Factors that influence the ability of a drug to translocate across biological membranes will affect its concentration in milk. Since milk is generally more acidic (pH 6.8) than plasma (pH 7.4), milk can act as an “ion trap” for weak bases and, at equilibrium, basic drugs may become more concentrated in milk than in the maternal circulation. Conversely, acidic drugs are limited in their ability to enter milk and a net transfer of the drug from milk to plasma occurs.
The 2014 amendment to the FDA’s regulations governing the content and format of labeling for human prescription drugs and biological products requires the inclusion of a “Lactation” subsection with a summary of the risks of using a drug during lactation.2,3 If data demonstrate that the drug is not absorbed systemically by the mother, it is so stated in the risk summary. If data demonstrate that the drug is absorbed systemically, the summary includes a risk statement.
The summary also provides, to the extent it is available, relevant information on the presence of the drug in human milk, effects of the drug on the breast-feeding child, and effects of the drug on milk production. The “Lactation” subsection also includes, to the extent it is available, relevant information concerning ways to minimize drug exposure in the breast-feeding child. In addition, the labeling includes data that are the basis for the risk summary.