Hypoglycemia (hypoglycaemia in British English) is the medical term for a pathologic state produced by a lower than normal level of glucose (sugar) in the blood. The term hypoglycemia literally means "under-sweet blood" (Gr. hypo-, glykys, haima). Hypoglycemia can produce a variety of symptoms and effects but the principal problems arise from an inadequate supply of glucose as fuel to the brain, resulting in impairment of function (neuroglycopenia). Derangements of function can range from vaguely "feeling bad" to coma and (rarely) permanent brain damage or death. Hypoglycemia can arise from many causes and can occur at any age.

The most common forms of moderate and severe hypoglycemia occur as a complication of treatment of diabetes mellitus with insulin or certain oral medications. Hypoglycemia is usually treated by the ingestion or administration of dextrose, or foods digestible to glucose.

Endocrinologists (specialists in disorders of glucose metabolism) typically consider the following criteria (referred to as Whipple's triad) as proving that an individual's symptoms can be attributed to hypoglycemia:

1. Symptoms known to be caused by hypoglycemia
2. Low glucose at the time the symptoms occur
3. Reversal or improvement of symptoms or problems when the glucose is restored to normal

However, not everyone has accepted these suggested diagnostic criteria, and even the level of glucose low enough to define hypoglycemia has been a source of controversy in several contexts. For many purposes, plasma glucose levels below 70 mg/dl or 3.9 mmol/L are considered hypoglycemic; these issues are detailed below.