Diabetes mellitus is a disease of the pancreas (an organ behind your stomach). Normally, the pancreas releases a hormone called insulin that helps your body store and use the sugar and fat from the food you eat. Diabetes occurs when the pancreas does not produce any insulin, or the pancreas produces very little insulin, or when the body does not respond appropriately to insulin, a condition called "insulin resistance."

To understand why insulin is important, it helps to know more about how the body uses food for energy. Your body is made up of millions of cells. To make energy, these cells need food in a very simple form. When you eat or drink, much of your food is broken down into a simple sugar called glucose. Glucose ("sugar") provides the energy your body needs for daily activities. Your bloodstream transports glucose both from where it is taken into the body after eating (the intestines) and where it is manufactured (in the liver) to the cells where it will be used (muscles, brain, etc.) or stored (in the liver) or converted to fat (also in the liver). When the amount of glucose in your blood reaches a certain level, your pancreas releases insulin. The insulin carries the glucose into the appropriate cells. As more glucose enters your cells, the level of glucose in your bloodstream drops. Without insulin, the glucose can't be stored - which allows the level of glucose in the blood to rise. Too much glucose in the blood is called "high blood sugar." By definition, diabetes is having a blood glucose level of 126 milligrams per deciliter (mg/dL) or more after an overnight fast (not eating anything).

Insulin is a hormone that controls blood glucose. It is injected instead of taken in pill form because when insulin is taken by mouth the acids in your stomach destroy most of it. Injecting insulin under the skin bypasses your stomach and allows it to stay in your body for different lengths of time, depending on the type of insulin used. Insulin comes in different forms. It can be injected using a syringe, cartridge or prefilled pen systems and in a recently developed needleless injection system.