International Unit
Meaning of International Unit: A measure of what a substance does rather than how much it weighs, agreed internationally so different products can be compared.
Definition of International Unit
An International Unit is a measure of the biological activity of a substance rather than its mass. It is defined by international agreement against a reference preparation held for that purpose, so one unit means the same amount of effect wherever it is measured. Vitamins, hormones, enzymes, vaccines and blood products are commonly expressed this way. The weight corresponding to one unit differs for every substance.
Why not simply weigh the substance?
Weighing works when a preparation is pure and its effect follows directly from its mass. Many biological products fail both conditions. A hormone extracted from tissue, an enzyme preparation or a vaccine contains a mixture, and what matters is what the preparation does. Activity can also be lost during storage while the weight stays unchanged. Measuring effect against a reference standard sidesteps these problems, which is why a biological drug is far more likely to be dosed in units than a simple chemical is.
Does one unit weigh the same for everything?
| Substance | What a unit reflects |
|---|---|
| Insulin | Effect on blood sugar in a standard test |
| Heparin | Effect on clotting time |
| Vitamin D | Activity against a reference preparation |
| Vitamin A | Activity, which differs between forms |
The answer is no. Each substance has its own conversion, and the figures are unrelated to one another. This is why a unit of one product tells nothing about a unit of another.
Where does confusion arise?
Several places. Supplement labels may state units, milligrams or micrograms, and converting between them requires the specific factor for that vitamin and sometimes for the particular chemical form, since natural and synthetic vitamin E differ in activity per unit of weight. Insulin is a recognized hazard, because concentrated preparations deliver more units in the same volume, so measuring by volume rather than by units can produce a large error. The abbreviation IU is also easily misread as IV or as the digit ten when handwritten, which is why writing the word in full is standard practice on prescriptions.
Also known as
Used in a sentence
The dose was recorded in International Units rather than milligrams.
Good to know
The mass equivalent of one International Unit is specific to each substance, so units of different products are not comparable and cannot be converted by a shared factor.