Prescribing Antiemetics

Introduction

Nausea and vomiting are common symptoms that have many causes, such as adverse effects of drugs, motion sickness and raised intracranial pressure. Although vomiting can be a protective reflex to remove substances that are harmful to the body, prescribing antiemetics is extremely useful.

The main classes of antiemetics include antagonists of the dopamine, muscarinic, histamine, serotonin and neurokinin systems, corticosteroids, benzodiazepines and medications for specific indications.

Causes of nausea and vomiting

The two main areas of the brain associated with the nausea and vomiting reflex are the chemoreceptor trigger zone (CTZ) in the base of the 4th ventricle, and the vomiting centre in the medulla.

The vomiting centre is responsible for most of the coordination of nausea and vomiting and receives input from the nucleus tractus solitarium and vagus nerve. It can be directly stimulated by stimuli or indirectly by the CTZ.

Visceral stimulation is one way the vomiting centre can be stimulated. This is mostly through afferent fibres in the gastrointestinal tract (mechanical and chemosensory receptors). These are triggered by direct visceral irritation or distension, and the impulse is conducted by the vagus nerve. The neurotransmitters involved are serotonin (5HT) and dopamine. This is the mechanism for nausea and vomiting from illnesses such as gastroenteritis.

Gastric stasis, radiotherapy and chemotherapy stimulate 5HT3, 5HT4 and D2 receptors within the peripheral gastrointestinal tract, as well as chemoreceptors and mechanoreceptors within the liver and gastrointestinal tract. This directly stimulates the vomiting centre and also indirectly via CTZ.

The vomiting centre can also be stimulated by inputs from other neurological centres, for example through different cranial nerves. The vestibular component of CN VIII is commonly implicated in nausea. There can often be a psychological component to nausea and common triggers that act via this mechanism include emotions, smells, vertigo, taste and anticipatory nausea (for example, following repeat cycles of chemotherapy). Common neurotransmitters involved in this pathway are acetylcholine, histamine and serotonin.

Anxiety and raised intracranial pressure interact with GABA and H1 receptors within the cerebral cortex which directly stimulates the vomiting centre. 

The CTZ lies outside the blood-brain barrier and is therefore exposed to more “systemic” causes of nausea and vomiting, such as medication or metabolic disturbances. It can be stimulated directly by hypercalcaemia or opioids in the blood or by peripheral receptors in the gastrointestinal tract or vestibular input. Common neurotransmitters involved in this are serotonin and dopamine.

After the vomiting centre is stimulated, vomiting may occur by the following steps:

• A deep breath is taken
• Relaxation of the upper oesophageal sphincter
• Closure of the glottis and elevation of the soft palate
• Contraction of the diaphragm and abdominal wall muscles
• Relaxation of the lower oesophageal sphincter

Prescribing antiemetics

Key prescribing points for antiemetic drugs:²

• When prescribing, always follow basic prescribing principles.
• Ask for allergies and other medications being taken.
• Why are they nauseated? The aetiology of nausea and vomiting guides the choice of anti-emetic.
• Any other medical conditions that will influence drug choice? For example, Parkinson’s disease or pregnancy.
• Describe what route is suitable for the patient and document on the prescription.
• Document a clinical review and decision date and rationale.

Route

The route of antiemetics is important in practice to achieve optimum therapeutic effect. For example, if someone is actively vomiting, then an oral antiemetic is unlikely to be effective and an alternative route should be considered.

The available routes for specific drugs can be found on the BNF and include oral (PO), intravenous (IV), intramuscular (IM), subcutaneous (SC), rectal (PR), and buccal.

Choice of antiemetic

While, in theory, any antiemetic can be prescribed for any cause of nausea and vomiting, certain classes will be more effective for some causes than others.

When considering indications for the classes of antiemetic medication in this article, the most common situations in which each medication is used will be listed.

Antihistamines

Examples of antihistamine antiemetics include cyclizine, promethazine hydrochloride, and doxylamine with pyridoxine.

Common indications

Antihistamines are generally used for nausea and vomiting caused by:

• Vertigo
• Labyrinthine disorders
• Meniere’s disease
• Pregnancy
• Motion sickness
• Post-operative nausea and vomiting

Cyclizine is also used to treat nausea and vomiting in bowel obstruction and in raised intracranial pressure (for example, in patients with brain metastases).

Contraindications

Cyclizine is contraindicated in heart failure as it can worsen cardiac oedema.

Antihistamine antiemetics should be used with caution in people with a higher risk of urinary retention, such as benign prostatic hypertrophy or bladder outlet obstruction due to the central anticholinergic effects. 

Mechanism of action

Antihistamines work on H1 receptors, which directly act on the vestibular system and the CTZ.

Doxylamine with pyridoxine is officially licenced for pregnancy-induced nausea and vomiting. Doxylamine is an antihistamine which selectively binds H1 receptors at the CTZ. Pyridoxine is vitamin B6 and is a water-soluble vitamin which is given as there could be a deficiency in pregnancy, which may be a factor in the cause of nausea and vomiting during pregnancy. 

Side effects

Antihistamines are usually well tolerated; however, the most common side effect is drowsiness, which is important to explain to patients. This can cause reduced coordination, reaction speed, and judgement if they drive or operate heavy machinery.

Anticholinergic side effects such as urinary retention, constipation, and blurred vision are common.

There is an increased antimuscarinic and sedative effect when cyclizine is given with tricyclic antidepressants and monoamine oxidase inhibitors, so this should be avoided if possible.

Phenothiazines and related drugs

Examples of phenothiazines are metoclopramide and prochlorperazine (can be given as buccal).

