Breastfeeding is an important aspect of mother-newborn relationship and is of great benefit for the baby. Unfortunately, many drugs taken by the mother may pass into her milk and exert an effect on the newborn. Very limited data is available and a cautionary approach is warranted especially when the woman receives anticancer treatment including chemotherapy, hormonal treatment and the recently introduced target agents as well as monoclonal antibodies. In all these conditions breastfeeding should be put on hold. More and more often physicians are faced with women that are pregnant years after the diagnosis of cancer: this has long been considered dangerous for the mother, but data show that prognosis is definitely not worse. If the woman is no longer being actively treated, breastfeeding is advisable every time it is possible, even if patients that received breast radiation may be unable to produce a sufficient amount of milk on that side.