Injected insulin mimics the action of the hormone normally produced by the body. Once it is injected under the skin and into the subcutaneous (below the skin) fat layer, it starts stimulating glucose uptake by both skeletal muscle and fat cells, while at the same time signaling the liver to slow or stop glucose production. So in effect, it closes one door (the liver) while opening another (the body cells).

The first insulin, isolated by Canadians Sir Frederick Banting and Charles Best (with help from fellow researchers J. J. R. Macleod and J. B. Collip), was extracted from the pancreas of a cow. The first few decades of insulin therapy used both bovine (cow) and porcine (pig) derived insulin. Today's insulins are created in the laboratory, cultured from bacteria and yeast with a technology known as recombinant DNA.