Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
In pharmacology, the International unit (IU, alternatively abbreviated UI, from French unité internationale) is a unit of measurement for the amount of a substance, based on measured biological activity (or effect). It is used for vitamins, hormones, some drugs, 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. 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 to the reference preparation. The goal here 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, and the IU is then officially abandoned for that substance. However, the unit 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 to simply specify the number of IUs of vitamin E.
The mass equivalents of 1 IU for selected substances:
- 1 IU Insulin: the biological equivalent of about 45.5 μg pure crystalline insulin (1/22 mg exactly)
- 1 IU Vitamin A: the biological equivalent of 0.3 μg retinol, or of 0.6 μg beta-carotene
- 1 IU Vitamin C: 50 μg L-ascorbic acid
- 1 IU Vitamin D: the biological equivalent of 0.025 μg cholecalciferol/ergocalciferol
- 1 IU Vitamin E: the biological equivalent of about 0.667 mg d-alpha-tocopherol (2/3 mg exactly), or of 1 mg of dl-alpha-tocopherol acetate
The IU should not be confused with the enzyme unit, which is also known as the "International unit of enzyme activity" and is abbreviated as U.
However IU is equivalent to IE the symbol for "international einheit" the German name for the international unit.
de:Internationale Einheit id:Satuan internasional nl:Internationale Eenheid nn:Internasjonal eining sr:Интернационална јединица