Is Zofran (ondansetron) safe while breastfeeding?

What does the evidence say about Zofran while breastfeeding?

1. In a pharmacokinetic study of women given a single 4 mg IV dose about 2 days postpartum, the relative infant dose was estimated at 3.3% of the mother's weight-adjusted dose, well under the roughly 10% level often used as a rough safety marker. 1
2. In a pharmacokinetic study of 78 women given ondansetron intravenously after birth, no adverse effects were reported in their breastfed infants, and computer modeling found milk amounts well below the dose already approved for infants as young as 1 month old. 2
3. Use of ondansetron in nursing mothers beyond the immediate postpartum setting hasn't been well studied, so the reassuring data mostly reflects standard short-course use right around delivery or a C-section rather than ongoing, repeated use. 2
4. Most studies of ondansetron in pregnancy haven't found a higher chance of birth defects, though some early research suggested a small possible increase in cleft palate or heart defects that other studies haven't confirmed. Side effects in breastfeeding infants have not been reported, though the evidence base there is more limited. 3
5. Ondansetron carries an FDA caution about QT interval changes, a heart rhythm effect, at higher intravenous doses, which is why a maximum single dose and monitoring are recommended for people with existing heart rhythm issues or electrolyte imbalances; separately, the drug does not appear to affect the onset of breastfeeding. 4

How Zofran changes by nursing stage

• Early weeks. Right around delivery or a C-section, this is the setting ondansetron is most studied in for breastfeeding: a pharmacokinetic study of 78 women given it intravenously after birth found no adverse effects in their nursing infants, and it doesn't appear to affect when breastfeeding gets going.
• Established nursing. If you need it again later while nursing, for a stomach bug or ongoing nausea, know that longer-term use beyond the immediate postpartum period hasn't been as well studied. The modeled infant dose stays low, well under the amount already considered acceptable for infants a month and older, but talk with your provider before repeat or ongoing use.
• Weaning. There's no specific cutoff where ondansetron becomes off-limits while nursing, but because higher doses carry an FDA caution about heart rhythm changes in the person taking it, mention any personal heart rhythm history or electrolyte issues to your provider so they can weigh the right dose and monitoring for you.