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Ondansetron blocks a receptor called 5-HT3 in the gut and brain. It is used to prevent nausea and vomiting from chemotherapy, radiation, anaesthesia and some surgeries.
What are the common side effects?
Headache, tiredness, and constipation or diarrhoea can occur.
What is the heart rhythm warning?
Ondansetron can affect heart rhythm. The risk is higher with older age, a family history, low potassium or magnesium, or some other medicines. Seek care for fainting or a fast or skipping heartbeat.
Who should tell their doctor first?
Tell your doctor if you have long QT syndrome, electrolyte problems, heart failure, a slow heartbeat, a recent heart attack, or recent gut surgery. Also tell your doctor if you are pregnant or breastfeeding. Some dissolving tablets contain phenylalanine.
Can it interact with other medicines?
Yes. Do not combine ondansetron with apomorphine because serious low blood pressure and loss of consciousness can occur. Medicines affecting heart rhythm and serotonergic medicines, including some antidepressants, triptans and tramadol, also need checking by your prescriber or pharmacist.
When should I get urgent help?
Get urgent help for trouble breathing, swelling of the face or throat, chest pain, or signs of serotonin syndrome, such as agitation, confusion, fast heartbeat, muscle twitching, sweating or seizures.
Doses and formulations vary. Follow your prescriber, pharmacist and the leaflet supplied with your exact medicine. In Australia, call 000 for a medical emergency or the Poisons Information Centre on 13 11 26 for suspected poisoning or overdose. Outside Australia, use your local emergency and poison-information services.
Source: MedlinePlus: Ondansetron and DailyMed: ondansetron prescribing information. General education does not replace individual medical advice. Updated November 2026.
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