Ondansetron — NZ Medication Guide
📋 Reviewed by a Registered Pharmacist NZ — This information is for general educational purposes only and does not replace professional medical advice. Always consult your doctor or pharmacist before starting, stopping, or changing any medication.
What is Ondansetron?
Ondansetron is a selective 5-HT3 receptor antagonist used as a highly effective antiemetic for the prevention and treatment of nausea and vomiting. This medication is commonly prescribed in New Zealand and is funded by Pharmac for chemotherapy-induced nausea and post-operative nausea.
What is Ondansetron Used For?
Ondansetron is used to prevent and treat nausea and vomiting caused by chemotherapy, radiotherapy, and post-operative states. It is also used off-label for nausea in pregnancy (hyperemesis gravidarum) and gastroenteritis.
How Does Ondansetron Work?
Ondansetron selectively blocks serotonin (5-HT3) receptors in the gut and brain, particularly in the chemoreceptor trigger zone and vagal afferents of the gastrointestinal tract. By blocking the serotonin signals that trigger nausea and vomiting, it effectively prevents these symptoms.
How to Take Ondansetron
Available as tablets (4 mg, 8 mg), wafer formulations that dissolve on the tongue (Zofran Melt®), and injection. For chemotherapy-induced nausea: typically 8 mg given 30 minutes before chemotherapy, then every 8–12 hours. For post-operative nausea: 4–8 mg. Take orally with or without food.
Common Side Effects of Ondansetron
- Headache
- Constipation
- Warm or flushing sensation
- Dizziness
- Fatigue
Serious Side Effects — Seek Medical Attention
Contact your doctor or call 111 immediately if you experience any of the following:
- QT prolongation (cardiac arrhythmia risk — particularly at higher doses or with other QT-prolonging medicines)
- Serotonin syndrome (rare — when combined with serotonergic agents)
- Severe allergic reaction (including anaphylaxis)
- Severe constipation
Drug Interactions
Ondansetron may interact with other medicines. Always inform your doctor and pharmacist of all medications, supplements, and herbal products you are taking. Key interactions include:
- QT-prolonging medicines (antiarrhythmics, haloperidol, certain antibiotics — additive QT risk)
- Serotonergic agents (SSRIs, tramadol — serotonin syndrome risk)
- Apomorphine (contraindicated — severe hypotension)
- Carbamazepine (reduces ondansetron levels)
New Zealand Prescribing Information
Ondansetron is funded by Pharmac primarily for chemotherapy-induced nausea and vomiting (CINV) and post-operative nausea. It is available as 4 mg and 8 mg tablets, 4 mg/5 mL oral solution, and 4 mg/2 mL injection. Ondansetron film (wafer) formulations (Zofran Melt®) are available but may not be funded — check with your pharmacist.
Frequently Asked Questions
Can ondansetron be used in pregnancy?
Ondansetron is sometimes used off-label for severe nausea and vomiting in pregnancy (hyperemesis gravidarum), particularly when other treatments have failed. While large studies are generally reassuring, there is some debate about a possible small increased risk of certain cardiac defects in early pregnancy. This should be discussed with your doctor on a case-by-case basis.
Why does ondansetron cause constipation?
By blocking 5-HT3 receptors in the gastrointestinal tract, ondansetron reduces gut motility alongside its antiemetic effect. Ensure adequate fluid intake and consider a laxative if constipation is problematic, especially with repeated dosing.
⚠️ Disclaimer: This page is for educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your doctor or a qualified health provider with any questions you may have regarding a medical condition or medication. In New Zealand, medication availability and funding may vary — check with Pharmac or your pharmacist for the most current information.
Reviewed by a Registered Pharmacist NZ
References & Further Information
The following New Zealand and international resources were used to inform this page: