All interferons, (including Betaseron) are safe during breastfeeding. One factor that determines whether a drug gets into the mother's breast milk or not is the size of the molecule. Any molecule with a molecular weight of greater than 600 to 1000 won’t get into the milk. Because interferons have molecular weights that are much larger (Betaseron has a molecular weight of 22,000), there is no issue. Furthermore, even if some interferon did get into the milk, which is very unlikely, it would be destroyed in the baby's stomach. Another drug sometimes used for multiple sclerosis, glatiramer (Copaxone) is also safe during breastfeeding for the same reasons.