The main purpose of this paper is to clarify the reasons for such a great international variety of actions by presenting some general rules on what happens to veterinary drugs administered to food-producing animals and emphasizing factors that determine concentrations of drug residues in milk and meat. Here, the term "residue" usually refers to the concentration of an active pharmaceutical ingredient in a food product, assuming that the substance is not present at a harmful level. Furthermore, the term "bioactive excretion" represents an important aspect of various drug residues, mainly related to the presence of pharmacologically active substances in animal excreta and their influence on the environment and non-target species.