Sounds like 'on-DAN-se-tron'

Easy-to-read medicine information about ondansetron – what it is, how to take ondansetron safely and possible side effects.

Type of medicine

Also called

  • Medicine used to treat nausea and vomiting
  • Belongs to a group of medicines known as antiemetics
  • Onrex®
  • Auro-Ondansetron®
  • Ondansetron-DRLA®
  • Zofran®
  • Zofran Zydis®

What is ondansetron?

Ondansetron is used to treat and prevent nausea and vomiting, such as when associated with a surgical operation, or due to cancer medication, radiation therapy or a migraine headache. It works by blocking a natural substance called serotonin, and in this way blocks the nerves that cause the nausea and vomiting reflex.

Read more about chemotherapy and its side effects.


  • The dose of ondansetron will be different for different people, depending on its use.
  • If you are due to have a 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 will then be asked to continue to take ondansetron for up to five more 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 couple of doses to take afterwards.
  • 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

  • Ondansetron is available in different forms such as tablets or wafers (also called oral dispersible tablets or oral disintegrating tablets).
  • Swallow the tablets with a glass of water.
  • If you are prescribed wafers or oral disintegrating tablets, place these on the tongue, allow to soften for a few seconds, and then swallow.  
  • It takes about 30 minutes for ondansetron to start working.
  • If you forget your dose, take it as soon as you remember that day. But, if it is nearly time for your next dose, just take the next dose at the right time. Do not take double the dose.

Possible side effects

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

Side effects What should I do?
  • Headache
  • Constipation
  • These are quite common when you first start taking ondansetron
  • Tell your doctor if troublesome
  • Drowsiness
  • Dizziness
  • Feeling tired
  • Be careful when driving or using tools until you know how this medicine affects you
  • Changes to your heart beat (fast or irregular)
  • Tell your doctor immediately or ring HealthLine 0800 611 116
  • Signs of movement changes such as shaking (tremors), slow or difficult movements, muscle stiffness, restlessness
  • Tell your doctor immediately or ring HealthLine 0800 611 116
  • Signs of serotonin syndrome such as feeling agitated and restless, heavy sweating, shivering, fast heart rate or irregular heart beat, headache, diarrhoea and rigid or twitching muscles
  • You are at increased risk of serotonin syndrome if you recently started taking ondansetron or recently increased the dose
  • Tell your doctor immediately or ring HealthLine 0800 611 116


Ondansetron may interact with a few medications and herbal supplements, so check with your doctor or pharmacist.

Learn more

The following links provide further information on ondansetron.

Medsafe Consumer Information Sheets:


Credits: Sandra Ponen, Pharmacist. Last reviewed: 14 Apr 2015