Insulin is a hormone that lowers the level of glucose (a type of sugar) in the blood by helping glucose enter the body's cells. Doctors use this hormone to treat diabetes when the body can't make enough insulin on its own. Although researchers are testing other ways to give insulin, it's only available now in a form that must be injected just under the skin. There are many types of injectable insulin, both short- and long-acting. Most people with diabetes take insulin injections based on their blood glucose levels, according to a plan that they've worked out with their doctor.