Probiotics: so hot right now.

Recent reports have linked probiotics, or “good bacteria,” to everything from fighting liver cancer to reversing childhood malnutrition to getting rid of yeast infections. But probiotics have not been proven effective at any of these things. That liver-cancer-fighting study was done on mice, as were two out of the three malnutrition studies cited at that link. And despite the pervasive gynecology advice to eat more yogurt, there has yet to be any conclusive evidence that it helps women with that pesky vaginal itch.

Doctors in US hospitals, it turns out, are also big fans of these almost magical-sounding bacteria: Between 2006 and 2012, probiotic use in hospitals nearly tripled, according to a new study from the US Centers for Disease Control and Prevention (CDC) published in the American Journal of Infection Control. The 2.9-fold increase in probiotic use in hospitals, the study warns, comes “despite inadequate evidence to support their use.”

Looking at approximately 1 million hospitalizations at 139 hospitals in 2012, the CDC researchers found that probiotics were prescribed in 2.6% of cases—approximately 52,000—compared to 2006, when it was only 1.0%. (The CDC study looked at oral probiotics only, not the increasingly popular fecal transplants.)

Outside of hospitals there’s been a similar increase in the use of probiotics, the CDC notes, citing other studies. In a 2007 survey, 0.4% of US adults reported taking probiotics in the prior 30 days; in 2012, that went up to 1.6%.

Probiotics are often prescribed with antibiotics, lead researcher Sarah Yi told Quartz. “It makes sense if you wipe out the bacteria in your intestinal tract [with the antibiotics], why not replace them?” she said.

As rational as that thinking sounds, though, the data don’t necessarily support it, she says. While some systematic reviews find that certain kinds of probiotic treatment help with specific illnesses, it’s still not conclusive—and doctors don’t necessarily prescribe in line with those reviews. “Some use is in accordance with evidence,” Yi says, “but not all of it.”

While most people view probiotics as benign at worst, it’s not always that simple. “Most hospitalized patients would not be at high risk for infection related to probiotics,” Yi says, “but there would be patients that could be, especially in the ICU.” That includes people with weak immune systems, such as those waiting for an organ transplant.

Probiotics are undoubtedly a promising new area for research. But most of that research has yet to be done.