If your milk production is low, formula may be necessary. However, see a board-certified lactation consultant to find out if there's a way to increase your milk production so you don't have to use formula. Usually this is possible. In answer to your question, exclusive breastfeeding for the first six months (solid foods are started then) is necessary for your baby to receive the full effect of breastfeeding from a health standpoint. But some breastfeeding is always better than none. For example, when researchers studied rates of ear infections, they found exclusively formula-fed babies had twice as many cases as exclusively breastfed babies. Babies receiving both breast and formula were in the middle.