No information is available on the clinical use of trastuzumab during breastfeeding. Because trastuzumab is a large protein molecule with a molecular weight of 145,531 Da, the amount in milk is likely to be very low. It is also likely to be partially destroyed in the infant's gastrointestinal tract and absorption by the infant is probably minimal. Until more data become available, trastuzumab should be used with caution during breastfeeding, especially while nursing a newborn or preterm infant. The manufacturer recommends that breastfeeding be discontinued during trastuzumab therapy and for 7 months after the last dose.