Why is Insulin Measured in Units

Most medications are dosed based on milligrams or grams because it is a standard quantity of weight. For example, if you are on 500mg metformin tablets, you expect 500mg of metformin in each tablet.

Have you ever wondered why is insulin dosed in units? What is a unit of insulin anyways and how come it is not dosed in the conventional milligrams of milliliters? Every time you twist the dial on your insulin syringe pen, the number you see in the viewing window is neither in milligrams or milliliters, but it represents the number of international units of insulin.

Medications are already confusing with the brand and generic names, perhaps this is a way to confuse patients even more and make medicine ever more mystically ambiguous . . . rest assure that there is a reason to this madness (even though this reason is more historically based rather than logically calculated).

What is an International Unit?

To understand the units of insulin, you need to first understand what is an international unit. The international unit is a standardized way to quantify the effect of a medication. In the case of insulin, it is the standard amount required for a precise measured of activity. In plain English, one international unit of insulin the amount required to lower the blood sugar by a standard amount.

A History Lesson

Historically when insulin was discovered at the turn of the 20th century by Banting and Best, they did not have the advanced technological luxury we have today to readily measure the weight of an insulin molecule. Furthermore, insulin was available in various strengths and concentrations so it was difficult to measure the mass or volume of insulin.

Scientists in the past however were smart people and they came up with creative ways to do things. Therefore, an indirect model was used to quantify a standard amount of insulin. Like most historical scientific studies, we used animals to test a hypothesis. In the case of insulin, we used rabbits. One international unit of insulin was the amount of insulin required to lower the fasting blood sugar of a rabbit by 2.5 mmol/L. With the advanced technology nowadays, we now know one unit of insulin is equivalent to 0.0347mg of pure crystalline of insulin.

Now is it easier to dose insulin based on 0.0347mg of insulin or 1 unit?

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