Reheating a bottle of breast milk or formula more than once is not recommended due to the risk of bacterial growth. Once a baby has drunk from a bottle, bacteria from their mouth can get into the milk, which multiplies rapidly at room temperature. If you reheat this milk, it doesn't kill all the bacteria, potentially leading to food poisoning. That's why discarding any leftover milk your baby doesn't finish during a feed is advised. For formula, it's best only to mix as much as you anticipate your baby will consume in one feeding, and for breast milk, it's better to store it in smaller quantities to avoid wastage.