A 5-HT3 anti-sickness medicine
Ondansetron
A powerful anti-sickness medicine for nausea from chemotherapy, surgery or severe pregnancy sickness — not intended for ordinary travel sickness.
What is Ondansetron?
Ondansetron is a strong anti-sickness, or antiemetic, medicine of the 5-HT3 antagonist type. It is used for the kinds of nausea and vomiting that simpler remedies cannot control, such as after surgery or during chemotherapy.
Education and reference only. This is a plain-language guide to Ondansetron — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Ondansetron is a strong anti-sickness (antiemetic) medicine of the "5-HT3 antagonist" type. It is used for the kinds of nausea and vomiting that simpler remedies struggle with: sickness caused by chemotherapy and radiotherapy, sickness after operations, and — under specialist guidance — severe pregnancy sickness (hyperemesis) that is not controlled by safer first-choice options. It is not designed for everyday motion sickness or mild tummy upsets, where other medicines are more suitable.
How it works
Vomiting is partly triggered by the chemical messenger serotonin (5-HT) acting on 5-HT3 receptors in the gut and in the brain's vomiting centre. Treatments like chemotherapy and surgery release a surge of serotonin that sets off this reflex. Ondansetron blocks these 5-HT3 receptors, breaking the signal that drives the sickness — which is why it is so effective for these specific, serotonin-driven causes of nausea, but less useful for nausea driven by other pathways such as motion sickness.
Company & origin
Originated / developed by: Glaxo (now GlaxoSmithKline).
Ondansetron, the first selective 5-HT3 receptor antagonist antiemetic, was developed by Glaxo (UK, now GSK) in the 1980s and first approved around 1990-1991, marketed as Zofran.
Practical use
How to take Ondansetron
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Taken as prescribed, often before or around the time sickness is expected, with or without food.
- Available as tablets, including melt-in-the-mouth forms, which can help when swallowing is difficult.
- Tell your doctor if you become constipated, which is a common effect.
- Mention any heart rhythm problems or other medicines, as ondansetron can affect heart rhythm.
- Tell your doctor if you are or might be pregnant.
- Use it only for the type of sickness it has been prescribed for.
Weighing it up
Advantages & disadvantages of Ondansetron
Advantages
- Effective for nausea and vomiting that milder treatments cannot control.
- Available in melt-in-the-mouth forms useful when vomiting makes swallowing hard.
- Generally well tolerated for short-term use.
Disadvantages
- Commonly causes constipation and sometimes headache.
- Can affect heart rhythm, so caution is needed in people with certain heart conditions.
- Interacts with some other medicines that also affect heart rhythm.
- Used to relieve symptoms rather than treat the underlying cause of the sickness.
Practical use
Good to know
It is usually taken before chemotherapy or surgery and continued for a short while afterwards, rather than long-term. The most predictable side effect is constipation, so keeping well hydrated and taking in fibre helps. It can affect the heart's electrical rhythm (QT prolongation), so it is used carefully in people with heart-rhythm conditions or low blood salts. In pregnancy it is reserved for severe sickness under specialist advice, after safer options, with the small possible risks discussed. Orodispersible (melt-in-the-mouth) forms can be useful when swallowing makes the sickness worse.
Who should not take it / use with caution
- People with a heart-rhythm condition that lengthens the QT interval (including congenital long-QT syndrome), as ondansetron can add to this.
- Used with caution in people with low blood potassium or magnesium, significant heart disease, or those taking other medicines that affect heart rhythm.
- In early pregnancy it is used cautiously and only when needed for severe sickness, under specialist guidance, after first-choice anti-sickness options.
Monitoring
- Heart rhythm and blood salts (potassium and magnesium) in those at risk
- Bowel habit — watching for constipation
- How well the sickness is controlled
Side effects
- Constipation is the most common and predictable effect.
- Headache and flushing or a sensation of warmth.
- Less commonly, changes to the heart's electrical rhythm (QT prolongation), which is why heart rhythm and blood salts are considered; rarely, allergic reactions.
Key interactions
- Other medicines that lengthen the QT interval (some antibiotics, antipsychotics and antidepressants) add to the heart-rhythm risk.
- Medicines or conditions that lower blood potassium or magnesium increase that risk further.
- Care with certain chemotherapy drugs and with tramadol, where the combined effect on serotonin and rhythm is considered.
Available as: Tablets, orodispersible tablets that melt on the tongue, an oral liquid, and an injection used in hospital.
Answers
Ondansetron: frequently asked questions
Can I use ondansetron for travel sickness?
It is not the right choice for ordinary motion sickness. Ondansetron blocks a serotonin pathway that drives chemotherapy, post-operative and severe pregnancy sickness, but motion sickness works through a different pathway, so other medicines (such as antihistamines or hyoscine) are far more suitable for travel.
Why does it make me constipated?
Slowing the gut's serotonin signalling, which is how ondansetron stops sickness, also slows bowel movement — so constipation is its most predictable side effect. Drinking plenty of fluids, eating fibre and staying active help; if it becomes troublesome, your pharmacist can suggest a gentle laxative.
Is it safe in pregnancy?
It is generally reserved for severe pregnancy sickness (hyperemesis) that hasn't responded to safer first-choice medicines, and is used under specialist guidance after weighing the small possible risks. If you are pregnant and struggling with sickness, your maternity team can advise on the safest stepwise approach.
I have a heart condition — is ondansetron okay?
It can affect the heart's electrical rhythm (QT prolongation), so it is used carefully if you have a heart-rhythm condition, low blood salts, or take other medicines that affect rhythm. Tell your prescriber about any heart problems so they can decide whether it suits you and monitor appropriately.
What is the difference between ondansetron and Zofran?
They are the same medicine — ondansetron is the generic (active-ingredient) name and Zofran is the brand. Generic ondansetron contains the identical active ingredient and works the same way; it is usually cheaper.
The wider class
About Anti-sickness medicines
Ondansetron belongs to the anti-sickness medicines class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
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Authoritative sources
- BNF: Ondansetron.
- electronic Medicines Compendium (SmPC): Ondansetron (Zofran).
- NICE: Nausea and vomiting in pregnancy.
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