A blood glucose level less than 50 mg/dl (3.3 mmol/L) that occurs before or between meals. Its most common cause in diabetic patients is an excessive dose of insulin or of another medication used to control high blood glucose. It may also be caused by some other medications, e.g., pentamidine or quinine, or by alcohol consumption, critical illnesses, prolonged exercise, or, rarely, insulin-secreting tumors.