At present a specific group of medicines - highly variable medicines - is distinguished based on intraindividual variability data (CVintra > 30%). It is quite difficult to confirm therapeutic equivalence of highly variable medicines by pharmacokinetic bioequivalence studies, and quite a large number of subjects need to be included into the study in order to confirm bioequivalence within standard limits of 80-125%. Variability may be caused by many factors which include physiological and pathophysiological differences in absorption and metabolism processes; factors associated with properties of the active substance and factors associated with the finished product. In general factors impacting variability in bioequivalence studies could be divided into controllable and uncontrollable. The influence of controllable factors can be neutralized by proper performance of the bioequivalence study or proper development of the finished product. The influence of uncontrollable factors cannot be neutralized, and due to these factors medicines can be recognized as highly variable. This article provides a definition of a highly variable medicine, describes reasons for high variability and outlines current regulatory recommendations for and approaches to studying bioequivalence of highly variable medicines, proposes recommendations for designing such studies
