thumb|250px|Multivitamins [[nutrition facts label showing that the international unit of, for example, vitamins D and E correspond to different gram values]]

In pharmacology, the international unit (IU) is a unit of measurement for the effect or biological activity of a substance, for the purpose of easier comparison across similar forms of substances. International units are used to quantify vitamins and biologics (hormones, some medications, vaccines, blood products and similar biologically active substances).

International units as used in pharmacology are not part of the International System of Units (SI). The main goal of the study is to reach a consensus regarding methods of analysis and the approach to standardization so that results can be compared.

The study is performed using highly purified preparations of the substance, typically in lyophilized form, called international reference preparations, or IRPs.

Examples

  • 1 IU of rhEGF is defined as the potency of 0.001 μg of a rhEGF in the "91/530" standard vial. One manufacturer reports that its rhEGF is 1.4 times as potent as the 91/530 standard.
  • 12.5 IU of oxytocin is defined as the potency of 21 μg of pure peptide in the "76/575" standard vial.

Before the purification of penicillin was perfected, the amounts were also described in "International Units" (and some non-international arbitrary "units" before them) to account for batch-to-batch variation. See .

For vitamins

A number of vitamins have distinct vitamers of distinct biological potencies. There is a system in place also called the international unit to describe the relative strength of these different molecular forms. Like the biological international unit, the IU for vitamins is also an arbitrary number that cannot be interconverted among different types of activities. By 1934, the WHO (then the League of Nations Health Organisation) had already defined the international units for vitamins A, B1, C, and D.

Unlike biologic preparations, small molecules like vitamins can be very easily purified to yield products that are equivalent in potency. As a result, a simple mass conversion (as opposed to an assay) suffices to obtain the IU:

  • 1 IU Vitamin A = 0.3 μg retinol (~0.1 nmol) = 0.6 μg beta-carotene