Jill:  I'm no pharmacologist, but I did attend a lecture by Cheston
Berlin, one of the members of the AAP's Committee on Drugs in Human Milk,
about 2-3 years ago at a LLL Physician's Seminar.  He said that the
concept of the drug being stored in the breast is incorrect.  The
production of milk is a very dynamic process, with reabsorption and
diffusion back into the bloodstream.  Although a small amount of milk is
stored over time, the bulk of the milk produced is being made at the time
the baby nurses, therefore it is the concentration in the milk at that
time that counts.
It's important to keep in mind that there is a big
difference between drugs used acutely for a few days, which peak after a
dose and drop to fairly low concentrations at the end of a dosing
interval (known as the trough), compared with chronic medications, which
after the initial several half-lives reach what's known as steady-state
concentration.  This means that although the concentration of the drug in
the mother's bloodstream goes up a little with each dose and drops
slightly at the trough, the drug has a significant steady concentration
in the mother's blood.  This would be a situation where the timing of the
dose probably wouldn't make much difference.  I don't have a reference,
but I can try to get one if you need.  Alicia.  [log in to unmask]