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What is ondansetron?

Ondansetron is an antiemetic, meaning it prevents emesis (vomiting). It is typically prescribed for the prevention of symptoms associated with chemotherapy or radiation treatment for cancer. Doctors also prescribe it to prevent and treat nausea and vomiting related to surgery.¹

Ondansetron is a serotonin 5-HT3 receptor antagonist. It works by blocking the actions of serotonin, thus preventing nausea and vomiting.²

The drug is available by its generic name and under the brand names Zofran and Zuplenz. 

What is ondansetron used to treat?

Doctors prescribe ondansetron to treat symptoms that usually occur among patients undergoing cancer therapy and after surgery. Specifically, ondansetron is approved by the US Food and Drug Administration (FDA) for the prevention and treatment of nausea and vomiting with the following indications:³

  • Moderate and high doses of chemotherapy for cancer

  • Radiation treatment of the abdomen or the whole body

  • After surgical procedures

The medication is also prescribed off-label for other causes of nausea and vomiting, such as pregnancy, gastroenteritis, and certain medical conditions.⁴

Ondansetron is available in several forms and is approved for use in adults and children (ages specific for the condition being treated).⁵ ⁶ ⁷

How do you take ondansetron?

Ondansetron is available as an injectable solution, a tablet, an oral solution, an oral soluble film, and an orally disintegrating tablet (ODT). How you take the drug will depend on the form you were prescribed and the cause of your nausea and vomiting.

Dosage forms and strengths

The available forms and dosages are as follows:⁸

  • Oral solution (Zofran): 4mg/5mL

  • Oral soluble film (Zuplenz): 4mg, 8mg

  • Tablet (generic, Zofran): 4mg, 8mg, 24mg

  • ODT (Zofran ODT): 4mg, 8mg

  • Injectable solution (Zofran): 2mg/mL

The tablet, ODT, and oral solution are interchangeable in terms of dosing. These forms are only approved for use in children receiving moderate doses of chemotherapy.

People taking the ODT form of ondansetron will remove a tablet from the blister packaging with dry hands immediately before taking it. Then, the ODT is placed directly on the tongue, where it dissolves.

Like the ODT, the orally soluble film is placed onto the tongue, where it dissolves within 20 seconds. It’s important not to chew or swallow the film. Once the film dissolves, you can swallow with or without water. This form is only approved for children if symptoms are caused by chemotherapy.

The injectable solution form is administered by a qualified healthcare professional in a medical setting.

Seeing results

All forms of ondansetron start working quickly — in as little as thirty minutes. The duration of effectiveness varies based on the type prescribed.⁹

Who should not take ondansetron?

Ondansetron is contraindicated for people with known hypersensitivity to the drug and those currently taking apomorphine, as these drugs may trigger severe hypotension and loss of consciousness when taken concurrently. It should also be avoided in patients with congenital long QT syndrome.¹⁰

Additionally, the drug should be prescribed at lower doses in people with severe liver dysfunction and should be prescribed cautiously in people with any of the following conditions:¹¹ ¹²

  • QT prolongation (or a family history of the condition)

  • Ventricular arrhythmia

  • Bradycardia

  • A history of torsades de pointes (a specific abnormal heart rhythm)

  • Congestive heart failure

  • Electrolyte abnormalities

  • Recent heart attack

  • Past abdominal surgery

  • Diarrhea predominant gastroenteritis (children)

  • Phenylketonuria (should not take the ODT form)

  • Risk factors for gastrointestinal obstruction 

Potential side effects of ondansetron

Ondansetron has known side effects, but they don’t occur in everyone who takes the drug. Most side effects are mild and resolve without intervention. However, some can be severe.¹³ ¹⁴

Common side effects are as follows:

  • Chills

  • Weakness

  • Drowsiness or fatigue

  • Headache

  • Constipation

Less common side effects include the following:

  • Visual changes, including blurred vision or vision loss

  • Chest pain

  • Shortness of breath

  • Difficulty breathing or swallowing

  • Hoarseness

  • Swelling of the face, eyes, lips, tongue, throat, or extremities 

  • Rash, hives, or itching 

  • Severe rash with peeling of the skin

  • Dizziness, poor coordination

  • Lightheadedness or fainting

  • Seizures

  • Sore, stiff, or twitching muscles

  • Numbness, tingling, or a sensation of cold

  • Changes in the heart rate (fast, slow, or irregular) or rhythm

  • Excessive sweating, flushing, or fever

  • Confusion, anxiety, agitation, hallucinations, or coma

  • Nausea, diarrhea, or vomiting

  • Urinary retention

  • Pain, redness, or burning at the injection site

Risk of serotonin syndrome

Taking ondansetron alone or with other medications can lead to serotonin syndrome, which can be fatal if left untreated. Some of the uncommon side effects listed above may indicate the presence of this condition. Seek urgent medical care if you experience any of the following symptoms of serotonin syndrome:¹⁵

