Ondansetron

Pronunciation: on-DAN-seh-tron.

Generic name
ondansetron
Drug class
Serotonin 5-HT3 receptor antagonist (antiemetic)
Form
Tablet, orally disintegrating tablet, oral solution, and solution for injection, in multiple strengths depending on market and formulation
Route
Oral (tablet, orally disintegrating tablet, oral solution) or intravenous/intramuscular injection
Legal status
Prescription-only

Availability and supply rules vary by country

Trade names by country: United States · United Kingdom · Canada · Australia · New Zealand

Australia: Zofran / Zofran Tablets / Zofran Syrup (7) · Full directory

What ondansetron is and how it works

Ondansetron is a prescription medicine that blocks serotonin (5-HT3) receptors involved in the nausea/vomiting reflex. It is used to prevent and treat nausea and vomiting caused by cancer chemotherapy, radiotherapy (depending on formulation/market), and surgery. (1)

Ondansetron carries a genuine, dose-dependent risk of QT interval prolongation (a change in the heart's electrical rhythm that can, rarely, lead to a dangerous arrhythmia called torsades de pointes), which is why it is avoided in people with congenital long QT syndrome and why maximum intravenous doses are capped -- and the exact cap genuinely differs between countries. It should only be given and dosed by or under the direction of a healthcare professional, especially for the injectable form.

Mechanism of action: how ondansetron works

Ondansetron is a selective antagonist of serotonin 5-HT3 receptors, both peripherally (on vagal nerve terminals in the gut) and centrally (in the chemoreceptor trigger zone), blocking the serotonin release that triggers the vomiting reflex after emetogenic chemotherapy, radiotherapy, or surgery. (4) (1)

What ondansetron is used for

Chemotherapy-induced nausea and vomiting (CINV)

Used to prevent nausea and vomiting from moderately to highly emetogenic cancer chemotherapy, in both initial and repeat treatment courses. (1) (2)

Postoperative nausea and vomiting (PONV)

Used to prevent and/or treat nausea and vomiting after surgery. (1) (4)

Radiotherapy-induced nausea and vomiting

Used to prevent nausea and vomiting from certain types of radiotherapy (such as total body irradiation or abdominal radiotherapy). In the United States this indication applies to the oral tablet label specifically, not the injectable label; the UK SmPC groups this with the chemotherapy indication. (2) (4)

Before taking ondansetron

Do not take / not appropriate for

  • Do not take ondansetron if you have congenital long QT syndrome. (all countries) (1) (4)
  • Do not take ondansetron together with apomorphine (a risk of profound low blood pressure and loss of consciousness has been reported with this combination). (all countries) (4)
  • Do not take ondansetron if you have a known serious allergic reaction to it or to other serotonin 5-HT3 receptor antagonists. (all countries) (1)

Tell your clinician about

  • Any personal or family history of long QT syndrome, or any heart rhythm problems.
  • Any electrolyte abnormalities (especially low potassium or magnesium), heart failure, or slow heart rhythms.
  • All other medicines you take, especially other medicines that can affect heart rhythm (QT-prolonging medicines).
  • Whether you are pregnant, especially in the first trimester, planning pregnancy, or breastfeeding.
  • Any liver problems, since severe liver impairment caps the total daily dose in some markets.
(1) (4)

How to take ondansetron

  • Ondansetron is given by mouth (tablet, orally disintegrating tablet, or oral solution) or by injection, timed around chemotherapy, radiotherapy, or surgery as directed by your care team.
  • The maximum single intravenous dose genuinely differs between countries -- follow your local product label and your clinician's instructions rather than assuming one figure applies everywhere.
  • Never increase your own dose or give yourself extra doses, particularly of the injectable form, because of the genuine, dose-dependent QT-prolongation risk.
(1) (4) (7)

Procedures and treatment changes

Tell the treating team before surgery, procedures, or any medicine change. Do not alter treatment without the responsible clinician. (1) (2) (3) (4) (5) (6) (7) (8)

Missed dose

Ondansetron is typically given around a specific chemotherapy, radiotherapy, or surgical event rather than on a fixed daily schedule at home; if you are on a multi-dose oral course and miss a dose, contact your care team or pharmacist for guidance rather than doubling your next dose.

