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Anti-emetics (anti sickness)

These are drugs used to treat nausea and vomiting. They work in a number of different ways but there is considerable overlap in their effects and many drugs work on a number of areas both in the gut and the central nervous system.

Domperidone and Metoclopramide

Are both dopamine receptor antagonists which aim to increase gastric peristalsis and thence gastric emptying. Because they work by antagonising dopamine receptors they may cause side-effects relating to their antagonism of these receptors, these include fatigue, dyskinesia and parkinsonism. For this reason their longterm use is discouraged. Domperidone is available by mouth or by suppository, metoclopramide can be given subcutaneously, by intra-venously, by intra muscular injection or by mouth. Both drugs are used in patients with nausea and vomiting where delayed gastric emptying is thought to be a factor. They may also be used in gastro-paresis.


This is a drug which works through its effects on dopamine. It is used for nausea caused by a range of disease, but it is also used in the treatment of schizophrenia. It may cause significant side effects including low blood pressure, postural hypotension and dyskinesia, as well as neuroleptic malignant syndrome, a serious but rare neurological condition.


Mirtazapine is an antihistamine, which also works on serotonin receptors though exactly how it effects nausea is unclear. It is also used to treat depression. Of note it does appear to increase appetite so weight gain is a potential side-effect as are drowsiness, fatigue and mental clouding.



Cyclizine is an anticholinergic and antihistamine that is best used for treating nausea induced by motion sickness and vertigo. It has also been used for nausea in a variety of other settings including post-anaesthesia and in nausea related to drugs and functional gastro-intestinal disease. As well as its effects on the balance centre it may also have other effects on the central processing of nausea. Interestingly it tends to delay gastro-intestinal transit.



Ondansetron is a 5HT3receptor antagonist and appears to work on the chemo-receptor trigger zone in the medulla (part of the brain). Certain stimuli appear to promote the release of serotonin from the enterochromaffin cells in the gut stimulating a response mediated via the vagal nerve which leads to stimulation of the chemoreceptor trigger zone and thence the vomiting reflex. Whilst it is often effective for chemotherapy and drug induced nausea it seems, perhaps not surprisingly given it’s postulated mechanism of action to have little effect on nausea related to vertigo or motion sickness. It may also promote gastric (stomach) emptying, although like many other ant-emetics one of the commonest side effects is constipation. Ondansetron is metabolised in the liver so doses are sometimes reduced in patients with liver disease.


This is a synthetic cannabinoid (derived from cannabis), not widely available in the UK. It is used in Canada and the US and also as an adjunct in the management of chronic pain. It’s principle side effects are drowsiness and altered mental state.

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