Measurable, but low, amounts of natalizumab are excreted into breastmilk in some, but not all, women. The time of the peak level in breastmilk is often in the first week after the dose, but might be as long as 6 months with repeated dosing. Because natalizumab is a large protein molecule with a molecular weight of about 149,000 Da, it is likely to be partially destroyed in the infant's gastrointestinal tract and absorption by the infant is probably minimal. Most expert guidelines do not recommend avoiding breastfeeding with natalizumab;[1-4] however, waiting for at least 2 weeks postpartum to resume therapy may minimize transfer to the infant.[5] No other precautions appear to be required during breastfeeding while using natalizumab and breastfeeding can resume immediately after injection.[5]