Ondansetron for sickness

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Ondansetron is an anti-sickness medicine.

It will start to work within 1-2 hours.

The most common side-effects are constipation, a headache, and feeling flushed.

Type of medicine5HT3-receptor antagonist anti-sickness medicine
Used forPrevention or treatment of sickness associated with chemotherapy, radiotherapy or surgery
Also calledDemorem®; Ondemet®; Setofilm®; Zofran®
Available asTablets, orodispersible (melt in the mouth) tablets, oral liquid medicine, suppositories, and injection

Ondansetron is known as a 5HT3-receptor antagonist. It is prescribed to stop you from feeling sick. It works by blocking the effect of a naturally produced chemical in your body, called serotonin. Serotonin is also referred to as 5HT.

Chemotherapy, radiotherapy and surgery may all cause the release of an increased amount of serotonin in your body. There are small receptors in your small intestine and brain called 5HT3 receptors. Serotonin acts on these receptors and causes you to feel sick. Ondansetron works by blocking serotonin from acting on 5HT3 receptors, and this stops you from feeling (or being) sick.

Some medicines are not suitable for people with certain conditions, and sometimes a medicine may only be used if extra care is taken. For these reasons, before you start taking ondansetron it is important that your doctor knows:

  • If you are pregnant or breast-feeding.
  • If you have any problems with the way your liver works.
  • If you have been told you have an irregular heartbeat.
  • If you think you may have a blockage in your bowel, or if you are severely constipated.
  • If you are taking any other medicines. This includes any medicines you are taking which are available to buy without a prescription, such as herbal and complementary medicines.
  • If you have ever had an allergic reaction to a medicine.
  • Before you start this treatment, read the manufacturer's printed information leaflet from inside your pack. The manufacturer's leaflet will give you more information about ondansetron and a full list of the side-effects which you may experience from taking it.
  • Take ondansetron exactly as your doctor tells you to. The way you have been told to take it will depend upon the reason why you are taking it. 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 - tablets are usually prescribed twice daily (every 12 hours), whereas suppositories are prescribed just once daily. 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 at eight-hourly intervals afterwards. If you are unsure about which is right for you, ask your doctor, nurse, or pharmacist for advice.
  • If you have been prescribed Zofran® Melt or Setofilm® tablets, these tablets dissolve very quickly in your mouth when you put them on the top of your tongue. The tablets are quite fragile so you need to peel off the foil packaging rather than try to push the tablet through it. Place the tablet on the top of your tongue and allow it to dissolve before you swallow.
  • If you forget to take a dose, take it as soon as you remember. If it is nearly time to take your next dose when you remember, skip the missed dose and take your next dose when it is due. Do not take two doses together to make up for a forgotten dose.

How to use ondansetron suppositories:

  1. Remove the suppository from the wrapping.
  2. You may find that inserting the suppository is easier if you squat or bend forward.
  3. Using a finger, gently push the suppository into your rectum (back passage) as far as possible, pointed end first.
  4. Remove your finger and then remain still for a little while to help you hold the suppository in place.
  5. Wash your hands afterwards.
  • Ondansetron should start to work within an hour or so. If you are taking tablets or syrup and you are sick within one hour of taking a dose, you should take the same dose again. This does not apply, however, if you are using suppositories or if you have been given an injection, as being sick will not prevent these from being absorbed by your body.
  • Even if you do not feel like eating or drinking, try to sip water regularly to prevent you from becoming dehydrated.
  • Rather than trying to eat three main meals a day, try eating small, simple but nourishing snacks, every few hours.

Along with their useful effects, most medicines can cause unwanted side-effects although not everyone experiences them. The table below contains some of the most common ones associated with ondansetron. You will find a full list in the manufacturer's information leaflet supplied with your medicine. The unwanted effects often improve as your body adjusts to the new medicine, but speak with your doctor or pharmacist if any of the following continue or become troublesome.

Common ondansetron side-effects - these affect less than 1 in 10 people who take this medicine
What can I do if I experience this?
HeadacheAsk your doctor to prescribe/recommend a suitable painkiller
ConstipationTry to eat a well-balanced diet, and drink several glasses of water each day
Feeling warm or flushedWear light, cool clothes
Other less common side-effects include: hiccups, chest pain, slow heart rate, feeling dizzy when getting up, and unusual body movementsIf any of these become troublesome, speak with your doctor

If you experience any other symptoms which you think may be due to this medicine, speak with your doctor or pharmacist.

  • Keep all medicines out of the reach and sight of children.
  • Store in a cool, dry place, away from direct heat and light.

Never take more than the prescribed dose. If you suspect that you or someone else might have taken an overdose of this medicine, go to the accident and emergency department of your local hospital. Take the container with you, even if it is empty.

This medicine is for you. Never give it to other people even if their condition appears to be the same as yours.

If you are having an operation or any dental treatment, tell the person carrying out the treatment which medicines you are taking.

If you buy any medicines, check with a pharmacist that they are suitable to take with your other medicines.

Do not keep out-of-date or unwanted medicines. Take them to your local pharmacy which will dispose of them for you.

If you have any questions about this medicine ask your pharmacist.

Further reading & references

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.

Original Author:
Helen Allen
Current Version:
Peer Reviewer:
Dr John Cox
Document ID:
1434 (v24)
Last Checked:
Next Review:
The Information Standard - certified member

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