Hello Jean,

The excerpts below are from the website of the InfantRisk Center at Texas Tech University School of Medicine, where Thomas Hale, PhD RPh serves as professor of pediatrics and associate dean of research at Texas Tech University Health Sciences Center, as well as acting executive director of the InfantRisk Center.  The lactation and infant feeding community know Dr. Hale best as the author of Hale's Medications & Mothers' Milk: A Manual of Lactational Pharmacology.  We often refer to this invaluable manual simply as Hale, as in "What does Hale say?" 

"The amount of drug excreted into milk depends on a number of kinetic factors:  1) the lipid solubility of the drug, 2) the molecular size of the drug, 3) the blood level attained in the maternal circulation, 
4) protein binding in the maternal circulation, 5) oral bioavailability in the infant and the mother, and 6) the half-life in the maternal and infant's plasma compartments.

"Once a drug has entered the mother's milk and has been ingested by the infant, it must traverse through the infant's GI tract and be absorbed. The term 'Oral Bioavailability' refers to the amount of a drug that actually reaches the circulation of the individual. While many drugs have poor oral absorption kinetics and are poorly absorbed into the infant's bloodstream, others are destroyed in the acidic milieu of the stomach, while others are rapidly extracted by the liver and fail to ever reach the infant's circulation. Although we understand oral bioavailability in adults reasonably well, our understanding of the specifics of oral absorption in infants is somewhat rudimentary. Still, many drugs that are poorly bioavailable in adults, are similarly so in infants. Oral bioavailability is a useful tool to estimate just how much of the drug will be absorbed by the mother or infant. In general, the infant's stomach is quite acidic and can denature many drugs. Further, some drugs are poorly absorbed when ingested with calcium-rich foods and particularly milk. In addition, many drugs are sequestered in the liver and never actually reach the plasma compartment where they are active. These absorption problems tend ultimately to reduce the overall effect of many drugs. Classic illustrations of poor oral bioavailability include the aminoglycoside antibiotic Gentamycin, the third-generation cephalosporins such as Rocephin, and morphine. There are certainly exceptions to this rule, and one must always be aware that the action of a drug in the GI tract can also be profound such as with the antibiotics, producing diarrhea, constipation, and sometimes syndromes such as pseudomembranous colitis with the overgrowth of the bacteria C. Difficile. Although there are many exceptions, a good rule of thumb is that far less than 1% of the maternal dose of a drug will ultimately find its way into the milk and subsequently the infant (although there are wide variations)."

With warm regards from the cool mountains of West Virginia,

Debbie

Debra Swank, RN BSN IBCLC
Program Director
More Than Reflexes Education
Elkins, West Virginia USA
http://www.MoreThanReflexes.org

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