In pharmacology, the International Unit is a unit of measurement for the amount of a substance, based on biological activity or effect. It is abbreviated as IU, as UI (Spanish unidad internacional or French unité internationale or Italian unità internazionale), or as IE (German Internationale Einheit, Danish International Enhed, Swedish Internationell Enhet). It is used to quantify vitamins, hormones, some medications, vaccines, blood products, and similar biologically active substances.
Many biological agents exist in different forms or preparations (e.g. vitamin A in the form of retinol or beta-carotene). 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 do so, the WHO Expert Committee on Biological Standardization provides a reference preparation of the agent, arbitrarily sets the number of IUs contained in that preparation, and specifies a biological procedure to compare other preparations of the same agent to the reference preparation. Since the number of IUs contained in a new substance is arbitrarily set, there is no equivalence between IU measurements of different biological agents. For instance, one IU of vitamin E cannot be equated with one IU of vitamin A in any way, including mass or efficacy.
Despite its name, IU is not part of the International System of Units used in physics and chemistry. The IU should not be confused with the enzyme unit, also known as the International unit of enzyme activity and abbreviated as U.
In order to remove the possibility of having the letter "I" confused with the digit "1", some hospitals have it as a stated policy to omit the "I", that is, to only use U or E when talking and writing about dosages, while other hospitals require the word "Units" to be written out entirely.
Read more about International Unit: Equality and Equivalency of IU For Different Substances, Mass Equivalents of 1 IU
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