Form and Function

For many years women were told that they should not breastfeed while taking medications. But today, with consideration for the risks of not breastfeeding as well as information provided by the study of transfer of medications from mother to baby, most women are able to safely combine medications and breastfeeding.

There are multiple factors that determine infant exposure to medications through breastmilk. Some of the factors that occur at the site of the milk production cells are described below:

1. Mother's stage of lactation. In the early days of breastfeeding, the specialized epithelial cells that produce milk (i.e. lactocytes) are small and unjoined, allowing free intercellular transfer of medications from the mother's plasma to the milk. However, because total amounts of available milk are small during this time, total transfer to the baby is lessened. Between 3 and 5 days postpartum, as milk production increases, the lactocytes increase in size and form tight junctions, and all further transfer must occur transcellularly, primarily through passive diffusion.

2. Maternal plasma concentration of the drug. As maternal plasma concentration increases, the drug moves through the lactocyte and into the milk along its concentration gradient, with maximum transfer occurring when levels peak in the maternal plasma. As the mother's body metabolizes the medication and plasma concentration levels fall below milk concentration levels, many drugs will pass back into the mother's blood.

3. Molecular weight of the drug. Transcellular diffusion requires that the drug compound pass through the cell membrane of the lactocyte, a process that is more difficult for larger molecules or molecules that are not protein bound and therefore less lipid soluble. At molecular weights greater than 500 daltons, diffusion through the cell becomes more difficult, and drugs with molecular weights greater than 1000 daltons do not pass in clinically significant amounts. One very common medication, insulin, falls into this category.

Resources

Thomas Hale, Chapter 5: Drug Therapy and Breastfeeding in Breastfeeding and Human Lactation, edited by Jan Riordan.