Medications and Lactation: What PNPs Need to Know

Jennifer M. Marks, BS; Diane L. Spatz, PhD, RNC


J Pediatr Health Care. 2003;17(6) 

In This Article

Protein-Binding of the Drug in Maternal Plasma

The foremost parameter that determines the degree of drug penetration into breast milk is plasma protein-binding (Hale, 2002). Drugs circulate in maternal plasma either bound to the plasma protein, albumin, or remain freely soluble in the plasma. The protein-bound fraction stays in the maternal circulation, while the medication fraction that is unbound can be transferred into human milk. Since more than half of maternal plasma protein is albumin, high protein-binding effectively restricts many drugs to maternal plasma and prevents their transfer into breast milk (Banta-Wright, 1997). The higher the percentage of protein-binding, the less likely the drug is to enter maternal milk (Hale, 2002). Thus, highly protein-bound drugs (e.g., warfarin) are excreted into the breast milk in low amounts and this reduces the possibility of the infant's exposure to the medication (Hale, 2002). A drug is considered significantly protein-bound if its binding to protein is greater than 90% (Hale, 2002).


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