Ondansetron (Zofran) for Nausea and Vomiting: Uses, Side Effects and Safety
Written by Cherlie Joseph, MSN, FNP-BC · Medically reviewed by Naurah Gaspard, DNP, FNP-C, FNP-BC · Updated November 2026
Ondansetron (brand name Zofran) is a prescription medicine that prevents and eases nausea and vomiting by blocking serotonin signals in the gut and brain. It is FDA-approved for nausea from chemotherapy, radiation and surgery, and providers also commonly prescribe it for stomach bugs. It is generally well tolerated, but it can affect heart rhythm and interact with some medicines.
In this guide
How ondansetron works and what it is used for
Ondansetron belongs to a group called 5-HT3 receptor blockers. Serotonin released in the gut and brain can trigger the vomiting reflex, and ondansetron blocks that signal.
Its FDA-approved uses are preventing nausea and vomiting from cancer chemotherapy, radiation therapy and surgery. In everyday practice, providers also prescribe it "off-label" for other causes, such as viral gastroenteritis, when vomiting makes it hard to keep fluids down. Off-label prescribing is common and legal when a provider judges it appropriate.
Ondansetron treats the symptom, not the cause. With a stomach bug, the goal is to settle vomiting enough that you can rehydrate. It is not a controlled substance.
Forms and how it is taken
| Form | Good to know |
|---|---|
| Orally disintegrating tablet (ODT) | Dissolves on the tongue without water, which helps when you cannot keep anything down. Some versions contain phenylalanine, important for people with PKU. |
| Regular tablet | Swallowed with water |
| Oral solution | Liquid form, measured with a dosing device |
| Injection | Given in hospitals and clinics |
Take ondansetron exactly as your prescription label says. Do not take extra doses if you still feel sick; contact your provider instead, because persistent vomiting may need a different plan. Generic ondansetron is widely available and usually inexpensive, but prices vary by pharmacy, so compare with discount programs.
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Most people tolerate ondansetron well. According to MedlinePlus and the prescribing information, common side effects include:
- Headache
- Constipation (or sometimes diarrhea)
- Tiredness or drowsiness
- Dizziness
Because constipation is common, drink fluids and let your provider know if it becomes uncomfortable.
Important safety warnings and interactions
- Heart rhythm (QT prolongation). Ondansetron can lengthen the QT interval on an EKG, which can rarely lead to a dangerous irregular heartbeat. Tell your provider if you have congenital long QT syndrome, heart failure, a slow heartbeat, or low potassium or magnesium (which can happen after a lot of vomiting or diarrhea), or if you take other medicines that affect heart rhythm, such as certain antibiotics, antidepressants or antipsychotics.
- Serotonin syndrome. Combining ondansetron with other serotonin-affecting medicines, such as SSRIs (sertraline, escitalopram, fluoxetine), SNRIs, triptans or tramadol, can rarely cause serotonin syndrome. Watch for agitation, fast heartbeat, fever, sweating, muscle twitching or stiffness, and get help right away if they occur.
- Apomorphine. Ondansetron must not be taken with apomorphine (used for Parkinson's disease) because the combination can cause very low blood pressure and fainting.
- Can mask other problems. Ondansetron can hide symptoms of a bowel blockage or other serious belly problems. Severe or worsening belly pain or swelling needs in-person evaluation.
- Allergy. Do not take it if you have had an allergic reaction to ondansetron or similar medicines (granisetron, palonosetron).
- Pregnancy. Ondansetron is sometimes used for pregnancy nausea, but the decision should be made with your obstetric provider, who can weigh the options.
Share a full list of your medicines and supplements with any provider prescribing ondansetron.
Getting ondansetron through an online visit
At OurTopClinic, a nurse practitioner licensed in your state reviews your symptoms, health history and medicines to decide whether ondansetron or another option is a safe fit. A prescription is never guaranteed, and some situations, like severe pain, signs of dehydration or vomiting blood, need in-person care instead.
Get in-person care or call 911 if you have:
- Fainting, a pounding or irregular heartbeat, or chest pain
- Fever, confusion, muscle stiffness or twitching while taking ondansetron with other medicines
- Hives, swelling of the face or throat, or trouble breathing
- Severe or worsening belly pain or a swollen belly
- Vomiting blood or material that looks like coffee grounds
- Unable to keep any fluids down for more than a day
Frequently asked questions
How fast does ondansetron work?
Many people notice relief within about 30 minutes to an hour, though it varies. If vomiting continues, contact your provider rather than taking extra doses.
Can I take ondansetron with sertraline or other antidepressants?
Sometimes, but the combination carries a small risk of serotonin syndrome and, with some antidepressants, heart rhythm effects. Your provider needs to know every medicine you take before prescribing it.
Does ondansetron make you sleepy?
Some people feel tired or drowsy, but it is usually less sedating than older anti-nausea medicines like promethazine.
Sources
Keep reading
- What to eat with a stomach bug
- Norovirus: symptoms, how long it lasts and how to stop the spread
- Heartburn on GLP-1 medicines
- Online prescription refills in Florida
- Browse all conditions and treatments A to Z
Sick to your stomach and need a plan?
$79 visit, no appointment needed. Adults 18+ in FL, NM, OR and WA.
Start a nausea visit →
Cherlie Joseph, MSN, FNP-BC
Board-certified family nurse practitioner licensed in Florida, caring for adults online at OurTopClinic. Meet our providers.
This article is for general education and is not a substitute for medical advice. Treatment, including any prescription, is decided by a licensed provider after reviewing your health history. OurTopClinic is not for emergencies; if you have a medical emergency, call 911. OurTopClinic serves adults located in Florida, New Mexico, Oregon and Washington. Learn more at ourtopclinic.com.
