But experts are increasingly shifting away from recommending the supplements—the American Gastroenterological Association does not recommend using probiotics for most digestive conditions, nor has there been sufficient evidence for the AGA to recommend it to the general population.

“I don’t advocate for daily use of probiotics if you’re healthy and you’re eating well. I think it’s just a waste of money,” says professor of microbiology at UMass Chan Medical School, Vanni Bucci.

Should you take probiotics?

There are only three situations in which the AGA recommends taking probiotics, all under clinical supervision: 

• Preventing bacterial infection in adults and children taking antibiotics.
• Preventing necrotizing enterocolitis, a life-threatening illness in preterm, low birthweight infants
• Managing pouchitis, a complication of inflammatory bowel disease.

“They are definitely good at restoring and temporarily relieving problems until your good bacteria that were disturbed can come back and reestablish the healthy ecosystem,” Bucci tells Fortune. He advises using them for temporary relief, but research has shown that taking probiotics after a course of antibiotics might not do much—and could delay gut health recovery.

Some people may find that probiotics make them feel better, but it’s certainly not to transform your gut health in isolation.

“It will ameliorate some distress, but it’s not the solution,” he says. What’s much more important, Bucci explains, is eating a healthy diet. “That’s much better than any probiotic out there,” he says.

King’s College London professor of epidemiology Tim Spector, who has done substantial research on the gut microbiome, encourages people to shift their diet before thinking about probiotics. He advises eating at least three servings of fermented foods per day and 30 different plants per week—fruits, vegetables, whole grains, and legumes—which contain essential fiber and prebiotics to feed your gut bacteria.

“I think we’ve underestimated [fermented foods] and overestimated the effect of commercial probiotics,” he told Fortune.

“Your microbiome is a record of how you’re living,” Bucci says. If you have a poor diet—high in red meat and low in fiber, fruits, whole grains, and vegetables—and frequently take antibiotics, you’ll likely have poor gut health, and probiotics can’t fully fix that.

When probiotics make things worse

Some research indicates that probiotics could disrupt the balance of your microbiome—which has its own unique ecosystem—and interfere with your good gut bacteria. 

And for for those dealing with—or who have previously had—small intestinal bacterial overgrowth, also known as SIBO, taking a probiotic supplement may actually worsen their symptoms.

“You’re holding too many bacteria in your gut and it leads to inflammation,” Bucci says. “If you’re suffering from this and you’re overloading with extra bacteria that you’re ingesting, it might lead to some inflammatory exacerbation.”

SIBO arises when there is an abnormal increase in the amount of bacteria in the overall bacterial population in the small intestine, especially bacteria that is not normally found there, according to the Mayo Clinic. It can cause diarrhea, loss of appetite, abdominal pain, nausea, bloating, malnutrition, and unintentional weight loss.

People can develop SIBO from complications after abdominal surgery, structural problems in and around the small intestine, and from certain medical conditions like Crohn’s disease, celiac disease, diabetes, or other conditions that slow movement of food through the small intestine.

A 2018 study found that some probiotics may provoke symptoms for SIBO patients including gas, bloating, and brain fogginess, suggesting that the supplements should be used with caution among people with the condition.

The type of probiotic you take is important

If you and your doctor agree that probiotics could be helpful, here’s some guidance on how to pick a one: Bucci says that all of the probiotics on the market are more or less the same, because they contain the same bacterial strains. However, each brand may vary in quality. You should check the label of a probiotic for the following:

• CFU count, or Colony-Forming Units, at the time of expiration to ensure there are enough live cultures through the product’s shelf life.
• Storage requirements—Because probiotics are live bacteria, some need to be refrigerated, but others are manufactured to be shelf-stable.
• Third-party testing for potency and purity.
• Delayed-release capsules, enteric coating, or spore-based strains, which will ensure the bacteria can survive in stomach acid.

For more on probiotics:

• Dr. Oz says probiotic supplements have wide-ranging health benefits. Here’s what science says
• Coca-Cola is Olipop’s and Poppi’s latest prebiotic soda competitor. But are ‘healthy’ sodas actually good for you?
• Probiotics and prebiotics are essential for gut health. Here’s how to get the best of both