Why do we measure insulin in units?

When I first got diabetes, I found it weird that we don’t deal in milliliters or milligrams as we do with most other drugs. What was the story with the insulin unit? A unit of what? As it turns out, we’re talking about rabbit units. 

When Banting and Best were experimenting with insulin in the 1920’s they tested the substance on rabbits. They soon started to express the potency of insulin in rabbit units. Normal blood sugar level of a fasting rabbit weighing 2 kg stood at 0.10 percent, and one unit of insulin was the amount required to lower the blood sugar to 0.045 percent or convulsion level in the rabbit when injected. Lab-tests showed that rabbits on a high-carbohydrate diet were more resistant to insulin than those on a low-carbohydrate diet. 

We also used mice, where one mouse unit was the amount of insulin that when injected caused hypoglycemia in half of the mice tested. The mouse unit is estimated to have one six hundredths the value of the rabbit unit so it’s really tiny.

From here we moved on to clinical units, as opposed to the original rabbit units, but the unit nomenclature still stuck through the years.