Other antipsychotics which are commonly used as antiemetics include haloperidol (unlicensed use), levomepromazine and domperidone.

Common indications

Phenothiazines and antipsychotics are often used for:

• Chemotherapy-induced nausea and vomiting
• Radiotherapy-induced nausea and vomiting
• Prevention and treatment of acute nausea and vomiting
• Labyrinthine disorders
• Migraine-related nausea and vomiting
• Nausea and vomiting in palliative care

Metoclopramide specifically can be used in patients with delayed gastric emptying or constipation.

Levomepromazine and haloperidol are often used for nausea and vomiting associated with hypercalcaemia, non-chemotherapy drugs and metabolic causes, e.g. hyponatraemia.

Contraindications

Metoclopramide is contraindicated in bowel obstruction due to its prokinetic effect as it can increase the risk of bowel perforation. It can be useful to confirm with patients that they passed flatus or faeces recently, and if still unsure, then an abdominal x-ray may be indicated.

This class of antiemetics should also be avoided in Parkinson’s disease, as the mechanism of action can worsen Parkinsonian symptoms.

Domperidone should not be used in patients with disorders of cardiac conduction, arrhythmias and congestive cardiac failure, as there is a small risk of cardiac side effects.

Mechanism of action

These are dopamine (D2) antagonists and work on D2 receptors within the CTZ.

Levomepromazine is a broad-spectrum anti-emetic working at multiple receptor sites, including dopamine type 2 (D2) antagonist, H1, 5HT-2 receptors and serotonergic sites. This makes it useful in situations such as palliative care as it can cover many aetiologies of nausea and vomiting and works secondarily as a sedative.

Domperidone is less likely to cause central effects, such as sedation and dystonic reactions, than other phenothiazines because it does not readily cross the blood-brain barrier.

Side effects

Due to their action at dopamine (D2) receptors, movement disorders can be a side effect of this class of antiemetics. This can precipitate acute dystonic reactions even in patients without underlying dopaminergic disorders like Parkinson’s disease. This side effect means that where possible, metoclopramide should only be prescribed for short periods of time.

Other common side effects include constipation, drowsiness, dry mouth, galactorrhoea and gynaecomastia, hyperglycaemia, urinary retention, and weight gain.

Levomepromazine can cause a prolonged QT interval so you should be cautious prescribing this alongside other medications that can affect the QT interval such as citalopram, fluconazole and sotalol.

5HT3 receptor antagonists

Examples of 5HT3 antagonists are ondansetron and granisetron.

Common indications

5HT3 receptor antagonists are commonly used in:

• Patients receiving chemotherapy
• Post-operative patients
• Patients with Parkinson’s disease: due to other drug classes being associated with worsening of Parkinson’s symptoms such as bradykinesia, shuffling gate and tremor.
• Acute gastroenteritis

Contraindications

Due to the specific side effect of QT prolongation, these antiemetics should be avoided with anyone at risk of prolonged QT.

There has also been shown to be a “small increased risk of oral clefts following use in the first 12 weeks of pregnancy (January 2020)”⁹

Mechanism of action

These work on 5HT3 receptors found in the vomiting centre within the medulla and the CTZ, blockade of these receptors suppresses the vomiting reflex.

Side effects

The side effects of 5HT3 antagonists are constipation, prolonged QT interval, headache, and abnormal sensation.

Antiemetics in specific circumstances

Parkinson’s disease^10,11

Consider using low-dose domperidone (does not cross the blood-brain barrier), cyclizine or ondansetron.

Metoclopramide and prochlorperazine are contraindicated due to the risk of exacerbation of parkinsonism.

Pregnancy^12,13

The only drug officially licenced currently, for nausea and vomiting in pregnancy >18 years old is doxylamine and pyridoxine. However, prochlorperazine, promethazine, metoclopramide, cyclizine, ondansetron, and chlorpromazine have established use in practice.

Meniere’s disease⁶

Antihistamines and phenothiazines are recommended due to the vestibular disruption.

Motion sickness

Hyoscine hydrobromide (an anti-muscarinic) is licensed to prevent motion sickness-induced nausea, vomiting and vertigo, and less sedating antihistamines such as cyclizine can also be used.

Post-operative nausea and vomiting (PONV)^6,14

Antiemetic therapy after surgical procedures is an important part of enhanced recovery programs (ERPs) and helps prevent and manage postoperative ileus.

5HT3 receptor antagonists such as ondansetron are often used first-line. Histamine (H1) receptor antagonists and dopamine (D2) receptor antagonists can also be used.

Within the GI tract, 5-HT3 receptors are found on macrophages. During gastrointestinal (GI) surgery, inflammatory cells infiltrate when the viscera is mobilised, reducing intestinal motility and leading to postoperative ileus. Ondansetron, which is a 5-HT3 receptor antagonist, helps improve GI motility by preventing inflammatory cells from infiltrating.

Palliative care¹⁵

It is more likely to see medications for end-of-life symptoms, including nausea and vomiting, to be given through continuous subcutaneous infusion as patients may no longer be able to manage medications orally, and this prevents having to put the patient through multiple injections and needles. 

Levomepromazine is a typical antipsychotic drug that acts by mainly blocking dopamine type 2 (D₂) receptors in the brain. Adverse effects include sedation, which can help in palliative care to ease agitation. This should be used with caution in people with cardiovascular disease, therefore an echocardiogram may be required.

Summary

Table 1. Summary of commonly prescribed antiemetics.

Reviewer

Dr Amir Guirguis

Editor

Dr Jess Speller

References

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