In pharmacology, the International Unit is a unit of measurement for the amount of a substance, based on measured biological activity or effect. "International Unit" is abbreviated as IU, or as UI from the Frenchunité internationale, or as IE from the GermanInternationale Einheit.
The precise definition of one IU differs from substance to substance and is established by international agreement for each substance. There is no equivalence among different substances; for instance, one IU of vitamin E does not contain the same number of milligrams as one IU of vitamin A.
To define an IU of a substance, the Committee on Biological Standardization of the World Health Organization provides a reference preparation of the substance, arbitrarily sets the number of IUs contained in that preparation, and specifies a biological procedure to compare other preparations of that substance to the reference preparation. The goal in setting the standard is that different preparations with the same biological effect will contain the same number of IUs.
For some substances, the equivalent mass of one IU is later established. If that happens, the former IU definition for that substance is officially abandoned, in favor of a newly established weight. However, the unit count often remains in use nevertheless, because it is convenient. For example, vitamin E exists in a number of different forms, all having different biological activities. Rather than specifying the precise type and mass of vitamin E in a preparation, for the purposes of pharmacology it is sufficient, simply, to specify the number of IUs of vitamin E.
The mass equivalents of 1 IU for selected substances are:
Insulin: 1 IU is the biological equivalent of about 45.5 μg pure crystalline insulin (1/22 mg exactly)
