Avonex (beta interferon) has been discussed on Lactnet.  An archives search
will be useful.  Interferon is a huge molecule and won't cross into the
breastmilk; additionally it isn't absorbed orally (that's why it's given
parenterally).  In the last Lactnet discussion, Newman, Hale and Nice all
agreed that Avonex is no problem to use in a breastfeeding mother.   I just
went through this research recently for a mom in my practice.
Linda L. Shaw MD FAAP

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