Ondansetron – Powerful 5-HT3 antagonist for severe nausea and vomiting.
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What is Ondansetron?
Ondansetron is a highly effective anti-nausea medication that’s particularly valuable for severe nausea and vomiting that doesn’t respond to conventional treatments. Its specific mechanism makes it especially useful for chemotherapy, post-operative, and other treatment-related nausea.
Book an online consultation if you’re experiencing severe nausea and vomiting that hasn’t responded to other treatments, need anti-nausea medication for medical procedures, or require assessment for specialized nausea management. Our GPs can evaluate your symptoms and determine if Ondansetron is appropriate for your specific condition.
What is Ondansetron used for?
- Chemotherapy-induced nausea and vomiting
- Post-operative nausea and vomiting
- Radiotherapy-induced nausea
- Severe morning sickness (hyperemesis gravidarum)
- Gastroenteritis with severe vomiting
- Severe migraine-associated nausea
- Palliative care nausea management
How does Ondansetron work?
A selective 5-HT3 receptor antagonist that blocks serotonin receptors in the chemoreceptor trigger zone and gut, preventing the cascade of events that leads to nausea and vomiting.
Dosage and Administration
Adults
4-8mg orally, or 4mg by injection. Can be repeated every 8 hours as needed. Maximum 32mg daily.
Children
150 micrograms per kg up to maximum adult dose—specialist supervision usually required.
Important Notes
- Can be taken with or without food.
- Dissolving tablets (wafers) available for patients who cannot swallow.
- Injection form available for severe cases or when oral route not possible.
- Take 30 minutes before chemotherapy when used for prevention.
Side Effects
Common Side Effects
- Headache
- Constipation
- Fatigue
- Dizziness
- Feeling warm or flushing
- Hiccups
Serious Side Effects
Seek immediate medical attention if you experience:
- Heart rhythm changes (QT prolongation)
- Severe allergic reactions
- Serotonin syndrome (rare)
- Severe constipation
- Liver problems (rare)
- Involuntary movements (rare)
Important Warnings
- Tell your GP if you have heart rhythm problems or take medications affecting heart rhythm.
- May cause constipation—ensure adequate fluid intake and consider laxatives.
- Use with caution in patients with liver problems.
- Not recommended for routine use in children under 2 years.
- Monitor for heart rhythm changes with high doses or IV administration.
Drug Interactions
Tell your GP if you're taking any of the following medications:
- Tramadol—may reduce effectiveness of tramadol for pain relief.
- Carbamazepine, phenytoin, rifampicin—may reduce ondansetron effectiveness.
- Apomorphine—dangerous interaction, must not be used together.
- Other 5-HT3 antagonists—avoid duplication.
- QT-prolonging drugs—increased risk of heart rhythm problems.
Frequently Asked Questions
How is Ondansetron different from other anti-sickness medications?
Ondansetron is much more effective for severe nausea, especially from chemotherapy or surgery, as it blocks the specific serotonin pathway involved in these types of vomiting.
Can Ondansetron be used for morning sickness?
It can be used for severe morning sickness (hyperemesis gravidarum) under specialist supervision when other treatments haven't worked, but it's not first-line.
Why might I become constipated with Ondansetron?
Ondansetron blocks serotonin receptors in the gut, which normally help with bowel movements. This can lead to constipation, especially with regular use.
How quickly does Ondansetron work?
It usually starts working within 30 minutes to 1 hour when taken orally, and within 15 minutes when given by injection.
Medically Reviewed by: Mohammed Hassan - Independent Prescriber
Last Updated: 2025-01-14