Ondansetron is an anti-sickness medicine used to prevent nausea and vomiting caused by chemotherapy, radiotherapy and surgery. It belongs to the 5-HT3 receptor antagonist group, and the main cautions are an effect on the heart rhythm (QT prolongation) and use in early pregnancy.
An NHS prescribing sheet says low potassium and magnesium are corrected before use. A prescriber considers heart rhythm risk when other QT-prolonging medicines are used.
Legal status differs between countries and can change; each row cites the document it was taken from and applies only to the country named.
What it is used for
Preventing nausea and vomiting caused by chemotherapy or radiotherapy, with age limits that differ between the two uses.
Preventing nausea and vomiting after surgery.
How it works in the body
Ondansetron belongs to a group called serotonin 5-HT3 receptor antagonists, which interfere with serotonin, a natural substance that can cause nausea and vomiting.
Common side effects
Headache is very common with ondansetron, according to a UK leaflet.
Constipation and flushing or a feeling of warmth are also listed as common.
Hiccups and low blood pressure are listed as uncommon effects.
Serious: seek help
Allergic reactions and sudden chest pain or chest tightness are among the serious effects listed in a UK leaflet.
In an emergency the route to care is 999 or A&E; for an urgent concern that is not an emergency it is NHS 111 or a same-day GP appointment, and for routine questions a GP appointment or a pharmacist.
Cautions and interactions
A UK leaflet says ondansetron must not be used with apomorphine (a Parkinson's disease medicine) or by anyone allergic to it.
The solution leaflet rules out use in the first three months of pregnancy.
The MHRA has warned of a small increased risk of cleft lip or palate after use in the first 12 weeks of pregnancy, and says women should be counselled on the benefits and risks if it is offered.
An NHS Specialist Pharmacy Service page says ondansetron can be used with caution during breastfeeding, with monitoring of the baby. UK manufacturer leaflets, by contrast, advise against breastfeeding during treatment.
Leaflets advise caution in people with a bowel blockage or severe constipation, heart failure, heart rhythm problems, liver problems or abnormal blood levels of potassium, sodium or magnesium.
An NHS prescribing sheet says ondansetron lengthens the QT interval (a measure of the heart's electrical cycle) in a dose-related way, and that torsade de pointes, a dangerous heart rhythm, has been reported.
The same sheet advises avoiding it in congenital long QT syndrome, and caution with heart failure, slow heart rates, other QT-prolonging medicines, and low potassium or magnesium, which are corrected before use.
The sheet also warns of serotonin syndrome when ondansetron is combined with serotonergic medicines such as SSRIs and SNRIs.
Interactions
Check interactions
Taking Ondansetron with other medicines? Add them to the checker to see what the official sources say about each pair: the mechanism, general management and monitoring, never a bare "safe".
Interactions documented in the official sources cited on each card. The list covers the medicines represented on this site and is not exhaustive; how the interaction data is compiled.
Azithromycin with ondansetron, another medicine that prolongs the QT interval, increases the risk of dangerous heart rhythms.
Why it can occur
QT prolongation
What official information says
US information advises telling a doctor about medicines for irregular heartbeat and other listed drugs, whose doses may need changing or monitoring; the BNF advises caution or avoidance depending on the drug.
What to discuss with a clinician
Official information: an alternative antibiotic where possible; otherwise ECG and electrolyte monitoring.
A UK tablet leaflet lists two licensed uses for ondansetron: preventing nausea and vomiting caused by chemotherapy or radiotherapy, and preventing it after surgery. MedlinePlus gives the same two groups of uses for adults taking the tablets. Great Ormond Street Hospital describes it as a medicine that stops a child feeling sick and being sick. It may be given by mouth as a tablet, melt or syrup, or as an injection into a vein.
How does ondansetron work?
Serotonin is a natural substance that can trigger nausea and vomiting. Ondansetron interferes with serotonin, which can trigger nausea and vomiting, by acting on 5-HT3 receptors. Medicines in this group are called 5-HT3 receptor antagonists. Great Ormond Street Hospital notes that some medicines can react with ondansetron and change how well it works, so a doctor or pharmacist is asked about other medicines first.
Side effects and when to get help
Headache is listed as very common, and constipation and flushing as common. Allergic reactions and sudden chest pain or chest tightness are among the serious effects listed in a UK leaflet. These effects need prompt medical assessment: the route to care is 999 for an emergency, or NHS 111 for urgent advice that is not an emergency.
Heart rhythm, pregnancy and other cautions
Ondansetron lengthens the QT interval in a dose-related way, according to an NHS prescribing sheet. The sheet advises avoiding it in congenital long QT syndrome and taking care with low potassium or magnesium, heart failure, slow heart rates and other QT-prolonging medicines. The MHRA has reported a small increased risk of cleft lip or palate with use in the first 12 weeks of pregnancy. Apomorphine must not be used with ondansetron, according to a UK leaflet.
Frequently asked questions
Is ondansetron available without a prescription?
No. Ondansetron is a prescription-only medicine in the UK; the emc lists its oral solution as POM.
Can ondansetron affect the heart?
Yes. An NHS prescribing sheet says it lengthens the QT interval in a dose-related way. Torsade de pointes, a dangerous heart rhythm, has been reported after the medicine came into use.
Can ondansetron be used while breastfeeding?
An NHS Specialist Pharmacy Service page says it can be used with caution during breastfeeding, with monitoring of the baby. UK manufacturer leaflets, by contrast, advise against breastfeeding during treatment.
Connected pages in detail
What each linked page on Anatomy Nexus covers, in a line, for Ondansetron.
Symptoms
NauseaThe unpleasant sensation of being about to vomit.
VomitingForceful emptying of the stomach through the mouth, a protective reflex run by the brainstem that closes the airway.
Drug classes
5-HT3 receptor antagonistsAnti-sickness medicines that block 5-HT3 receptors; ondansetron is the member covered on this site.
Educational content. Anatomy Nexus provides medical education and does not replace professional medical advice, diagnosis, treatment, prescribing or pharmacist review. Doses, choices and monitoring are decided by a prescriber for an individual; never start, stop or change a medicine on the basis of this page.
