If the baby is older than three or four months, and supplementation appears to be necessary, formula is not necessary and extra calories can be given to the baby as solid foods. First solids may include: mashed banana, mashed avocado, mashed potato or sweet potato, infant cereals, as much as the baby will take, and after the baby has nursed, if he is still hungry. Even at this age giving bottles when the baby is not getting much from the breast will often result in breast rejection. If you must give formula, mix it with the baby’s solids. Giving solids at three or four months if everything is going well is not recommended, and even if the weight gain is slow, there are several ways of getting the baby more breastmilk that can be tried before adding solids. Solids should normally be started when the baby is showing interest in eating solids (usually around five or six months of age).