ANTIEMETICS are used to prevent vomiting. They are thus related to antinauseant drugs which are used to reduce or prevent the feeling of nausea that very often precedes the physical process of vomiting (emesis). Commonly, the terms are used synonymously, though it is usually an antinauseant action that is being sought. The type of antinauseant drugs used, and the likelihood of success, depends on the mechanism and origin of the nauseous sensation, and there are a number of ways it can be triggered. Motion sickness (travel sickness) can often be prevented by taking antinauseant drugs before travelling, e.g. the antihistamines meclozine and dimenhydrinate, and the anticholinergic hyoscine. Probably all these drugs act as central MUSCARINIC CHOLINOCEPTOR ANTAGONISTS. Similar drugs may be used to treat nausea and some other symptoms of labyrinthine disease (where the vestibular balance mechanisms of the inner ear are disturbed, e.g. in Meniere’s disease), though other antinauseant drugs may also be necessary, e.g. cinnarizine or phenothiazine derivatives such as chlorpromazine and prochlorperazine. Steroids, such as dexamethasone and methylprednisolone, are effective antiemetics that work by an undefined mechanism. In view of their marked side-effects they are for emergency use only.
Antiemetics include the prokinetic drugs, which are used to enhance the strength of oesophageal sphincter contraction, stimulate gastric emptying and facilitating small intestine transit; e.g., to help reduce the vomiting that accompanies radiotherapy and chemotherapy: e.g. cisapride, domperidone and metoclopramide (see GASTRIC MOTILITY STIMULANTS).
A number of chemicals and drugs induce nausea and vomiting by an action involving the so-called chemoreceptor trigger zone (CTZ) within the area postrema of the brain. For instance, opioid analgesic drugs, e.g. morphine, cause nausea as a very frequent side-effect, and this may be reduced by giving it in combination with an antinauseant such as cinnarizine. The nausea component that precedes the vomiting that commonly accompanies radiotherapy and chemotherapy, can be difficult to treat, though some recently developed drugs of the 5-HT3 antagonists type are proving to be valuable, e.g. granisetron, ondansetron, tropisetron (see 5-HYDROXYTRYPTAMINE RECEPTOR ANTAGONISTS). Also, the cannabis derivative nabilone may be tried in difficult cases (see CANNABINOID RECEPTOR AGONISTS).