International Units is an internationally accepted measure defined by the International Conference for Unification of Formulae 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 in food and supplement packaging, but it is also applied to biologicals, such as vaccines, enzymes, and hormones. The WHO sample is used to assess other preparations of the substance to ensure consistency and serve as a standard for establishing dosages.
The abbreviation for International Unit is I.U. 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, a cup of one item and a cup of another item to be an identical amount. International units, however, refer to the biological activity that results from administration from 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 of IU to milligrams or any other measure of mass. Furthermore, 2 IU of a vaccine from a particular manufacturer may have a different mass than 2 IU from another manufacturer, because there could, for example, be other ingredients that do not figure in the biological potency.
The USP unit — the unit of the United States Pharmacopeia — is similar, expressing the potency of drugs, rather than their amount. USP units are usually identical to International Units. The lack of such a unit of measure on many natural supplements is one of the factors that leads to warnings that dosages may not be equivalent on different batches.
Some substances that were once specified by IU measurements have now undergone a more thorough chemical analysis that has allowed a better understanding of their properties. In such cases, the measure of biological potency has been able to be replaced with a weight, or both may be used. For example, rather than simply specifying IU, the U.S. Institute of Medicine of the National Academy of Sciences recommends 5 micrograms or 200 IU of Vitamin D for everyone under age 50.