Infant formula is milk-based and is modified to resemble breast milk. Some infants are allergic or have an intolerance to the protein, or more rarely, the sugar that is in the milk. Half of all infants who are sensitive to cow's milk are also sensitive to soy-based formula.
Some of the most obvious symptoms of this sensitivity are rashes, hives, difficulty breathing, eczema (dry, flaky patches of skin especially around the forehead), spitting up excessively, or vomiting. Other signs to watch for are fussiness, crying, and discomfort during or after a feeding. Some infants are very gassy and act colicky. A infant may also have a red ring around the rectum, may have stools that are unusually hard, watery, loose, or foul-smelling.
There are formulas, such as Alimentum and Nutramigen, which have the protein already broken down so it is easier to digest.
If allergies run in your family, make sure you alert your healthcare provider during your pregnancy.