The agency is concerned that probiotics are being illegally sold to hospitals to treat diseases in preterm infants.
Probiotics can be extremely risky for very young babies and have been linked to at least one infant death, the Food and Drug Administration said Thursday. 
The infant death linked to probiotics occurred this year, the FDA said. Since 2018, there have been more than two dozen adverse events in infants linked to the products, and the agency is investigating whether probiotics have contributed to other cases in babies.
There are no FDA-approved probiotics for infants of any age, the agency said. Some products, however, are illegally sold to treat diseases in preterm infants in hospital settings, the FDA said.
Probiotics — which contain live bacteria or other microorganisms — are commonly used as dietary supplements in the U.S. Because they are supplements, not drugs, the FDA doesn’t review them for their safety or effectiveness.
Last month, the agency sent a letter to health care providers, warning them about the use of probiotics in preterm infants, due to the risk of “invasive, potentially fatal” infections.
The letter mentioned a preterm infant who, while still in the hospital after birth, had been given a probiotic product called Evivo with MCT Oil, made by the company Infinant Health.
The baby developed sepsis, a life-threatening condition that happens when the body’s immune system has an extreme response to an infection, and subsequently died, the letter said.
The letter didn’t say where the death occurred.
Infinant Health did not immediately respond to a request for comment.
The FDA also said Thursday that it sent a warning letter to Abbott Laboratories on Tuesday about one of its products, Similac Probiotic Tri-Blend, which was being sold for use in preterm infants in hospitals. It contains three bacterial strains: B. infantis, S. thermophilus and B. lactis.
It’s unclear whether the product led to any adverse events in babies, but Abbott agreed to discontinue sales and is working with the FDA to “take additional corrective actions,” according to the agency.
Abbott did not immediately respond to a request for comment.
The FDA has previously warned that probiotics have been documented in medical literature to cause serious bacterial or fungal infections, particularly in very preterm or very low birthweight infants.
“We especially want to make clear that products containing live microorganisms may present serious risks to preterm infants in hospital settings,” Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, said in a statement Thursday.
“We want to caution parents, caregivers and healthcare providers about these products when used for the prevention or treatment of disease, as they have not undergone the agency’s rigorous premarket process to evaluate their safety and effectiveness for these uses,” Marks said.
The American Academy of Pediatrics does not recommend the use of probiotics in preterm infants, although the organization says about 10% of preterm infants receive a probiotic during their stay in the neonatal intensive care unit.

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