If you've been given free samples or have discount coupons for a brand of formula that's different from the one you've been feeding your baby, it's tempting to switch. In most cases, it's fine to change formulas provided you stick with the same type. For instance, the ingredients in all cows' milk-based, iron-fortified infant formulas (recommend for most babies) are essentially the same. The taste may vary slightly and your baby may balk when you switch, but you won't harm her by changing brands. Likewise, if your baby is on a pediatrician-recommended soy-based, iron-fortified formula, switching to a different brand of soy-based, iron-fortified formula won't harm her. But if your baby is on a special type of formula, such as a hydrolysate formula, or you're considering switching formula types, check with your pediatrician first.
You may be worried that switching formulas will upset your baby's stomach, but that's unlikely. All babies have gas – a lot of gas – so gas alone doesn't mean your baby is unable to tolerate whatever kind of formula you're giving her. Nor is it a sign of trouble if your baby's bowel habits change in other ways, such as frequency or color. If her stool suddenly becomes very firm, she may be constipated, in which case you should tell your pediatrician (Don't assume the constipation is due to iron-fortified formula and switch to a low-iron formula. Studies show that iron doesn't cause constipation or any other digestive upsets.) If there's blood in your baby's stool or she's vomiting and there's blood in the vomit, call your baby's doctor as soon as possible. These are signs of a true formula intolerance.
Finally, I don't recommend switching from cows' milk-based formula to a soy-based formula without consulting with your pediatrician first. If you have a conversation with your child's doctor about the pros and cons of cows' milk formula versus soy, you'll then be able to make an informed decision together. Some parents worry that excessive gas or other bowel oddities indicate an allergy to cows' milk, but true cows' milk allergies are extremely rare. I've cared for more than 1,000 newborns in my practice and treated only two for milk-based protein intolerance. Incidentally, many children who are allergic to cows' milk-based formulas are also allergic to soy-based formulas, so they're not a cure-all.
Watch a pediatrician demonstrate how to make baby formula and store it safely.