Nausea and vomiting and antiemetics

If a patient presents with nausea and vomiting, a clear cause should be established before perscribing antiemetics, as nausea and vomiting may be a sign of serious illness such as diabetic ketoacidosis, and may be reversed by treating the cause, for example in digoxin overdose. The cause of nausea and vomiting may be apparent with history and examination.

patients with persistent nausea and vomiting should also be assessed for their fluid status, and for any electrolyte abnormalities caused by the vomiting.

Causes of nausea and vomiting

Nausea and vomiting may be caused centrally through stimulation of the vomiting center and the chemoreceptor trigger zone in the medulla, or caused afferent signals due to irritation in other areas of the body, usually in the gastrointestinal tract, but also due to disease in the central nervous system or the vestibular system in the ear. 

There are several types of antiemetics, some of which act more centrally and some which act more peripherally.

Group	Action	Suitable for
Antihistamines Cyclizine Cinnarizine Meclizine Promethazine	Antagonises H1 receptors	More effective against nausea and vomiting due to vestibular causes or peripheral causes such as irritants in the Gastrointestinal tract. Promethazine is first line in severe morning sickness.
Muscarinic antagonists hyoscine	Antagonises muscarinic receptors	Mostly used for motion sickness
Serotonin (5-HT3) antagonists Odansetron	Acts both peripherally and centrally	Useful in postoperative nausea and vomiting and nausea and vomiting due to chemotherapy.
Dopamine antagonists Metoclopamide Domperidone	Acts centrally but also has an effect on the gastrointestinal tract.	Metoclopamide may be used for nausea and vomiting associated with the GI system but has many side effects. Domperidone  also acts on the chemoreceptor trigger zone, but does not cross the blood brain barrier as easily and has less side effects. It may be used to treat GI symptoms as well as nausea and vomiting due to cytotoxic therapy.
Antipsychotics Chlorpromazine Perphenazine Prochlorperazine Trifluoperazine Haloperidol	Dopamine antagonists, acts on the chemoreceptor trigger zone.	Used in diffuse metastatic disease, radiation sickness, and nausea and vomiting caused by drugs. Haloperidol and levomepromazine is used in palliative care.
Steroids Dexamethasone	Mode of action not known	Often used synergistically with dopamine antagonists or 5-HT3 antagonists, but has some antiemetic effects by itself.

There are different guidelines for which antiemetics to use for specific scenarios.

http://www.patient.co.uk/doctor/persistent-nausea-or-vomiting
http://www.medscape.org/viewarticle/559665
BNF september 2012
Rang and Dale pharmacology
Churchill's pocketbooks differential diagnosis.