The two most common forms of diabetes are due to either a diminished production of insulin (type 1), or diminished response by the body to insulin (type 2 and gestational). Both lead to hyperglycemia, which largely causes the acute signs of diabetes: excessive urine production, resulting compensatory thirst and increased fluid intake, blurred vision, unexplained weight loss, lethargy, and changes in energy metabolism.

All types of diabetes have been treatable since insulin became medically available in 1921, but there is no widely available cure for any form of diabetes. Type 1 diabetes can only be treated with injected insulin, with dietary and other lifestyle adjustments commonly being part of the treatment. Type 2 is usually managed with a combination of dietary treatment, tablets and, frequently, insulin supplementation. Insulin can also be delivered continuously by a specialized pump which provides subcutaneous insulin through a temporary catheter.

Diabetes and its treatments can cause many complications. Acute complications (hypoglycemia, ketoacidosis, or nonketotic hyperosmolar coma) may occur if the disease is not adequately controlled. Serious long-term complications (i.e., chronic side effects) include cardiovascular disease (doubled risk), chronic renal failure, retinal damage (which can lead to blindness), nerve damage (of several kinds), and microvascular damage, which may cause impotence and poor wound healing. Poor healing of wounds, particularly of the feet, can lead to gangrene, and possibly to amputation.

Adequate treatment of diabetes, as well as increased emphasis on blood pressure control and lifestyle factors (such as not smoking and maintaining a healthy body weight), may improve the risk profile of most of the chronic complications. In the developed world, diabetes is the most significant cause of adult blindness in the non-elderly and the leading cause of non-traumatic amputation in adults, and diabetic nephropathy is the main illness requiring renal dialysis in the United States.