No information is available on the clinical use of loncastuximab tesirine during breastfeeding. Because loncastuximab is a large protein molecule with a molecular weight of about 151,000 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] However, loncastuximab is conjugated with SG3249, a small-molecule alkylating agent that might enter milk and be absorbed by the infant. Because of the potential for serious adverse reactions in the breastfed infant, the manufacturer recommends that women not breastfeed during treatment and for 3 months after the final dose.