Gastrointestinal
Antiemetics
Anti-sickness drugs — Drugs that prevent and treat nausea and vomiting — the right choice depends on the cause.
Education and reference only. This is a plain-language class overview — it deliberately contains no doses. Always check the current Summary of Product Characteristics (SmPC), the BNF and your local formulary before prescribing or administering any medicine.
What it is
Antiemetics control nausea and vomiting from many causes — surgery, chemotherapy, migraine, gastroenteritis, pregnancy and motion sickness. Several different classes act on different pathways.
How it works
Nausea is triggered through several receptor systems (serotonin, dopamine, histamine and others), so each antiemetic class blocks a particular pathway. Matching the drug to the likely trigger is what makes treatment effective.
In practice
In practice antiemetics are chosen by mechanism and cause rather than picked at random: a 5-HT3 blocker (e.g. ondansetron) suits chemotherapy and post-operative sickness, a dopamine antagonist (e.g. metoclopramide) suits gastric stasis, and an antihistamine suits motion sickness. Watch the QT interval with ondansetron, limit metoclopramide courses because of movement side-effects, and always treat the underlying cause alongside.
Examples
Practical use
How to take it & use it well
- Take antiemetics such as ondansetron, metoclopramide or cyclizine as prescribed for nausea and vomiting, often shortly before food or before a known trigger like travel or chemotherapy.
- Some come as tablets that dissolve on the tongue, which can help if swallowing is difficult when you feel sick.
- Use metoclopramide only for short courses, as longer use raises the risk of movement-related side effects.
- If you miss a dose and are still feeling sick, take it when you remember; if not, skip it and continue as needed.
- Tell your prescriber about your other medicines, as some antiemetics interact or are best avoided together.
- Seek advice if vomiting continues despite treatment, as you may need a different approach or fluids.
Common uses
- Post-operative and chemotherapy-induced nausea
- Nausea from gastroenteritis or migraine
- Motion sickness
Monitoring
- Response and the underlying cause
- QT interval where risk factors exist
- Movement side-effects with dopamine antagonists
Weighing it up
Advantages & disadvantages
Advantages
- They relieve nausea and vomiting from many causes, including illness, surgery, travel and cancer treatment.
- Different types work in different ways, so there is usually an option to suit the cause.
- Several come in forms that dissolve in the mouth, useful when swallowing is hard.
- They can prevent dehydration by helping you keep fluids down.
- They often act quickly to bring relief.
Disadvantages
- Ondansetron commonly causes constipation and can affect heart rhythm in some people.
- Metoclopramide can cause restlessness and movement problems, especially with longer use or in younger people.
- Cyclizine can cause drowsiness and a dry mouth.
- They treat the symptom rather than the underlying cause of the sickness.
- Some are not suitable in certain conditions or for particular age groups, so the choice needs care.
Key safety principles
What to watch for
- Ondansetron can prolong the QT interval — caution with other QT-prolonging drugs.
- Metoclopramide and prochlorperazine can cause movement disorders; metoclopramide is limited to short courses, especially in the young.
- Choose by cause and by patient factors (e.g. avoid some agents in Parkinson's disease).
Key interactions
What to avoid or check alongside
- Ondansetron with other medicines that affect heart rhythm can increase the risk of a dangerous rhythm change.
- Metoclopramide with certain mental health and Parkinson's medicines can worsen movement-related side effects.
- Cyclizine and other sedating medicines or alcohol add to drowsiness.
- Some antiemetics interact with strong painkillers and antidepressants, so a medicines check is wise.
- Combining several antiemetics without advice can increase side effects rather than help.
Patient & carer advice
- Tell us what seems to trigger the sickness
- Report any restlessness or abnormal movements
- Stay hydrated and seek help if you cannot keep fluids down
Use with
Related clinical calculators
Dose and risk decisions for this class often depend on renal function, weight or bleeding/stroke risk. These tools help:
Answers
Antiemetics: frequently asked questions
How quickly do antiemetics work?
Many start to ease nausea within an hour or so, though this varies by type and how it is given. Dissolvable tablets can be handy when swallowing is hard.
Why is metoclopramide only for short-term use?
Longer use raises the risk of movement-related side effects such as restlessness or muscle problems, so it is usually limited to short courses.
Which antiemetic is best for travel sickness?
Cyclizine is one commonly used option for motion sickness. A pharmacist can recommend the most suitable choice for your situation.
Can these medicines cause constipation?
Ondansetron in particular commonly causes constipation. Keeping hydrated and eating fibre can help, and a pharmacist can advise if it persists.
What if I am still being sick after taking it?
If vomiting continues, you cannot keep fluids down, or you feel increasingly unwell, seek medical advice, as you may need a different treatment or fluids.
Authoritative sources
Always verify against the source
This overview is for orientation. For doses, interactions, contra-indications and the full monograph, use:
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