International unity

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The international unit (abbreviation IE or IE ; English international unit , IU or U ) is a measure of the amount of a medically used substance measured by means of a biological or biochemical test . The measured variable is based on the biological activity of the substance, which can be measured reproducibly using reference standards .

The international unit is used as a measure of biologically active and complex substances such as hormones , antibiotics , vaccines , blood products , immunological preparations and some vitamins . Different forms or preparations of one and the same substance are therefore comparable with regard to their biological effects.

The international unit is not part of the international system of units (SI units) , nor is it a physical unit in this sense. The relationship between international units and mass or international units and amount of substance is different for each substance . For many substances these ratios are chosen arbitrarily.


Using the example of antimicrobial agents, the use of biological activity instead of the amount of a substance to measure its potency can be explained: The first antibiotic agents (especially penicillins ) were isolates from fungal cultures, some of which had very different effects with the same amount of substance . However, since the effectiveness of a certain substance was the decisive factor for its use in therapy, microbiological valuation methods were introduced.

In some cases, due to improved chemical analysis methods, the mass data prevail over the specification of a biological activity, or both are used.

Reference standards

Reference standard preparations for active substances whose activity is defined by a microbiological content determination can be found in the “WHO International Biological Reference Preparations” of the World Health Organization . There are also standards at the European Directorate for the Quality of Medicines (EDQM).


  • Hormones
1 IU human insulin ≙ 34.7  µ g
  • Antibiotics
940 IU amphotericin B 1 mg
  • Vitamins
1 IU of vitamin A ≙ 0.3 µg retinol ≙ 0.6 µg beta-carotene
1 IU vitamin D 3 ≙ 0.025 µg vitamin D 3 ≙ 65.0 p mol vitamin D 3
1 IU vitamin E 910 µg DL -α-tocopherol 670 µg D -α-tocopherol ≙ 1 mg DL -α-tocopherol acetate


The international unit corresponds to the international unit (IU) in English and the unité internationale (UI) in French . Due to the ambiguity of the term unit or the symbol U , confusion may arise, for example with the enzyme unit (U).

Individual evidence

  1. ^ Dictionary of Medicine , p. 349. dtv, 1994.
  2. a b International units or milligrams? Deutsche Apotheker Zeitung 2008, No. 50, p. 58, December 11, 2008.
  3. Insulin, human . European Pharmacopoeia , 8th Edition, 6th Supplement, Monograph 0838.
  4. Vitamin conversion tables of the GDCh (PDF)

Web links