Partitioning of drugs into human milk

Breast fed infants ingest drugs present in the milk. All drugs pass from plasma into milk to some extent. The milk to plasma concentration ratio (M/P) is the most commonly quoted index of drug distribution into milk. This ratio is determined by many factors such as maternal plasma protein binding, the protein binding in the milk, solubility of drug in lipid and physicochemical factors affecting diffusion of drug through biological barriers. Most drugs have M/P ratios less than 1. The M/P ratio is used to calculate the likely infant dose from the likely maternal plasma concentration. The infant dose can then be compared with maternal doses, or with doses used therapeutically in infants. The plasma clearance of the drug by the infant compared with that of the mother will determine the mean concentrations finally achieved in the infant's plasma. Assessment of these factors enables decisions to be made regarding the safety of breast feeding during maternal ingestion of drugs.