to be effective in treating or lessening symptoms of certain gastrointestinal diseases, their function in healthy adults has been surprisingly underexplored by scientists. Despite a lack of evidence, many healthy adults have begun taking them, likely inspired by a wealth of research and media coverage that suggests a healthier gut (“microbiome”) makes for a healthier individual.

The trend isn’t necessarily far-fetched. It’s reasonable, perhaps even smart, to assume that healthy individuals can strengthen their guts by adding more good bacteria (“microbiota”). But the truth may be a little more complicated.

To dig into whether or not probiotics are benefitting the healthy individuals who take them, researchers from the University of Copenhagen performed a systematic review of seven clinical trials. Each trial analyzed the short-term effect of probiotics on the fecal microbiota of healthy adults.

The studies were made up of small sample sizes ranging from 21 to 81 adults, ages 19 to 88. None of those studied had gastrointestinal disorders, nor were taking other forms of supplements. Over the course several weeks, researchers analyzed the guts of two groups: One was taking probiotics (in the form of biscuits, milk-based drinks, or capsules); the other was taking nothing.

Despite what the widespread use of probiotics would suggest, the majority of those taking the probiotics showed little to no change in their gut bacteria. Out of the seven trials the researchers studied, just one reflected evidence of a significant change.

“According to our systematic review, no convincing evidence exists for consistent effects of examined probiotics on fecal microbiota composition in healthy adults, despite probiotic products being consumed to a large extent by the general population,” said Nadja Buus Kristensen, one of the study’s authors.

The Food and Drug Administration classifies probiotics as dietary supplements, meaning that they are widely available to the public. With few side effects and no regulation on their use, consumption is soaring. From 2013-2014 alone, global sales increased 14 percent, garnering $2.4 billion worldwide. In the U.S. (which accounted for $1.5 billion of total 2014 sales), the growth has been explosive, jumping 23 percent in one year.

Researchers, whether intentionally or not, seem to be contributing to the growth of the industry. Studies on potential benefits of the supplements are released almost daily. Hours before the systematic review, for example, researchers from Tokyo released data showing that they may help “mitigate stress” in students at exam time.

While it’s comforting to think that a small capsule of bacteria has the potential to improve our lives, it’s a theory that’s not yet supported by science—and one the researchers from Copenhagen hope people will consider more carefully.

“While there is some evidence from previous reviews that probiotic interventions may benefit those with disease-associated imbalances of the gut microbiota, there is little evidence of an effect in healthy individuals,” said senior author Oluf Pedersen. “To explore the potential of probiotics to contribute to disease prevention in healthy people there is a major need for much larger, carefully designed and carefully conducted clinical trials.”