Antiemetics are used to treat nausea and vomiting. Vomiting is controlled by the vomiting center in the medulla. The vomiting center is activated by either one of four trigger zones: chemoreceptor trigger zone, vestibular nuclei, cerebral cortex and gastrointestinal tract. The cerebral cortex activates vomiting from smell, thought and so on. Different classes of antiemetics drugs are classified by their different mechanism of actions. The classes include 5HT3 receptor antagonist, anticholinergic antiemetics, NK1 receptor antagonist and phenothiazine antiemetics. Antiemetics that do not fall into these classes are classified as miscellaneous antiemetics.