Ondansetron is an anti-sickness medicine and is sometimes called an antiemetic. This is mainly used in hospitals to help with nausea and vomiting which is associated with cancer treatments or operations. The Food and Drug Administration (FDA) has already approved this medication. 

This is a type of medicine called a 5HT3 receptor agonist. It works by blocking 5HT3 receptors that are found in the brain and gut. Chemotherapy, radiotherapy, and surgery can cause a substance called serotonin (5HT) to be released in the body. This 5HT stimulates the 5HT3 receptors that are found in the brain and gut. It also causes nerve messages to be sent to the vomiting center. The vomiting center is an area of the brain that makes you feel sick and controls vomiting.

Ondansetron stops 5HT from stimulating the 5HT3 receptors and so stops nerve messages being sent to the vomiting center. This stops feelings of sickness which is nausea and vomiting that could otherwise be caused by cancer treatments or surgery.

Nausea is an overall term that defines a queasy stomach with or without the feeling that you are about to vomit. At some time, almost everyone experiences nausea. This is not a disease, but a symptom of many different disorders. Nausea is difficult to describe. It is a very uncomfortable but not painful feeling that is felt in the back of the throat or the upper abdomen. When to body prepares to vomit, the following sequence may occur:

  • The abdominal muscles and diaphragm contract 
  • The muscular ring between the esophagus and stomach relaxes
  • The windpipe closes
  • The lower portion of the stomach contracts 

Vomiting is the forcible emptying of the stomach in which the stomach has to overcome the pressures that are normally in place to keep food. This is not a condition but rather a symptom of other conditions the same as with nausea. Vomiting can be a one-time event especially when it is caused by eating or drinking something that doesn’t settle right in the stomach. However, vomiting repeatedly can be a sign of an emergency or a serious underlying condition. 

Nausea and vomiting associated with cancer treatments

Nausea and vomiting are side effects of cancer therapy and affect most patients who have chemotherapy. It is very important that nausea and vomiting are controlled. It is to let them continue treatment and perform activities of daily life. Nausea and vomiting that are not controlled can cause the following:

  • Mental changes
  • Chemical changes in the body
  • Malnutrition 
  • Loss of appetite
  • Dehydration 
  • Broken bones 
  • A torn esophagus 
  • Reopening of surgical wounds 

In some patients, after they have had several courses of treatment, nausea and vomiting may occur before a treatment session. This is called anticipatory nausea and vomiting. It is caused by triggers such as odors in the therapy room. The more chemotherapy a patient has, the more possible it is that anticipatory nausea and vomiting will occur. 

How to use Ondansetron?

This is dissolved on top of the tongue and not meant to be chewed or swallowed like other tablet forms. Dry your hands before using this medication. Allow the medicine to dissolve completely then swallow it with saliva. You don’t need to take this product with water. Doing so may increase your chance of getting a headache. 

To prevent nausea and vomiting from chemotherapy, take this medication usually 30 minutes before treatment begins. This medication may be taken either with or without a meal. However, your doctor will tell you not to eat before chemotherapy. 

Take any other doses as directed by your doctor. This may be taken up to 3 times a day for 1 to 2 days after your chemotherapy treatment is finished. To get the most benefit from it, take it regularly if you are taking this on a prescribed schedule. The dosage is based on your medical condition and response to the treatment. The dosage for children may also be based on age and weight. Take this drug exactly as it is directed. You should not take more medication or take it more often than it is prescribed. Inform your doctor if your condition does not improve or if it gets worse. 

What are the side effects of Ondansetron?

Common side effects:

  • Fatigue
  • Headache
  • Diarrhea
  • Constipation
  • A general feeling of being unwell
  • Dizziness
  • Fever
  • Agitation
  • Anxiety
  • Itching
  • Urinary retention 

Let your doctor know if these side effects are severe or do not go away:

  • Weakness
  • Drowsiness
  • Chills
  • Headache 
  • Blurred vision
  • Hives
  • Shortness of breath 
  • Light-headedness
  • Fainting
  • Excessive sweating
  • Confusion 
  • Loss of coordination 
  • Stiff or twitching muscles 
  • Coma or loss of consciousness 


  • Tell your doctor if you are receiving Apomorphine. Your doctor will probably tell you not to take Ondansetron if you are receiving this medication.
  • Tell as well to your doctor and pharmacist if you are allergic to Ondansetron, any other medications, or any of the ingredients in Ondansetron products. Ask your pharmacist for a list of the ingredients.
  • Inform your doctor and pharmacist what prescription and non-prescription medications, vitamins, food supplements, and herbal products you are taking or plan to take. 
  • Let your doctor know if you or anyone in your family has or has ever had long QT syndrome or another type of irregular heartbeat or heart rhythm problem, or if you have or have ever had low blood levels of magnesium or potassium in your blood, heart failure, or liver disease.
  • Tell your doctor if you are pregnant, plan to become pregnant, or are breastfeeding. If you become pregnant while taking Ondansetron, call your doctor right away.
  • Take the missed dose as soon as you remember it. However, if it is almost time for the next dose, skip the missed dose and continue your regular dosing schedule. Do not take a double dose to make up for a missed one.
  • Keep this medication in the container it came in, tightly closed, and out of reach of children. 
  • Store the tablets and rapidly disintegrating tablets away from light, at room temperature. Store the solution in the bottle upright at room temperature and away from light, excess heat, and moisture.