Each breast is different in shape, size and anatomical make-up, and babies often develop some degree of feeding preference. Reassure the mother that most of the reasons for persistent breast refusal are harmless and temporary, and can be rectified with patience, persistence and good support.
Physiological differences between the mother’s nipples or breasts, or differences in the let-down reflex may cause the baby to develop a preference for a specific side. This may lead to a cycle of events whereby the baby feeds less frequently and the supply drops, to the point that the baby refuses to feed from one breast. Similarly, if one breast has been injured or has had surgery, a reduction in the number of functioning milk ducts may cause the baby to receive less milk. In other cases, using a deodorant or perfume may discourage the baby.
In the rare cases when a baby consistently refuses one breast from birth, it is advisable to arrange GP or specialist examination to investigate any physiological or medical reason for the refusal, in either the mother or child (perhaps as a result of position in utero, birth trauma, or physiological factors involving the breast, milk ducts or nipple). Sudden refusal in an older baby may occur if the mother has mastitis, because of reduced milk flow and altered taste of the milk.
Treatment if the unfavourable breast has a lower or different shaped nipple, suggest different a different feeding position or use a suction device, breast shell or pump to draw out the nipple until the baby learns to latch on by himself. If the milk flow is suboptimal from one breast, express a little milk before the baby latches on, so that the feed begins with less effort. Nursing in different positions and regularly offering the less-preferred breast at the start of a feed may be helpful; regular expression also enables the milk supply to be maintained.
Certainly a baby can grow and develop normally by feeding exclusively from one breast, and expressed breast milk from the unfavourable side can be given in top-up feeds. There is no need to offer other supplements unless the baby’s growth is affected.

From: Journal of Family Health Care Bulletin. Directory of Breast-Feeding Advice. December 2009. Published with JFHC 2009; 19(6). http://www.jfhc.co.uk/images/stories/breastfeeding.pdf