Sounds like 'on-DAN-se-tron'

Ondansetron is used to treat nausea and vomiting. Find out how to take it safely and possible side effects. Ondansetron is also called Onrex.

Type of medicine Also called
  • Medicine used to treat nausea and vomiting
  • Belongs to a group of medicines known as antiemetics
  • Onrex®
  • Apo-Ondansetron®
  • Ondansetron-DRLA®

What is ondansetron?

Ondansetron is used to treat and prevent nausea and vomiting, such as following a surgical operation, or due to cancer medication or radiation therapy. It blocks the actions of chemicals in your body that can trigger nausea and vomiting.


In New Zealand, ondansetron is available as tablets (4 mg and 8 mg) and can be given as an injection in the hospital.

  • The dose of ondansetron will be different for different people, depending on its use.
  • If you are due to have chemotherapy or radiotherapy treatment that could cause you to feel sick, your doctor will prescribe you a dose of ondansetron an hour or so before the treatment. You may then need to continue taking ondansetron for up to 5 days.
  • If you are being prescribed ondansetron because you are due to have an operation, you will be given a dose shortly before the surgery, and then prescribed a few doses to take afterwards if you need it.
  • Always take your ondansetron exactly as your doctor has told you. The pharmacy label on your medicine will tell you how much to take, how often to take it, and any special instructions.

How to take ondansetron

You can take ondansetron with or without food. It should start to work within 30 minutes to an hour. Ondansetron tablets are available in 2 different forms – tablets or oral disintegrating tablets (also called ODT). 

  • Tablets: Swallow the tablets with a glass of water. Don't halve or crush the tablets. 
  • Oral disintegrating tablets: These tablets dissolve in your mouth very quickly. Place the tablet on your tongue, allow it to dissolve or melt, then swallow.  

Things to consider while you are taking ondansetron 

  • Ondansetron can interact with other medicines. Tell your doctor or pharmacist about all medicines you are taking including over the counter medicines, herbal and complementary medicines or recreational drugs. 
  • Tell your doctor if you are pregnant, planning to become pregnant, or breastfeeding.

What are the side effects of ondansetron?

Like all medicines, ondansetron can cause side effects, although not everyone gets them. Often side effects improve as your body gets used to the new medicine.

Serotonin syndrome

Serotonin syndrome is a very rare but serious side effect of ondansetron. Serotonin syndrome occurs when the level of serotonin in your brain gets too high. You are at risk of serotonin syndrome if you just started taking or increased the dose of your ondansetron. You are also at risk if you take other medicines or herbal supplements that increase serotonin levels such as triptans for migraines or St John's Wort. Symptoms can range from mild (eg, shivering and diarrhoea/runny poo) to severe (eg, muscle rigidity, fever and seizures). Milder forms of serotonin syndrome may go away within a few days of stopping the medicines that caused your symptoms. Severe serotonin syndrome needs hospital admission and can be fatal if not treated. Read more about serotonin syndrome.

Other possible side effects

Side effects What should I do?
  • Headache
  • Constipation
  • These are quite common when you first start taking ondansetron.
  • Tell your doctor if they bother you.
  • Dizziness
  • Feeling faint
  • Be careful when driving or using tools until you know how this medicine affects you.
  • Take care when getting out of bed to make sure you don’t fall.
  • Avoid alcohol.
  • Flushing, fever
  • Tingling or numbness
  • Tell your doctor if these bother you.
  • Changes to your heartbeat (fast or irregular)
  • Tell your doctor immediately or ring Healthline 0800 611 116.
Did you know that you can report a side effect to a medicine to CARM (Centre for Adverse Reactions Monitoring)? Report a side effect to a product.

Learn more

The following links have more information on ondansetron.

Ondansetron New Zealand Formulary Patient Information


  1. Ondansetron New Zealand Formulary

Additional resources for healthcare professionals

Onrex Medsafe, NZ
Ondansetron ODT-DRLA Medsafe, NZ
Ondansetron – new dose restrictions Medsafe, NZ, 2012
Ondansetron – safe prescribing – relieve the heave SafeRx, NZ, 2019
Irritable bowel syndrome in adults BPAC, NZ, 2014
Nausea and vomiting in pregnancy BPAC, NZ, 2011

Credits: Sandra Ponen, Pharmacist. Reviewed By: Angela Lambie, Pharmacist, Auckland Last reviewed: 12 Aug 2022