Rules of Thumb for Estimating Drug Levels in Breast Milk: How Well Do They Work?

The American Academy of Pediatrics and the World Health Organization recommend exclusive breastfeeding for the first 6 months of life due to extensive benefits for the maternal-infant dyad. While over 90% of mothers initiate breastfeeding, continuation drops to 35% by 6 months, often due to concerns about medication safety. Clinicians often face decisions about medication use during lactation in the absence of robust data. Factors such as molecular weight (MW), lipid solubility, protein binding and a drug's acid-base status are routinely stated as the determinants of drug passage into milk and used as "rules of thumb" (e.g., MW <500 Da). However, these have not been rigorously examined for clinical applicability. This study investigates MW, protein binding, milk-to-plasma ratios (M/P), and relative infant dose (RID) with the aim of assisting clinicians' estimation of medication safety during breastfeeding. Small-molecule drugs with well-documented M/P were selected using predefined criteria. Physicochemical properties and active transporter status were compiled from multiple databases. Analyses investigated relationships between physicochemical properties, M/P, and RID. A total of 94 drugs were included in the physicochemicalM/P analyses, 91 for the M/P-RID analysis and 91 for the protein binding-RID analysis. Our study highlights the complexities of predicting drug passage into breast milk, finding no straightforward MW cutoff for passage of small molecules. These findings challenge common assumptions and emphasize the importance of considering active transport and other physicochemical properties. While protein binding >75% can serve as a preliminary guide, slow maternal drug clearance and active metabolites can decrease its utility.