 Emesis is a complex phenomenon characterized by three components: nausea, vomiting, and retching. Nausea is a subjective phenomenon of an unpleasant sensation in the epigastrium and in the back of the throat that may or may not culminate in vomiting; it is also described as feeling sick at the stomach. Nausea exists only insofar as it is defined by the patient. Vomiting is the forceful expulsion of the contents of the stomach, duodenum and jejunum through the oral cavity as a result of changes in intrathoracic positive pressure. It is also described as throwing up. Retching, also called dry heaves, is the attempt to vomit without expelling any material. 