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International Units (IU) is a measure of biological activity defined by the WHO and used for fat-soluble vitamins, biologics, and drugs. It differs from standard units of mass and may vary between manufacturers. Some substances now use weight instead of IU.
International Units is an internationally accepted measure defined by the International Conference for the Unification of Formulas based on a reference preparation of a substance provided by the World Health Organization (WHO) Expert Committee on Biological Standardization. It is most often seen as a measure of fat-soluble vitamins on food and supplement packaging, but is also applied to biologics, such as vaccines, enzymes, and hormones. The WHO sample is used to evaluate other preparations of the substance to ensure consistency and serve as a standard for establishing doses.
The abbreviation for International Unit is UI. International units differ from standard units of other types, such as kilograms, pounds, liters, or quarts. Working with these other types of measurements, one would expect, say, one cup of one item and one cup of another item to be an identical amount. However, international units refer to the biological activity that results from the administration of the substance in question.
As a result, 2 International Units of one substance may be a different amount of that substance than 2 International Units of another substance. In fact, this difference is more likely than not, and there is no set conversion from IU to milligrams or any other measurement of mass. Also, 2 IU of a vaccine from a particular manufacturer may have a different mass than 2 IU from another manufacturer, because there could be, for example, other ingredients that are not listed in the biologic potency.
The USP unit, the unit of the United States Pharmacopeia, is similar and expresses the strength of drugs, rather than their quantity. USP units are usually identical to international units. The lack of such a unit of measure in many natural supplements is one of the factors leading to warnings that dosages may not be equivalent in different batches.
Some substances that were once specified by UI measurements have now undergone a more thorough chemical analysis that has allowed for a better understanding of their properties. In such cases, the measure of biological potency may have been replaced with a weight, or both may be used. For example, rather than simply specifying IU, the US Institute of Medicine of the National Academy of Sciences recommends 5 micrograms or 200 IU of vitamin D for everyone under 50 years of age.
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