Probiotics are commonly known as “friendly bacteria.”
Found in certain foods and supplements, these live microorganisms are similar to the beneficial microbes that naturally colonize the human body.
While yogurt and other probiotic products have existed for millennia, the market is rapidly growing for probiotic supplements and commercially produced probiotic foods.
Many of these products also contain prebiotics, or ingredients that promote the growth or activity of beneficial microbes. So-called synbiotics contain both probiotic and prebiotic components.
Proponents of probiotic and prebiotic products often suggest they can help prevent or alleviate a wide array of health problems, from irritable bowel syndrome to yeast infections and more.
But how much do we really know about them?
New research on probiotic safety
A new literature review published in the journal Annals of Internal Medicine has found that many studies report little to no information about the safety of these products.
The researchers were from the French National Institute of Health and Medical Research and other institutions.
The authors of the review assessed the published findings of 384 randomized controlled trials on probiotics, prebiotics, and synbiotics.
“One-third of the trials gave no information on harms,” the study authors wrote. “Only 2 percent adequately reported key safety components.”
Most of the studies included in the review did not report how the researchers defined adverse events or serious adverse events.
Moreover, the vast majority did not describe the methods used to collect harm-related data.
“One strong belief about probiotics and prebiotics is that they are safe,” noted the study authors. “Yet adverse events (AEs) arising from their use are poorly understood.”
“We believe that researchers must clearly describe the incidence and severity of AEs related to probiotics, prebiotics, and synbiotics,” they added, “particularly when they are used to treat severe disease or are used by high-risk patients.”
Lack of safety data contributes to funding concerns
Dr. Shira Doron, an associate professor at Tufts University School of Medicine and attending physician at Tufts Medical Center, has helped to conduct multiple studies on probiotic products.
She was not surprised by the study’s findings.
“This was a well-done, systematic review demonstrating something that those of us who do research in the field already know — that the vast majority of published studies do not adequately evaluate the safety of probiotics, prebiotics, and synbiotics,” Doron told Healthline.
“The biggest implication is something you might not think of — without that safety data, the FDA has been reluctant to allow government-funded clinical trials,” she added.
This means that most research on probiotics is conducted overseas or funded by manufacturers of probiotic products.
When research is conducted in other countries, it raises questions and concerns “over the ability to extrapolate results to U.S. patients,” Doron said.
When research is funded by manufacturers, it calls into question the validity of the results. After all, manufacturers have a vested interest in finding their products and prospective products to be safe and effective.
“Researchers in the U.S. without ties to manufacturers who seek government funding to do clinical trials face difficulty in obtaining approval by the FDA to conduct the types of studies that consumers need in order to know which probiotics are effective for their particular condition,” Doron said.
This poses significant barriers to what Doron calls “the really interesting work” — researching the efficacy of using probiotics to treat specific health conditions.
Taking probiotics may have benefits and risks
According to Doron, some people may benefit from taking certain strains and doses of probiotics.
“There are several conditions for which probiotics have been shown to be beneficial in well-done clinical trials,” she said. “These include prevention of allergies in children and prevention of antibiotic-associated diarrhea.
“There are other conditions for which people take probiotics with little evidence to support that practice,” she added.
Although probiotics have not been shown to improve general health or alleviate many of the conditions for which people take them, most people can safely consume them, Doron said.
However, there have been reports of infections caused by organisms in probiotic products. And in some cases, those infections have been serious.
“Usually there is an underlying medical condition to explain these infections,” Doron explained, “such as an impaired immune system or disruption in the lining of the intestine.”
Doron advises against taking probiotics if you have an impaired immune system or a condition that disrupts the integrity of your intestinal tract.
She also recommends against using them if you have an intravenous catheter, which can provide a pathway for organisms from probiotic products to enter your bloodstream and cause an infection.
If you do take probiotic products, it’s best to use a strain and dose of probiotics that has been shown to be effective for treating your specific condition or symptoms.
“When you choose a probiotic, make sure you are using one that was shown to be effective in a clinical trial treating the particular condition or symptom you are looking to ameliorate,” Doron advised.