Overdose

Seek urgent medical attention for a suspected overdose. ECG monitoring is specifically recommended in cases of overdose because of the dose-dependent QT-prolongation risk. Symptoms of overdose can include vision changes, severe constipation, low blood pressure, and fainting episodes. (4)

Side effects

Side effects and their urgency vary; report severe, persistent, or concerning symptoms.

Headache.

What to notice: Headache.

What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (2)

Constipation.

What to notice: Constipation.

What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (2)

Fatigue or feeling generally unwell (malaise).

What to notice: Fatigue or feeling generally unwell (malaise).

What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (2)

Diarrhoea.

What to notice: Diarrhoea.

What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (1)

Serious warnings

Signs of a dangerous heart rhythm problem (QT prolongation / torsades de pointes) — high caution

Symptoms: Fainting, severe dizziness, a racing or irregular heartbeat, or seizures.

Action: Seek emergency medical help immediately -- ondansetron prolongs the QT interval in a dose-dependent manner, and postmarketing cases of torsades de pointes (a potentially fatal heart rhythm) have been reported. Risk is higher with higher doses, faster IV infusion, electrolyte abnormalities (low potassium or magnesium), heart failure, slow heart rhythms, or other QT-prolonging medicines. (1) (3)

Signs of serotonin syndrome (when combined with other serotonergic medicines) — high caution

Symptoms: Agitation, hallucinations, fast heartbeat, high blood pressure, dizziness, sweating, muscle twitching or rigidity, loss of coordination, nausea, vomiting, or diarrhoea.

Action: Seek urgent medical help -- ondansetron affects serotonin activity and, combined with other serotonergic medicines, has been associated with serotonin syndrome in postmarketing reports. (1)

Signs of a serious allergic reaction — emergency

Symptoms: Swelling of the face, lips, tongue, or throat; difficulty breathing; widespread rash or hives; bronchospasm.

Action: Seek emergency medical help immediately. (1)

Interactions

  • Apomorphine: do not combine -- reports of profound hypotension and loss of consciousness with concurrent use. (4)
  • Other QT-prolonging medicines increase the risk of a dangerous heart rhythm when combined with ondansetron. (1) (6)
  • Strong CYP3A4 inducers (such as rifampicin/rifampin) can reduce ondansetron blood levels. (2)
  • Other serotonergic medicines (such as SSRIs/SNRIs) may increase the risk of serotonin syndrome when combined with ondansetron. (1)

Pregnancy, breastfeeding, kidney/liver function, age

Pregnancy

The UK SmPC specifically advises that ondansetron should not be used during the first trimester of pregnancy, because of a suspected small increased risk of orofacial malformations (such as cleft lip/palate); discuss any use in pregnancy, at any stage, with your clinician. (1) (2) (3) (4) (5) (6) (7) (8)

Breastfeeding

Not recommended -- sources reviewed advise against breastfeeding while taking ondansetron. (1) (2) (3) (4) (5) (6) (7) (8)

Children

Approved for CINV prevention in children (from 6 months of age in the US injectable label, and similarly in the UK from 6 months for chemotherapy-induced nausea/vomiting), with weight/body-surface-area-based dosing; not all indications are approved in children. (1) (2) (3) (4) (5) (6) (7) (8)

Elderly

Australia's TGA-approved product information specifically caps the maximum single IV dose at 8 mg for patients 75 years and older (versus 16 mg under 75), reflecting increased QT-prolongation caution with age in that market's official dosing guidance. (1) (2) (3) (4) (5) (6) (7) (8)

Hepatic_impairment

In severe hepatic impairment, the UK SmPC caps the total daily dose at 8 mg for both CINV and PONV indications. (1) (2) (3) (4) (5) (6) (7) (8)

Renal_impairment

No specific dose adjustment is described as required for renal impairment in the sources reviewed for this dossier, though your clinician will consider your overall clinical picture. (1) (2) (3) (4) (5) (6) (7) (8)

