Breast cancer resistance protein (BCRP/ABCG2) is an ABC (ATP-binding cassette) transport protein which extrudes xenotoxins from the cells in the intestine, liver, mammary gland, and other organs and allows many compounds to secrete into milk by affecting their pharmacological and toxicological behavior. Today, BCRP has gained relevance in veterinary medicine as well as in the human field. Many drugs have been identified as substrates of clinically relevant ABCG2 drugs in human and rodent studies. Studies in rodents and sheep also have shown that ABCG2, which is expressed in mammary tissue, is responsible for the transfer of many compounds into milk. The transport of drugs into milk results in residue problems that may cause various health risks for consumers. The purpose of this article is to provide information about the role of BCRP/ABCG2, which is overexpressed during lactation, in the transfer of drugs into milk, ABCG2 polymorphism, and ABCG2 inhibitors.