Levels which are significantly above or below this range are problematic and can in some cases be dangerous. A level of <3.8 mmol/L (<70 mg/dL) is usually described as a hypoglycemic attack (low blood sugar). Most diabetics know when they are going to "go hypo" and usually are able to eat some food or drink something sweet to raise levels. A patient who is hyperglycemic (high glucose) can also become temporarily hypoglycemic, under certain conditions (e.g. not eating regularly, or after strenuous exercise, followed by fatigue). Intensive efforts to achieve blood sugar levels close to normal have been shown to triple the risk of the most severe form of hypoglycemia, in which the patient requires assistance from by-standers in order to treat the episode.[8] In the United States, there were annually 48,500 hospitalizations for diabetic hypoglycemia and 13,100 for diabetic hypoglycemia resulting in coma in the period 1989 to 1991, before intensive blood sugar control was as widely recommended as today.[9] One study found that hospital admissions for diabetic hypoglycemia increased by 50% from 1990–1993 to 1997–2000, as strict blood sugar control efforts became more common.[10] Among intensively controlled type 1 diabetics, 55% of episodes of severe hypoglycemia occur during sleep, and 6% of all deaths in diabetics under the age of 40 are from nocturnal hypoglycemia in the so-called 'dead-in-bed syndrome,' while National Institute of Health statistics show that 2% to 4% of all deaths in diabetics are from hypoglycemia.[11] In children and adolescents following intensive blood sugar control, 21% of hypoglycemic episodes occurred without explanation.[12] In addition to the deaths caused by diabetic hypoglycemia, periods of severe low blood sugar can also cause permanent brain damage.[13] Although diabetic nerve disease is usually associated with hyperglycemia, hypoglycemia as well can initiate or worsen neuropathy in diabetics intensively struggling to reduce their hyperglycemia.[14]

Normally, blood glucose levels are tightly controlled by insulin, a hormone produced by the pancreas. Insulin lowers the blood glucose level. When the blood glucose elevates (for example, after eating food), insulin is released from the pancreas. This release of insulin promotes the uptake of glucose into body cells. In patients with diabetes, the absence of insufficient production of or lack of response to insulin causes hyperglycemia. Diabetes is a chronic medical condition, meaning that although it can be controlled, it lasts a lifetime.