IBCLC Practice Exam 2025 – Complete Preparation Guide

The most influential factor in determining whether a drug passes into breast milk is protein binding. Drugs that are highly protein-bound tend to have a lower concentration in breast milk because they are largely attached to plasma proteins in the mother's bloodstream and are less available to pass into breast milk. Conversely, drugs that have low protein binding are more likely to be present in higher concentrations in breast milk, as they can more readily diffuse into the milk.

While other factors such as fat solubility, half-life, and pH do play a role in the overall distribution and availability of drugs, it is the degree of protein binding that has the most significant impact on the extent to which a drug can enter breast milk. For example, lipophilic (fat-soluble) drugs may accumulate in breast milk, but if those drugs are also highly protein-bound, their actual transfer into milk will still be limited. Understanding protein binding is vital for lactation consultants and healthcare providers when considering medication use in breastfeeding mothers, ensuring the safety of the infant while providing necessary treatments to the mother.