People widely assume that probiotics and prebiotics are safe, but researchers in France said Monday that there’s very little research to show that’s true.

There’s also sparse data on whether the products work in the way people believe they do, other researchers noted. Despite the growing market for probiotics, evidence that they are either safe or effective is hard to come by.

Probiotics — products that deliver allegedly helpful bacteria to the gut — have been around for hundreds, if not thousands, of years. Yogurt is one example, with its heavy dose of lactobacilli and bifidobacteria. Prebiotics are similar products that aim to nurture and cultivate so-called good bacteria.

The growing body of research on the microbiome shows that the collection of bacteria, yeasts and perhaps viruses that live in and on the body definitely affect human health.

Less has been done on whether they could be harmful in some cases.

Aida Bafeta of the French research institute Inserm and colleagues looked at 384 separate studies of prebiotics, probiotics and synbiotics — products that include both — done since 2015.

Most research teams seemed to simply assume that the products were safe, Bafeta and colleagues reported in the Annals of Internal Medicine.

“One-third of the trials gave no information on harms, and only 2 percent adequately reported key safety components,” they wrote.

“We cannot broadly conclude that these interventions are safe without reporting safety data.”

Dr. Shira Doron of the Tufts University School of Medicine is one of the few researchers who has looked at the safety of probiotics.

It’s true, she says, that there are not many reports of adverse events in healthy people. But there have been some, especially in hospitalized patients.

“If you are immunocompromised, or if you are on chemotherapy or neutropenic — having a dangerously low white blood cell count — or your gut is leaky for whatever reason," Doron told NBC News, "it stands to reason that if you ingested probiotics, they could get into your blood.”

Bacteria that belong in the intestines may do damage if they get into the bloodstream, a condition called bacteremia.

Another potential risk concerns people who take probiotics to try to counter any damage done by antibiotics. The World Health Organization has pointed out that in theory, live bacteria could deliver antibiotic-resistance genes to infectious bacteria in the body, although that’s not been shown to have happened.

Probiotics might also overstimulate the immune system, but this is another poorly studied area.

It can be dangerous to assume that taking doses of bacteria, even if they are supposedly beneficial, is safe for everyone under all circumstances, Bafeta’s team cautioned.

“People have strong beliefs about the safety of probiotics, prebiotics and synbiotics,” they wrote.

“Many researchers in this area think that a detailed evaluation of potential harms is not necessary. Caution is needed, however, particularly when considering these interventions for vulnerable or critically ill persons.”

The bigger problem, said Doron, is that no one actually knows much about whether any of the products on the market are even helpful.

Some of the product labels strongly imply that they can help people live longer, healthier lives — suggesting “life extension” — or hinting at specific effects, such as “mood-boosting,” “comfortable digestion” or “nutrient absorption.”

But scientists who study the microbiome say they haven’t yet been able to figure out which bacteria people might need more of, which they might need less of, and whether simply eating yogurt or other fermented food or taking supplement capsules can even change that balance.

“We have way more questions that we have answers,” said Doron, who was not involved in the French team’s study. “They have never been proven to be good for general health.”

There are hundreds of different species of bacteria in the intestines, and different people — and sometimes completely different populations — have different balances.,

Medical studies have looked at using probiotics for specific cases. Perhaps the best known is the use of fecal transplants to treat deadly Clostridium difficile infections, but researchers have also looked at the use of probiotics to treat other infections, to help in weight loss, or to reverse conditions such as irritable bowel syndrome.

The Food and Drug Administration allows most probiotics to be considered generally recognized as safe when added to food. But the FDA does not regulate supplements.

Doron complained that the FDA’s regulations make it difficult for researchers such as herself to study either the safety or efficacy of probiotics. She says it’s difficult to get permission to study them in people with antibiotic-resistant infections, for example.

“It’s what the public wants to know,” Doron said. “They are spending their hard-earned money.”