IU stands for international unit, which is a unit of measurement for the effect or biological activity of a substance, for the purpose of easier comparison across similar forms of substances. International units are used to quantify vitamins, hormones, some medications, vaccines, blood products, and similar biologically active substances. Many biological agents exist in different forms or preparations, and the goal of the IU is to be able to compare these so that different forms or preparations with the same biological effect will contain the same number of IUs. To define the IU for a substance, an international collaborative study is organized by the WHO Expert Committee on Biological Standardization using various assay systems in several laboratories. The main goal of the study is to reach a consensus regarding methods of analysis and the approach to standardization so that results can be compared. The IU measurement is an international standard that you can see all over the world, and it is based on other international medication standards, so you’ll be able to find consistent information from country to country.