I believe that avonex is interferon.  If it is, there is no concern.
interferon is too big a molecule to enter the milk.  Furthermore, it
would be mostly destroyed in the baby's gastrointestinal tract.
Interferon actually is normally found in breastmilk anyway, though it
is somewhat different from the "drug" interferon.

When in doubt, continue breastfeeding, because the risks of not
breastfeeding are almost always greater than the risks of continuing
with a tiny amount of drug in the milk.

Jack Newman, MD, FRCPC