Probiotics are having a moment. The tiny buggers are marketed as health enhancers for adults, for kids, and even for dogs (in a beef flavor, of course). You can now pop a capsule, swig a fruit drink or eat an energy bar that’s been spiked with probiotics, which the World Health Organization defines as “live microorganisms” that “confer a health benefit on the host” as long as enough of them enter the system.

The concept of using “good” bacteria to improve health may feel like a recent idea, but it was first put forward by Ilya Metchnikoff, a Russian scientist born in 1845 who thought the colon was a “vestigial cesspool” and hypothesized that the friendly microbes in yogurt might help improve the population of the bacteria in your gut. A century after his death, consumers are snapping up the products based on his idea. According to the National Health Interview Survey, 3.9 million U.S. adults reported using probiotics or prebiotics in 2012, the most recent year for which data is available. That was almost 3 million more than in 2007, so it’s safe to assume that even more people are taking them now. Sales of probiotics worldwide passed $32 billion in 2013 and are likely to reach $52 billion by 2020, according to Grand View Research.

It’s the third day of this week’s series on gut science. We’ve written about whether gut science is biased, why we’re so obsessed with constipation, and we’ve made a video about what poop can tell us about our health — and there’s more to come later in the week.

But there is still so much we don’t know about whether and how the probiotic products now on the shelves — which most commonly contain bacteria from the Lactobacillus and Bifidobacterium genera — can improve health. Probiotic supplements (and to a lesser extent, prebiotics) have been studied for a host of ailments, including digestive problems, allergic disorders, obesity, dental problems, the common cold, high cholesterol and gestational diabetes. But there’s limited evidence that they work for any but a handful of conditions. The probiotic craze has gotten ahead of the science.

At the very basic level, we don’t understand how probiotics work. “There’s a black box in between giving the [probiotic] and the health effects,” said David Mills, a professor of food science and technology at the University of California, Davis. There are a variety of possible ways they might work, including competing with “bad” bacteria for nutrition and stimulating the immune system. But none of it is nailed down. “We are just realizing how complex the gut is,” Mills said. We still don’t even know what an ideal gut bacterial mix would look like, if there is such a thing.

There’s also no such thing as a standard probiotic. Instead there are many different kinds of microorganisms, and they likely work differently for different diseases or conditions. “The challenge for trials is how to decide what the right bacteria is to use,” said Dr. Matthew Ciorba, a gastroenterologist and assistant professor of medicine at Washington University in St. Louis. “And it’s not only choosing among different strains — should we combine it with something else? Or is that a detriment?”

This all means that the evidence surrounding probiotics for various conditions gets complicated, fast. Even with the health problems for which there is some suggestive evidence that probiotics can help, there’s a ton of research yet to be done about the best types of probiotics to use, what dosage is ideal, how long to take them, and which groups of people are most likely to see a benefit.

Consider one theoretically logical use for probiotics: the prevention of diarrhea caused by antibiotics, which can alter the balance of bacteria in your gut. It would make sense that putting back some of the “good” bacteria in the form of probiotics might be helpful. In a 2012 meta-analysis covering 63 randomized controlled trials, researchers found that probiotics reduced the risk of diarrhea by 42 percent. That sounds pretty good.

But the authors said there were information gaps in the studies they analyzed. For example, the studies rarely specified the type of antibiotic that caused the diarrhea, making it difficult to see whether that affected probiotic effectiveness. The probiotics used in the studies also often contained blends of different types of bacteria, but few described the specific strains, and even fewer said they’d tested the probiotic for potency before using it. That missing information makes the studies hard to translate into practice, said Sydne Newberry, project leader for the evidence-based practice center at RAND Corp. and an author of the study.

And to complicate things, in 2013, a study of almost 3,000 older adults, which the authors said appeared to be the largest of its kind at the time, looked at a high-dose, multistrain combination of lactobacilli and bifidobacteria to treat antibiotic-associated diarrhea. The group taking the probiotic fared no differently than the group taking a placebo.

See? Confusing. I’ll avoid getting too far into the weeds, but there’s strong evidence that probiotics can help prevent necrotizing enterocolitis, a serious bowel disease, in premature infants. There’s more limited evidence that certain probiotics can ameliorate infectious diarrhea, cutting the duration by about a day. And they may help with irritable bowel syndrome. For details on other conditions, here’s a 2015 consensus opinion from a Yale/Harvard workshop on probiotics, (though FYI, the meeting was sponsored by probiotics makers). And here’s a 2011 review of evidence by the World Gastroenterology Organization.

It can all be seriously hard, if not impossible, to decipher as a consumer. Ciorba recommended telling your doctor if you want to try a probiotic. She may be able to recommend the best brand for your problem based on the research or her experience. It’s also important to note that most rigorous clinical research for specific problems has been conducted with supplements, not probiotic-spiked food items. Although fermented foods containing live microbes (such as kombucha and yogurt) are often described as probiotics, the International Scientific Association for Probiotics and Prebiotics limits the term to the microbial species that have been shown in controlled trials to benefit health. Modern processed yogurts vary in bacterial strains and count as well as shelf life, which means there may not be a probiotic benefit from a typical serving by the time you eat it, even if it contains live cultures, says Wahida Karmally, director of nutrition for the Irving Institute for Clinical and Translational Research at Columbia University Medical Center.

Beyond the knowledge gaps in the current science, there’s the matter of quality control. Probiotics are regulated as dietary supplements or foods, which is why their labels can make only vague claims about health. If a manufacturer wanted to make a specific claim that a probiotic prevents or treats a disease, it would have to go through a formal (and very expensive) multistage Food and Drug Administration approval process, like a regular drug.

And the FDA doesn’t regulate supplements in the same way it does drugs. So consumers can’t be as sure that label claims are accurate. You want to know whether you’re getting the types of bacteria that the label promises, in the stated amounts. Mills’s research is among the studies that have found differences between the type of bacteria listed on the label and what was in the bottle. He and his colleagues looked at 16 types of bifidobacterial probiotic products and found that only one perfectly matched its label claims in all the samples tested. And an analysis by the independent testing group ConsumerLab found two brands out of 40 that didn’t contain the number of cells that was claimed.

There’s also the possibility of contamination. A study co-authored by Dr. Peter Green, a professor and director of the Celiac Disease Center at Columbia University, found that 14 percent of a group of 423 people with celiac disease reported using probiotics. And in another study presented at a meeting last year, he and his colleagues reported finding gluten in some probiotic supplements. That could cause stomach issues for the very people with celiac disease who are using them to try to alleviate their symptoms.

As for other potential harms of probiotics, the available studies don’t show safety issues for healthy people, though the authors of a government-sponsored review said future research should dig more deeply into whether certain groups of people might have problems with certain types of probiotics. There have been rare reports of an association between probiotics and infections in people who are already very ill or who have compromised immune systems.

But if you have a healthy immune system and aren’t seriously ill, an over-the-counter probiotic with some research behind it might be worth a shot if you’re trying to treat a specific condition. Ciorba suggested trying one for a month and moving on if it doesn’t help. (As you saw in the charts above, these things are expensive, in some cases costing upwards of a dollar a capsule.) There’s no evidence in humans, however, to support taking probiotics just to generally improve your gut health or support immune function — whatever that’s even supposed to mean. As Ciorba put it, “If you don’t have a disorder, why are you taking them?”

Additional reporting on probiotic costs by Leah Libresco.