A: Allergy symptoms can appear from the first few weeks to the first two months, depending on how sensitive your child is to the milk protein casein, which is usually the cause of an allergy to cow's milk formula. Typically babies with milk protein allergies will suffer from stomach cramps that make them cry, get cranky, and spit up after feedings. Loose poops and diarrhea, as well as blood in the stool, are also common symptoms. Some children may also vomit, break out in hives, or even have difficulty breathing in severe cases. In other instances the only sign of a cow's milk formula allergy is that your child becomes irritable after eating -- and offering another bottle to soothe him only makes things worse. If your doctor suspects a milk allergy, he will probably suggest you switch to a hydrolyzed cow's milk formula, in which the milk proteins are broken down enough so that your baby can tolerate them.
Originally published in American Baby magazine, April 2004. Updated 2009.