Source_ids

UK-EMC-ONDANSETRON-TABLET-SMPC-2026,AU-TGA-ZOFRAN-PI-2026,US-DAILYMED-ZOFRAN-INJECTION-2026 (1) (2) (3) (4) (5) (6) (7) (8)

Monitoring

Before starting

  • Personal/family history of long QT syndrome or heart rhythm problems
  • Electrolyte levels (potassium, magnesium), corrected before dosing if abnormal
  • Liver function, if severe hepatic impairment is suspected

During treatment

  • ECG monitoring recommended for at-risk patients (electrolyte abnormalities, heart failure, bradyarrhythmias, other QT-prolonging medicines) and specifically recommended in cases of overdose.
  • Watch for signs of a heart rhythm problem (fainting, palpitations) or serotonin syndrome if combined with other serotonergic medicines.
(1) (4)

Storage

Store tablets/oral solution at room temperature as directed on the product label, protected from light and moisture; injectable ondansetron is typically stored and prepared by a healthcare facility. Keep out of reach of children.

Professional information

Clinician-facing context only, sourced from the current US FDA prescribing information (injection and tablet) and UK SmPC, plus Canada/Australia/New Zealand regulator sources reviewed for this dossier; verify against the current full label before prescribing.

United States

Indication: CINV (IV) / PONV (IV) / CINV, PONV, radiotherapy (oral)

Dose summary: IV: 0.15 mg/kg x3 doses, no single dose >16 mg. Oral: 8-24 mg depending on indication/emetogenicity (see dose_context).

Renal context: No specific adjustment described in sources reviewed. (1) (2)

United Kingdom / New Zealand

Indication: CINV/radiotherapy, PONV

Dose summary: Single IV dose must not exceed 8 mg; severe hepatic impairment total daily dose capped at 8 mg (UK).

Renal context: No specific adjustment described in sources reviewed. (4) (8)

Australia

Indication: CINV (IV)

Dose summary: Single IV dose up to 16 mg under age 75; capped at 8 mg from age 75.

Renal context: No specific adjustment described in sources reviewed. (7)

Mechanism of action

Hepatic guidance

Severe hepatic impairment reduces ondansetron clearance; the UK SmPC caps total daily dose at 8 mg in this population.

Overdose note

No specific antidote; manage with supportive care and ECG monitoring given the dose-dependent QT-prolongation risk. (4) (1)

Frequently asked questions

Why was the 32 mg dose of ondansetron withdrawn?

In 2012, a dedicated FDA-required study found that a single 32 mg intravenous dose of ondansetron caused a clinically significant increase in the QT interval (a measure of the heart's electrical cycle), with a mean QTcF increase of about 19.6-21.5 milliseconds versus placebo, creating a risk of a dangerous heart rhythm called torsades de pointes. The FDA required the 32 mg single-dose vial to be removed from the market, and the label now caps single IV doses at 16 mg in the United States. (3)

Is the maximum ondansetron IV dose the same everywhere?

No -- this genuinely differs by country. The current US label allows up to 16 mg per single IV dose. The UK and New Zealand labels cap the single IV dose at 8 mg. Australia's approved product information uses an age-based cap: up to 16 mg for adults under 75, but only up to 8 mg from age 75 onward. Always follow the specific product label and your clinician's instructions for your country. (1) (4) (7) (8)

Can I take ondansetron if I have a heart rhythm problem?

Ondansetron is avoided in people with congenital long QT syndrome, and used with caution -- often with ECG monitoring -- in people with electrolyte abnormalities (low potassium or magnesium), heart failure, slow heart rhythms, or who take other medicines that also affect heart rhythm. Tell your clinician about any heart rhythm history before starting ondansetron. (1) (6)

What is the mechanism of action of ondansetron?

Ondansetron is a selective antagonist of serotonin 5-HT3 receptors, both peripherally (on vagal nerve terminals in the gut) and centrally (in the chemoreceptor trigger zone), blocking the serotonin release that triggers the vomiting reflex after emetogenic chemotherapy, radiotherapy, or surgery. (4) (1)

What is the generic name of Ondansetron?

