Ondansetron
Say it: on-DAN-seh-tron · Brand names: Zofran
Ondansetron is a medicine that helps prevent nausea and throwing up.
What it is
Chemotherapy and radiation can make your body release a chemical called serotonin in the gut. Serotonin sends signals to the brain that cause nausea. Ondansetron blocks those signals.
Often used for
- preventing nausea and throwing up from chemotherapy
- preventing nausea and throwing up from radiation
- preventing nausea and throwing up after surgery
How you get it
It comes as a pill, a tablet that melts on your tongue, a liquid, or a drip into a vein. It is often given before chemotherapy and then for a day or more after. Your schedule may be different.
Common side effects and what helps
- Constipation. Drink fluids, eat fiber if you can, and ask your team about a stool softener.
- Headache. Rest and ask your team which pain medicine is safe.
- Feeling tired or sleepy. Rest when you need to.
- Diarrhea. Drink plenty of fluids and tell your team if it does not stop.
- Feeling dizzy. Stand up slowly.
Call your care team right away if you have
- A fast, pounding, or uneven heartbeat, or fainting
- Feeling very restless, shaky, sweaty, or confused, with stiff muscles or a high fever
- Bad belly pain or swelling, or not passing gas or stool
- Signs of an allergic reaction, like a rash all over, swelling of the face or throat, or wheezing
- Nausea or throwing up that does not stop, or you cannot keep fluids down
Before you start, tell your team
- If you or anyone in your family has a heart rhythm problem called long QT
- If you have heart problems or liver problems
- All medicines you take, especially medicines for depression, migraines, or heart rhythm
- If you have phenylketonuria. The melting tablets may contain phenylalanine.
- If you are pregnant, may become pregnant, or are breastfeeding
Good to know
- Ondansetron can change your heart rhythm. Your team may check your potassium and magnesium, or do a heart tracing test.
- Taken with some other medicines, it can rarely cause serotonin syndrome, which causes agitation, fever, sweating, and stiff muscles.
- It should not be taken with apomorphine, a medicine for Parkinson disease.
At home
- Take it the way your team told you. It works best when taken before nausea starts.
- For the melting tablet, peel back the foil with dry hands. Do not push it through. Place it on your tongue and let it dissolve.
- If you miss a dose, ask your team what to do.
- Store at room temperature, away from children.
Questions to ask
- Should I take this on a schedule or only when I feel sick?
- What should I do if this medicine does not stop my nausea?
- What can I take for constipation?
Side effect guides
Sources
Quick answers
- What is Ondansetron?
- Ondansetron is a medicine that helps prevent nausea and throwing up. Brand names: Zofran.
- What is Ondansetron used for?
- preventing nausea and throwing up from chemotherapy; preventing nausea and throwing up from radiation; preventing nausea and throwing up after surgery.
- How is Ondansetron given?
- It comes as a pill, a tablet that melts on your tongue, a liquid, or a drip into a vein. It is often given before chemotherapy and then for a day or more after. Your schedule may be different.
- What are the common side effects of Ondansetron?
- Constipation. Headache. Feeling tired or sleepy. Diarrhea. Feeling dizzy.
- When should I call my care team while taking Ondansetron?
- A fast, pounding, or uneven heartbeat, or fainting; Feeling very restless, shaky, sweaty, or confused, with stiff muscles or a high fever; Bad belly pain or swelling, or not passing gas or stool; Signs of an allergic reaction, like a rash all over, swelling of the face or throat, or wheezing; Nausea or throwing up that does not stop, or you cannot keep fluids down.
Last updated: 2026-10-27. Medically reviewed by Veli Bakalov, MD, medical oncologist (October 2026). General education from Step by Step, the patient portal of HemeOncBuddy.