About 10% of breast-milk samples purchased via Internet-sharing sites contained significant amounts of cow’s milk or formula based on it, according to a study in the journal Pediatrics.

Most experts recommend waiting until a baby is at least 12 months old to introduce cow’s milk because it is too low in iron and too high in protein and minerals that are difficult for infants to digest. It also can cause allergic reactions ranging from mild distress to anaphylactic shock, according to the American Academy of Pediatrics.

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