Headache with blackness in front of the eyes may be hypoglycemia. In hypoglycemia, patients can experience lack of oxygen in the brain, and in severe cases, patients can have headache, nausea, vomiting, and also blackness in front of the eyes. Some patients can show discomfort such as palpitations, sweating, chest tightness and shortness of breath. Blood glucose should be checked. If blood glucose is lower than 3.9mmol/L, it is generally called hypoglycemia clinically and should be corrected. If necessary, oral sugar cubes, sugar saline or, if available, intravenous push of hypertonic glucose can be given. Intravenous push hypertonic glucose can correct hypoglycemia quickly. If the patient is severely hypoglycemic, it may cause hypoglycemic encephalopathy, and the patient may suffer from memory loss and inattention, which may even endanger the patient’s life. In addition, it may also be caused by insufficient blood supply to the brain. Insufficient blood supply to the brain can occur in young people or in middle-aged or elderly people, and patients can be caused by a variety of reasons, such as severe hypovolemia, which may be caused by severe hypotension. It can also be caused by prolonged mental tension, stress, and lack of sleep.