Hypoglycaemia is an abnormally low concentration of glucose in the blood — typically below 3.9 mmol/L — the brain’s fuel crisis. The brain cannot store glucose, so shortage announces itself fast: shakiness, sweating, pounding heart, and hunger (the autonomic alarm), progressing to confusion, slurred speech, and unconsciousness as neuroglycopenia sets in. In diabetes it is usually treatment-caused — too much insulin or sulphonylurea for the food eaten, or exercise without adjustment. Outside diabetes, causes include insulinomas, adrenal failure, and severe liver disease. Treatment is fast sugar — glucose tablets, juice — followed by longer-acting carbohydrate; unconscious patients need injected glucagon. Every insulin user must know its warning signs.