The generic name is ondansetron. (1) (6)

What class of drug is ondansetron?

Ondansetron is classed as: Serotonin 5-HT3 receptor antagonist (antiemetic). (1) (6)

References

  1. ZOFRAN (ondansetron hydrochloride) injection -- full prescribing informationU.S. Food and Drug Administration / DailyMed (NIH), full prescribing information (injection) · United States · accessed 2026-07-30 · source 1
  2. ZOFRAN (ondansetron hydrochloride) tablets -- full prescribing informationU.S. Food and Drug Administration / DailyMed (NIH), full prescribing information (oral tablet) · United States · accessed 2026-07-30 · source 2
  3. FDA Drug Safety Communication: New information regarding QT prolongation with ondansetron (Zofran) -- 2012, plus 2012 update on 32 mg single IV dose removalU.S. Food and Drug Administration, Drug Safety Communication · United States · accessed 2026-07-30 · source 3
  4. Ondansetron 4 mg film-coated tablets -- Summary of Product Characteristicselectronic medicines compendium (emc), United Kingdom · United Kingdom · accessed 2026-07-30 · source 4
  5. Ondansetron -- list of nationally authorised medicinal products (PSUSA/00002217/202102)European Medicines Agency (EMA) -- Periodic Safety Update Report Single Assessment (PSUSA) list of nationally authorised medicinal products · European Union · accessed 2026-07-30 · source 5
  6. ZOFRAN ODT (ondansetron) -- Health Canada Product MonographHealth Canada product monograph · Canada · accessed 2026-07-30 · source 6
  7. Zofran (ondansetron) -- TGA-approved Product Information (Medicine Finder)Therapeutic Goods Administration (TGA)-approved Product Information, as published by the Australian Commission on Safety and Quality in Health Care's Medicine Finder · Australia · accessed 2026-07-30 · source 7
  8. Ondansetron -- New Zealand Data Sheet(s) (multiple registered brands, e.g. Ondansetron-Baxter, Ondansetron-Claris, APO-Ondansetron)Medsafe (New Zealand Medicines and Medical Devices Safety Authority) · New Zealand · accessed 2026-07-30 · source 8

Brand names by country

Single-ingredient ondansetron names are shown separately by country. Product availability, strengths, and supply rules can change; combination products are not included here.

Priority countries

United States

  • Zofran (1) (2)FDA approved currently labeled generic available · 4 mg, 8 mg, 16 mg (oral, formulation-dependent), 2 mg/mL injection · Novartis/GSK (originator, historical); numerous generic manufacturers

United Kingdom

  • Ondansetron (generic; Zofran historically) (4)MHRA licensed emc smpc confirmed · 4 mg, 8 mg tablet, 8mg/5ml oral solution, 2 mg/mL injection · multiple UK generic manufacturers

Canada

  • ZOFRAN
  • ZOFRAN ODT
  • ONDANSETRON OMEGA -(WITH PRESERVATIVE - MULTIDOSE
  • ONDISSOLVE ODF
  • SANDOZ ONDANSETRON ODT

Australia

  • Zofran / Zofran Tablets / Zofran Syrup (7)TGA approved PI confirmed via medicine finder · 4 mg, 8 mg tablet, 4mg/5ml syrup, 2 mg/mL injection · Novartis Pharmaceuticals Australia (originator, historical); multiple generic manufacturers

New Zealand

  • Ondansetron-Baxter, Ondansetron-Claris, APO-Ondansetron, Onrex, Ondansetron-DRLA, Periset ODT (multiple registered brands) (8)Medsafe datasheets confirmed multiple brands · varies by brand/formulation · multiple NZ-registered suppliers
View brands in additional countries.

European Union

Ondansetron (generic; nationally authorised per member state) (5)

This page provides general reference information about ondansetron and is not a substitute for advice from the prescriber managing your treatment. All dosing decisions, especially for the injectable form, should be made with your prescriber or care team.

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