Vomiting, expulsion of the contents of the stomach through the mouth. Vomiting is often preceded by nausea, sweating, and excessive salivation, although it can occur without warning. It is controlled by a specific part of the brain stem, called the vomiting center, that can be stimulated in several ways. Most commonly, the center is excited by nerve impulses sent from the gastrointestinal tract (the mouth, esophagus, stomach, and intestines) when any part of the tract is overly distended (swollen), irritated, or excited. Nerve impulses may also come from the balancing mechanism of the inner ear. The vomiting center then sends impulses to the abdominal muscles involved in vomiting. The muscles of the abdomen contract and the diaphragm (the muscular partition between the chest and abdominal cavities) pushes downward. These contractions compress the stomach, raising the internal pressure. The esophageal sphincter (muscle between the stomach and the esophagus) then relaxes, forcing up the contents of the stomach.

Vomiting associated with irritation of the gastrointestinal tract can occur as the result of improper eating; food poisoning; stomach irritations brought about by chemicals, drugs, or excessive amounts of alcohol; gastrointestinal obstructions; and many infectious diseases. It also may occur during pregnancy, usually from the 5th or 6th week through the 12th week, and is most likely caused by fluctuating hormone levels.

Vomiting sometimes occurs because the vomiting center has been excited by impulses stimulated by stomach-turning sights or odors, by particular drugs and anesthesia, or by the motion of a ship, car, or airplane (see Motion Sickness).