There is no one answer to what you are asking.  The fact that the nursing
staff do not feel "comfortable" with not having milk levels just tells me
how wrong we go.  What difference does that make really?  The amount is
likely to be very small, and it is not necessarily so that the peak milk
levels will be in the four hours after the mother takes the medication.

Some drugs flow easily back and forth from maternal plasma to milk and back.
Some don't.  Alcohol is a typical one which is freely diffusable back and
forth, so as the plasma level drops, the alcohol will return into the
mother's plasma from the milk.  But other drugs are not like that,
especially if they are highly ionized, for example.

The point is that very few drugs get into the milk in such quantities that
we need to worry for the baby.  When the baby is premature, of course, there
is often more concern, but still, most drugs get into the milk in tiny
amounts, too tiny even for the premature baby to worry about.

Jack Newman, MD, FRCPC

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