Forcible ejection of the stomach contents from the mouth, usually following nausea .
Causes include illness, motion sickness , certain drugs, inner ear disorders, and head injury. Vomiting may occur without nausea (e.g., after extreme exertion). Two centres in the brain's medulla oblongata are believed to control it; the vomiting centre initiates and controls a series of muscle contractions beginning at the small intestine and moving through the stomach and esophagus. This reaction may be set off by the chemoreceptor trigger zone, stimulated by many toxins and drugs, to rid the body of them, or by stimuli from various parts of the body that may be stressed or diseased. Severe vomiting may cause dehydration, malnutrition, or esophageal wall rupture. Vomiting of blood may be a sign of bleeding ulcer or other upper digestive tract disorders. See also bulimia .