Various units of biological potency used in pharmacology. Symbol, IU. For example, a dose of 200 IU of an antibiotic has a certain amount of bacteria-fighting ability, but it might have a mass of 20 milligrams from one manufacturer and 40 from another. The size of the unit varies with the substance and is presently set by the Expert Committee on Biological Standardization of the World Health Organization (WHO).
An International Unit is standardized by defining a test method and providing a reference standard, an actual preparation of the substance having a known activity. Suppose, for example, one wants to determine how many IUs are in a given amount of a preparation of an antibiotic, and that WHO says one gram of the reference standard for that antibiotic has an activity of 500 IU. The amount of the preparation being tested that inhibits the growth of bacteria in the test to the same extent as one gram of the reference standard will contain 500 IU of the antibiotic. The assay is biological, not chemical.
In the United States, the reference standards are produced and released under the authority of the United States Pharmacopeia Convention. USP Units and the International Units of potency are usually identical.
Measuring the size of a dose in terms of its biological effect rather than as a specific weight of pure substance solved a number of problems: sometimes it wasn't known exactly which chemical had the effect; sometimes the preparation (for example, material produced by fermentation) contained a number of chemicals each of which produced the effect but in varying degrees; sometimes the amount was so small it could not be assayed quantitatively by the methods of the day.
In the 1950s quantitative analysis of pharmacologicals became much more precise and sensitive. With new methods it was possible to detect and determine the actual mass of chemicals in very small concentrations. Once the substance responsible for an effect has been isolated, identified, and prepared in a form that allows it to be completely characterized by chemical and physical properties, the biological assay is no longer needed and the International Unit for that substance is generally discontinued. For a decade or more stating the actual mass has been preferred to using IU; for example, they are no longer used in the U.S. Pharmacopeia. Some examples of old international units and their modern equivalents are:
One IU of vitamin A = 0.3 micrograms of retinol or 0.6 micrograms of beta-carotene. Quantities of vitamin A are also measured in “retinol equivalents.” One retinol equivalent = 3.33 IU of retinol, or 10 IU of beta-carotene.
One IU of vitamin E = 0.91 milligrams of synthetic dl-alpha-tocopherol, or about 0.67 milligrams of d-alpha-tocopherol.
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Last revised: 18 May 2004.