Antiemetics

Pharmacology

The antiemetics are used to treat nausea and vomiting (N&V). If antiemetic drug treatment is indicated, drug choice is guided by the aetiology of nausea and vomiting.

The mechanism of action of some of the commonly used antiemetics is summarised in the table below:

Antiemetic	Mode of Action
Cyclizine	Histamine H1-receptor antagonist
Promethazine	Histamine H1-receptor antagonist
Ondansetron	Serotonin 5-HT3 receptor antagonist
Prochlorperazine	Dopamine D2 receptor antagonist
Metoclopramide	Dopamine D2 receptor antagonist 5-HT3 receptor antagonist 5-HT4 receptor agonist

Antiemetics may be grouped into classes. These are shown below with type examples and common indications:

Class	Examples	Conditions	Notes
Antihistamines	Cinnarizine Cyclizine Promethazine	Several conditions e.g. motion sickness, Post-op N&V caused by opioids or GA	Adverse effects, e.g. drowsiness and antimuscarinic effects, differ between antihistamines
Phenothiazines	Chlorpromazine Prochlorperazine Trifluoperazine	Chemotherapy-induced N&V Radiation-induced N&V Post-op N&V caused by opioids or GA	Act centrally by blocking the chemoreceptor trigger zone. Severe dystonic reactions sometimes occur (especially in children). Prochlorperazine available as a buccal tablet.
Other antipsychotic	Haloperidol Levomepromazine	N&V in palliative care
Antiemetic & prokinetic	Metoclopramide	Superior to phenothiazines for emesis associated with gastro-intestinal and biliary disease	Associated with acute dystonic reactions Limit prescribing to no longer than 5 days
Dopamine antagonist	Domperidone	Parkinson’s disease	Acts at the chemoreceptor trigger zone. Less likely to cause central effects as does not readily cross the blood-brain barrier
5HT3-receptor antagonists	Granisetron Ondansetron Palonosetron	N&V in patients receiving cytotoxics	Common side effects include: constipation feeling hot headache abnormal sensation