No information is available on the clinical use of pembrolizumab during breastfeeding. Because pembrolizumab is a large protein molecule with a molecular weight of about 149,000 Da, the amount in milk is likely to be very low.[1] It is also likely to be partially destroyed in the infant's gastrointestinal tract and absorption by the infant is probably minimal.[2] Until more data become available, pembrolizumab should be used with caution during breastfeeding, especially while nursing a newborn or preterm infant. The manufacturer recommends that breastfeeding be discontinued during pembrolizumab therapy and for 4 months after the last dose.