What's UI stand for?

UI stands for Unit? International and is the established usage in most of the world excluding North America, where the alternative units of IU (international unit) are used. The units are however interchangable.

What are they?

UI refers to the activity of a substance. The substance is typically a vitamin or a drug, and the activity refers to the actual effectiveness of the substance. Each substance has an internationally agreed biological effect which relates to a dose of 1 UI.

How to calculate using UI

Most substances are shipped marked with a value of UI, IU, or Units per mg. For example a vial of collagenase may be marked 1601 U/mg, implying that 1601 units are present in each milligramme. Should one require 100,000 units, then it is a straight forward matter of ratios to find that 62.46mg are required.

Word of warning

Not all substances have a definate mass equivalent to 1 UI, due to variation between preparations of the substances, for example the enzyme collagenase may vary greatly in the mass quantity required to produce the same effect.

Selected References

How Many? A Dictionary of Units of Measurement