A standardised measure of the biological activity of certain vitamins, hormones, and other biologically active substances, abbreviated IU. One international unit represents the amount of a substance that produces a defined biological effect, allowing consistent dosing regardless of the preparation’s chemical form or purity. The measure is commonly used for vitamins A, D, and E on supplement labels and in clinical nutrition. Because different substances have different definitions of the unit, values cannot be compared across nutrients: 1 IU of vitamin D is not equivalent to 1 IU of vitamin A. Regulatory bodies are gradually replacing international units with metric mass units such as micrograms on labels.