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

Oral Bioavailability to the Infant

Oral bioavailability refers to the ability of a drug to reach the systemic circulation after an oral dosage. It is generally a good indication of the amount of medication that is absorbed into the bloodstream of the patient (Hale, 2002). Drugs with low oral bioavailability are generally either poorly absorbed in the gastrointestinal (GI) tract, or they are partially metabolized by the liver prior to entering maternal plasma. While available oral bioavailability values typically refer to adults, they can be useful in estimating whether a mother, or perhaps an infant, will absorb enough drug to provide clinically significant plasma concentrations to produce adverse consequences (Hale, 2002).

Once a drug enters mother's milk and is ingested by the infant, it must traverse through the infant's highly acidic GI tract prior to absorption. Many drugs, including aminoglycosides, omeprazole, and large peptide drugs, such as heparin or insulin, are quickly denatured in that environment (Hale, 2002). Drugs destroyed in the GI tract are not bioavailable to the infant—effectively negating their presence in breast milk (Auerbach, 1999). It is also important to recognize that the gastric emptying time of breastfed infants is short enough to conceivably reduce the exposure time of a drug that is not destroyed in the infant's GI tract, thus diminishing the impact of a potentially adverse drug on the infant (Auerbach, 1999). Finally, many drugs first pass through the liver and are partially metabolized, substantially decreasing the chance that they will ever actually be absorbed into the infant's plasma (Hale, 2002).

The frequency of feedings and volume of breast milk the infant ingests must be considered when determining bioavailability. The infant who is exclusively breastfed for long periods of time (e.g., eight or more times a day) is more likely to be exposed to maternal medications than is a child who breastfeeds only once or twice a day and takes supplemental nutrition such as solids and juices. Calculation of an infant's dose is often made on the assumption of exclusive breastfeeding (Riordan & Auerbach, 1998). This provides an estimate of the maximum likely medication dose per kilogram that the infant would ingest via human milk. Usually, the actual dose the infant receives is much lower and the hazard is exaggerated in the case of an infant who receives supplemental nutrition (Riordan & Auerbach, 1998).


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