The unit is used for vitamins, hormones, some medications, vaccines, blood products, and similar biologically active substances. Despite its name, the IU is not part of the International System of Units used in physics and chemistry.
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:
The IU should not be confused with the enzyme unit, also known as the International unit of enzyme activity and abbreviated as U.