Insulin injection. Close-up of a syringe of insulin and a child's abdomen being swabbed. Insulin is used to treat diabetes mellitus. Diabetes is a metabolic disorder in which either the production of the hormone insulin, which regulates blood glucose levels, is reduced, or the sensitivity of body tissues to insulin is reduced. This causes unused sugars to accumulate in the blood and urine. Blood glucose levels must be carefully monitored. Treatment is with a strict diet and daily insulin injections to maintain the correct blood-sugar balance.