Photo Illustration by The New York Times; Getty Images

Walk into a health food store, or even a drugstore, and you’re likely to find an entire aisle, maybe two, dedicated to probiotics. Probiotics are live micro-organisms, usually bacteria, that provide health benefits when consumed at appropriate doses.

According to some surveys, approximately four million Americans take probiotics, which are available as pills, powders, foods and drinks. Probiotics are a huge industry — at least a $40 billion dollar one, according to Zion Market Research — and popular brands sell for 35 cents to $1 a dose, with a shelf life of several months.

Proponents argue that they improve the composition of the gut microbiome, which is involved in many aspects of health including immunity, metabolism and mood.

Framed this way, probiotics may seem like a no-brainer. But before you reach for your wallet, keep in mind that while many scientists and doctors believe that probiotics have promise, they also say that a lot of products on the market don’t live up to the hype.

“The current evidence does not convince me to recommend probiotics for any of my healthy patients,” said Dr. Pieter Cohen, an assistant professor at Harvard Medical School and an internist at Cambridge Health Alliance.

In a review of the scientific literature on probiotics published in January 2019, researchers concluded that “the benefits and feasibility of probiotic consumption in healthy adults remain uncertain.” Recent research has raised questions about how well probiotics are tested for safety, too.

Are there benefits to probiotics?

Some clinical studies suggest that certain probiotics can help in certain contexts. According to the American College of Gastroenterology, the probiotic Bifidobacterium infantis 35624 can help treat irritable bowel syndrome (I.B.S.), while Saccharomyces boulardii, a yeast, and Lactobacillus rhamnosus GG can each reduce the risk of diarrhea in adults taking antibiotics.

Specific probiotics have also been shown to help treat pouchitis, ulcerative colitis, colic and infectious diarrhea, and to reduce the risk of developing Clostridium difficile infections after taking antibiotics.

To understand the various contexts in which probiotics could be useful, check out this guide recommended by Gregor Reid, a microbiologist and immunologist at Western University in Ontario, Canada, and the former president of the industry-funded International Scientific Association for Probiotics and Prebiotics (ISAPP), although some products on the list are backed by more research than others.

And no matter what the science says, probiotics sold as dietary supplements are not disease-treating drugs. If companies want to market their probiotics as medical treatments, they have to pursue a form of Food and Drug Administration approval to market them as live biotherapeutic products. Thus far, no probiotics have been given this designation.

Are probiotics a scam?

With probiotics, details matter. It’s silly to walk into a drugstore, grab a probiotic off the shelf and think it’s going to do you any good.

Be wary, too, of recommendations made by people who haven’t consulted the scientific literature. “Don’t blindly trust a pharmacist, doctor, health-food shop attendant or dietary book, as sadly, most are ill informed,” Dr. Reid said.

But if a doctor you trust recommends a particular strain that has been shown in clinical studies to help your condition, or if you have identified a strain that is backed up by solid clinical research and really want to give it a try, then sure, go ahead — after double-checking with your doctor first. Don’t expect miracles, however.

So what are the issues with probiotics?

There is still the question of whether some probiotics stick around long enough in the body to do anything.

In a 2018 study published in Cell, a team of scientists gave Supherb’s Bio-25, a mixture of 11 bacterial strains commercially available in Israel, to 10 healthy people for four weeks and found that the probiotics passed right through four of them.

This suggests that in some people, these strains “will not have an effect,” said Eran Segal, a computational biologist at the Weizmann Institute of Science in Israel and a co-author of the study.

Studies aside, though, the other major problem with probiotics is that they may not contain what they say they do. In the United States, the F.D.A. regulates most probiotics as dietary supplements, which means that their manufacturing and quality-control standards are far less stringent than standards for prescription and over-the-counter drugs.

In a 2016 study, researchers at the University of California-Davis and other institutions used DNA analysis to compare the bacterial strains listed on the labels of 16 commercially available probiotics with what the products actually had in them.

The researchers found that only one of the 16 products contained the strains listed on the label; some had entirely different bacterial species. Good quality control matters: In 2014, a premature baby died from what was believed to be mold contamination in a probiotic supplement.

Moreover, some probiotics don’t even say what strains they purportedly contain or how many colony-forming units are in each dose (a measure of how many viable bacteria each dose contains), because the F.D.A. doesn’t require it.

As a result, consumers “need to do a lot of homework,” Dr. Reid said, in order to make informed decisions. They will need to hunt for brands that include this information on the label, and cross-reference the labels with their doctors’ recommendations or published research. Ideally, they should take the dose shown to work in clinical studies.

What about taking probiotics in food?

The guide recommended by Dr. Reid mentions a handful of foods that have been shown to provide benefits: some Activia, Goodbelly, DanActive and Yakult drinks and a few infant formulas.

Keep in mind that some yogurts and fermented foods don’t contain live micro-organisms, and that even when they do, these bacteria may not do anything useful.

“Unfortunately, misuse of the term ‘probiotic’ has also become a major issue, with many products exploiting the term without meeting the requisite criteria,” according to a 2014 ISAPP consensus statement published in the journal Gastroenterology & Hepatology.

What about taking probiotics after a course of antibiotics?

Some doctors recommend taking probiotics after a course of antibiotics as a way of jump-starting microbial regrowth in the gut, but a study in Cell questioned the wisdom of that practice.

Researchers found that the same probiotic mixture, Bio-25, colonized the guts of a small group of healthy people who had recently taken antibiotics, but that those who took Bio-25 then regrew their normal gut bacteria more slowly than people who didn’t take Bio-25.

The Cell studies were small, though, and their findings apply only to the Bio-25 product; no one knows whether other strains have similar effects.

And this brings up another important point: Every probiotic strain is different, and one strain’s effects have no bearing on others. Put another way, probiotics “cannot be generalized,” Dr. Reid said.

Why aren’t doctors convinced of the benefits of probiotics?

It takes time, not to mention lots of money, to amass the evidence needed to blanket-recommend a product for a large swath of the population. (After all, doctors still argue over whether healthy people should take multivitamins.)

Thus far, many clinical studies on probiotics have been small, poorly designed or difficult to interpret. Many have focused on short-term outcomes, rather than looking at long-term effects (in part because long-term trials are so expensive).

Moreover, a 2018 systematic review published in the Annals of Internal Medicine found that probiotic trials often do not report adequate safety data, particularly when it comes to potential side effects, which raises questions about whether probiotics are as safe as they have been made out to be.

Among other things, Dr. Cohen wrote in a 2018 editorial, since probiotics are live organisms, they could potentially spread dangerous antibiotic resistance genes among people’s gut bacteria — although, thankfully, there is no evidence that this has happened in humans.

Still, if one thing is clear about probiotics, it is that we need more high-quality research to determine how safe and effective the products on store shelves are.