Protein binding is only one of several factors which determines how much of
a drug gets into the milk.  Almost all drugs are attached to proteins in the
mother's plasma, to a greater or lesser extent.  A drug attached to a
protein becomes, in effect, too big to get into the milk, so that only the
drug which is not bound to protein can get into the milk.  Thus when a drug
is 99% protein bound, only 1% of that which is in the mother's plasma can
get into the milk.  Of course, if 90% of the drug is sitting in the mother's
fat or tissue other than the plasma, then only 10% is in the mother's
plasma, and of that, only 1% gets into the milk.

Jack Newman, MD, FRCPC

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