Can you take fluorouracil while breastfeeding?
What LactMed says
The National Library of Medicine’s own summary, quoted in full. We do not write a verdict of our own on this question, and we do not compress theirs into a label.
Most sources consider breastfeeding to be contraindicated during maternal antineoplastic drug therapy. 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, while others have used a 72-hour abstinence period safely. 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. Women who receive chemotherapy during pregnancy are more likely to have difficulty nursing their infant.
Topical fluorouracil applied away from the breast should pose negligible risk for the breastfed infant. If the mother requires topical fluorouracil, 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
Quoted from Fluorouracil — Drugs and Lactation Database (LactMed®), National Institute of Child Health and Human Development, revised July 15, 2026.
How much fluorouracil gets into breastmilk
Maternal Levels. A mother who was 9 months postpartum was diagnosed with rectal cancer and began treatment with pelvic radiotherapy and a continuous intravenous infusion of fluorouracil 200 mg/square meter daily. Her plasma fluorouracil concentration was constant at about 110 micromoles/L (14.3 mg/L) from weeks 2 to 5 of therapy. She discontinued nursing her infant and pumped her breasts twice daily and collected 36 breastmilk samples before, during and for 10 days following fluorouracil therapy (exact times not specified). Fluorouracil was undetectable (<0.5 micromole/L [<65 mcg/L]) in all of the milk samples.
Infant Levels. Relevant published information was not found as of the revision date.
What has been seen in breastfed infants
A woman was diagnosed with colorectal cancer at 26 weeks of gestation. She was treated with unspecified doses of fluorouracil, oxaliplatin, and calcium folinate (leucovorin calcium) at 28 weeks in the first of 3 planned doses. However, the infant was born at 30 weeks gestation, 14 days after the first dose. On day 1 postpartum, the infant was given colostrum mouth care and the mother’s milk supply was initiated. On days 8 and 22 postpartum, the infusions of fluorouracil were given, without oxaliplatin. Pumped breastmilk was fed to the infant except for the 48 hours of fluorouracil infusion and for the 24 hours afterward. The infant was exclusively breastfed on discharged from the NICU on day 50 and at 11 months postpartum, the infant was thriving.
Effects on milk supply
A telephone follow-up study was conducted on 74 women who received cancer chemotherapy at one center during the second or third trimester of pregnancy to determine if they were successful at breastfeeding postpartum. Only 34% of the women were able to exclusively breastfeed their infants, and 66% of the women reported experiencing breastfeeding difficulties. This was in comparison to a 91% breastfeeding success rate in 22 other mothers diagnosed during pregnancy, but not treated with chemotherapy. Other statistically significant correlations included: 1. mothers with breastfeeding difficulties had an average of 5.5 cycles of chemotherapy compared with 3.8 cycles among mothers who had no difficulties; and 2. mothers with breastfeeding difficulties received their first cycle of chemotherapy on average 3.4 weeks earlier in pregnancy. Of the 9 women who received a fluorouracil-containing regimen, 8 had breastfeeding difficulties.
Frequently asked questions
Can you take fluorouracil while breastfeeding?
Most sources consider breastfeeding to be contraindicated during maternal antineoplastic drug therapy. The full record is quoted on this page, and the decision is one to make with the person who prescribed it — LactMed itself states it is not a substitute for professional judgement.
Does fluorouracil pass into breastmilk?
LactMed's measured drug levels for fluorouracil are quoted in full on this page, under "How much gets into breastmilk".
Do I need to pump and dump after taking fluorouracil?
LactMed does not frame its records that way — it reports measured drug levels in milk and what has been observed in breastfed infants, which is what this page quotes. "Pump and dump" advice for a specific drug and dose should come from your clinician or a pharmacist, not from a general rule.
More on fluorouracil
LactMed states that the information it presents is not a substitute for professional judgement, and that you should consult your healthcare provider for breastfeeding advice related to your particular situation. Nothing on this page is medical advice.