The drug you refer to is probably interferon.  Interferon normally is
found in human milk.  The amount that would get into the mother's milk
as a result of treatment is likely negligible since the molecular
weight of interferon beta-1b (Betaseron) is 15-20000 daltons (huge).
This drug is also given by injection, probably because the amount
absorbed from the gastrointestinal tract would be insignicant.

Given all the above, there should be no problem with taking Betaseron
and continuing breastfeeding.

Jack Newman, MD, FRCPC