Fainting is caused by a drop in blood flow to the brain leading to reduction of oxygen. Among the old and diabetic, sometimes the heart rate and blood vessels can't react fast enough when the body's need for oxygen changes. Other causes include:standing up too fast, working or playing hard when very hot, breathing too fast, getting very upset, some high blood pressure medicine, coughing, urinating and stretching, drop in blood sugar when diabetic, alcohol, marijuana, seizures and heart or blood vessel problems.
Attribution: Straight Talk Foundation