INTRODUCTION: Medication use is highly prevalent in breastfeeding persons, posing potential risks for drug exposure to nursing infants. Transporters in the lactating mammary gland carry pharmacological and toxicological significance, as they can mediate the active transfer of drugs and nutrients into breastmilk. AREAS COVERED: In this narrative review, we searched and compiled current knowledge on the transport of drugs in the human mammary gland from literature indexed in PubMed (current as of 25 October 2024), and clinical evidence demonstrating active transport of drugs into milk is provided. In vitro and in vivo models of the mammary gland are outlined in brief and known drug transporters at the blood-milk barrier and their potential relevance to drug concentrations in milk are described in detail. EXPERT OPINION: Although clinical data show that membrane transporters mediate the transfer of multiple drugs into breast milk, our ability to predict milk concentrations for these drugs is limited. Improving our understanding of the transporter biology and pharmacology in the mammary gland is crucial for developing models to predict drug concentrations in human milk, which will support clinicians and lactating individuals in making rational decisions to balance the benefits of breastfeeding and the risks of drug exposure to infants.