The evidence base to support prescribing of antiemetics in advanced cancer and other palliative conditions is not well developed. Much of the existing knowledge relates to chemotherapy induced nausea and vomiting, which involves a well-defined 'emetic pathway'. The physical causes in palliative care patients are likely to be more complex.
The main classes of antiemetics are:
- Dopamine antagonists (eg, metoclopramide, haloperidol, domperidone, levomepromazine, other antipsychotics)
- Antihistamines (eg, cyclizine, promethazine)
- Serotonin (5HT3) antagonists (eg, ondansetron, tropisetron, granisetron)
- Other agents (eg, anticholinergics, steroids, neurokinin-1 antagonists, benzodiazepines for anticipatory nausea).
Choosing antiemetics based on the presumed mechanism of the symptoms (55kb pdf) and the inferred neurotransmitter pathways thought to be involved in each mechanism, is a standard approach in palliative care. The effectiveness of this approach has been investigated in several small studies.  However the lack of strong evidence regarding the underlying assumptions means that antiemetic choices are still, in practice, derived mainly from expert opinion. 
There is some evidence that metoclopramide is effective in advanced cancer, and it is the best studied antiemetic in this setting.  It had a 75% response rate when used for chronic nausea thought to be due to gastroparesis. 
There is evidence to support the effectiveness of serotonin antagonists in palliative care and these medications remain the main treatment for nausea and vomiting associated with radiotherapy. 
Many antiemetics are commonly in use off-license in palliative care, such as:
- Cyclizine (an antihistamine available via the Special Access Scheme)
- Corticosteroids 
- Methotrimeprazine / levomepromazine (a broad spectrum antipsychotic available via the Special Access Scheme).
They are likely to be effective, although the research supporting their use is not extensive. 
There is moderate quality evidence that olanzipine may prevent nausea in patients receiving chemotherapy, but it’s use in palliative care is under studied. 
- Antiemetics that can be given by the subcutaneous route if nausea or vomiting are uncontrolled are used in palliative care for practical reasons.
- Prochlorperazine is not as frequently used because it is unsuitable for the subcutaneous route.
- Inferences about the main mechanism of nausea can be used to guide choice of antiemetic according to receptor profile in most instances.