In the letter, the FDA said it was concerned that preterm infants were at risk of potentially fatal diseases or infections caused by bacteria or yeast contained in the probiotics.

"The product is an unapproved new drug and an unlicensed biological product being sold in violation" of regulations, the FDA said on Thursday about Abbott's Similac probiotic product.

An Abbott spokesperson said the issue pertains to Similac Probiotic Tri-Blend, which is used by fewer than 200 hospitals, and that it does not apply to any of the company's infant formula products available at retail stores.