Does Formula Feeding Decrease Prescription Drug Risks While Breastfeeding?

I was reading a blog about pharmacology in nursing practice. The blog was hosted at the University of Texas at Arlington School of Nursing. The post I was reading asked why one would want to know if a mother is breastfeeding when prescribing drugs. The comments got me thinking.

The comments consisted of an interchange between what I assumed to be a nursing student and a teacher. The student rightly stated that the drug could have a negative pharmacological effect in the baby or decrease the milk’s nutritional value. The teacher agreed with her and asked what questions she would ask a breastfeeding mom in a clinical setting. Here are the questions she would ask:

1. How old is the baby?
2. How much does the baby weigh?
3. Does the baby have any other preexisting conditions?
4. Is the baby breastfed exclusively or is formula given as well?

It was the last question that got my attention, and got me thinking. What difference would that make?

The student’s justification for asking that question seemed reasonable at first. “If a baby is on breast milk and formula, their exposure to the drug is decreased, and this may be a factor in prescribing drugs.” The teacher agreed with her and told her that her question about whether the baby was exclusively breastfed or supplemented with formula is “the most important question” to ask the mom for that reason.

Again I asked myself, what difference does that make? After second thought, the students question did not seem reasonable at all to me. Assuming the mother said she was not exclusively breastfeeding, to what measurable extent is the baby’s exposure to the drug actually decreased by formula supplementation, and how exactly can that be measured? If it can’t be measured or known with certainty, it is just an untested assumption, and the answer to the question is virtually useless.