Most sources consider breastfeeding to be contraindicated during maternal antineoplastic drug therapy.[1] It might be possible to breastfeed safely during intermittent therapy with an appropriate period of breastfeeding abstinence. The duration of abstinence is not clear, but some have suggested a period of 24 hours before resuming nursing.[2] Limited information indicates that a maternal continuous intravenous fluorouracil infusion at a dose of 200 mg/square meter daily produces undetectable levels in milk. If fluorouracil use is undertaken, monitoring of the infant's complete blood count and differential is advisable. Chemotherapy may adversely affect the normal microbiome and chemical makeup of breastmilk.[3] Women who receive chemotherapy during pregnancy are more likely to have difficulty nursing their infant.[4]
Topical fluorouracil applied away from the breast should pose negligible risk for the breastfed infant. If topical fluorouracil is required by the mother, it is not a reason to discontinue breastfeeding. Do not apply fluorouracil to the breast or nipple and ensure that the infant's skin does not come into direct contact with the areas of skin that have been treated