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

Molecular Weight of the Drug

The molecular weight (MW) of a medication is a significant factor in determining the entry of a medication into human milk (Hale, 2002). The semi-permeable lipid membrane of the mammary epithelium contains small pores that allow medications with a low molecular weight (i.e., less than 200 kilodaltons) to traverse easily into breast milk. Conversely, drugs that have high molecular weights can only enter human milk by being dissolved in the lipid membrane, which can significantly reduce milk concentrations. Thus, the lower a drug's molecular weight, the more easily it passes into mother's milk. Drugs with molecular weights greater than 200, such as protein medications (e.g., heparin, insulin), pass through at such low concentrations that they are virtually excluded from human breast milk and are of little threat to a breastfeeding infant (Banta-Wright, 1997; Hale, 2002). Therefore, if possible, it is recommended that medications with higher molecular weights be given to lactating mothers (Hale, 2002).


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