  • Tachycardia

  • Unstable blood pressure

  • Dizziness, fainting

  • Flushing

  • High temperature

  • Tremors, twitching, or jerking

  • Rigid muscles, hyperactive reflexes

  • Poor coordination

  • Seizures

  • Mental status changes, including agitation or hallucinations

  • Gastrointestinal symptoms, including nausea, vomiting, or diarrhea


You must seek immediate medical help; call the National Poison Center helpline or 911 if you overdose on ondansetron.¹⁶

Symptoms of toxicity from ondansetron might include any of the adverse effects listed above or the following:¹⁷

  • Dizziness, lightheadedness

  • Low or high blood pressure

  • Loss of consciousness

  • Constipation

  • Irregular or rapid heartbeat

  • Sudden but temporary loss of vision

  • Flushing, sweating

  • Seizures

  • Symptoms of serotonin syndrome

Allergy information

In some cases, taking ondansetron could result in a severe allergic reaction or anaphylaxis. Symptoms of an allergic or anaphylactic reaction may include any of the following:¹⁸

  • Hives, redness, or itching

  • Chest discomfort or tightness

  • Difficulty breathing or swallowing

  • Slurred speech

  • Swelling of the face, eyes, or tongue

  • Nasal congestion

  • Heart palpitations

  • Diarrhea

  • Abdominal pain

  • Nausea or vomiting

  • Feeling anxious

  • Loss of consciousness

You must seek immediate medical attention if you suddenly experience symptoms of anaphylaxis, as it is a life-threatening condition.

Long-term use of ondansetron

Doctors prescribe ondansetron to patients who are undergoing chemotherapy or radiation treatment and to patients who have had or will have surgery. Depending on the indication, it could be taken for one or two days after chemotherapy or radiation therapy is finished. There’s a lack of evidence regarding the safety and efficacy of the drug beyond five days.¹⁹

Ondansetron in pregnancy and breastfeeding

Pregnancy category

The US FDA designated ondansetron as a category B drug in pregnancy. Studies show that using it does not increase the possibility of congenital disorders.²⁰

Sometimes, doctors prescribe ondansetron off-label for pregnancy-related nausea and vomiting. It may also be used for the same symptoms after delivery by cesarean section.

There are no adequate studies to determine exactly how much of the drug passes through human breast milk. However, limited research has shown no adverse effects on mothers’ milk production or their infants. Your doctor will weigh the potential risks and benefits to decide if you should continue taking ondansetron while nursing.²¹

Missed doses

If you missed a dose of ondansetron, take it as soon as you remember if it is not close to the next one. But it’s best not to take it if it is within two hours of your next dose. If that’s the case, skip the missed dose and resume your regular dosing schedule.

Drug interactions

A number of drugs are known to interact with ondansetron.²²

  • Apomorphine (Apokyn), as taking this drug alongside ondansetron may cause your blood pressure to drop to a dangerously low level, possibly leading to loss of consciousness. It also increases your risk of heart rate disturbances

  • Pain medicines, such as tramadol (Ultram, Conzip), hydromorphone (Dilaudid), hydrocodone (Vicodin, Lortab), meperidine (Demerol), oxycodone (Oxycontin)

  • Levoketoconazole (Recorlev)

  • Antiseizure medicines, like carbamazepine (Tegretol), phenytoin (Dilantin)

  • Lithium

  • Drugs for heart arrhythmias, such as dofetilide (Tikosyn), procainamide (Pronestyl), amiodarone (Pacerone), dronedarone (Multaq), sotalol (Betapace)

  • Antipsychotic drugs, such as pimozide (Orap), thioridazine (Mellaril)

  • Antibiotics, like erythromycin, azithromycin (Zithromax), clarithromycin (Biaxin), linezolid (Zyvox)

  • Antivirals, such as foscarnet (Foscavir)

  • Antidepressants, such as desipramine (Norpramin), imipramine (Tofranil), nortriptyline (Pamelor), trazodone (Desyrel), mirtazapine (Remeron)

  • Sleeping pills, including doxepin (Zonalon) 

  • Other antinausea medicines, like droperidol (Inapsine)

  • Migraine medications, such as eletriptan (Relpax), ergotamine (Ergomar), sumatriptan (Imitrex)

  • Loperamide (Imodium)

  • Drugs that increase serotonin:

  • St. John’s wort, yohimbe

This information on drug interactions is not exhaustive. Inform your doctor of all medications, herbs, and supplements you take regularly or occasionally.

Can I drink alcohol while taking ondansetron?

There are no direct interactions between ondansetron and alcohol, but drinking alcohol may increase the risk of side effects, such as dizziness or drowsiness. Notably, there’s evidence that ondansetron may be helpful in treating alcohol abuse disorder in people with early-onset alcoholism. However, more research is needed to establish the extent of the drug’s benefit (if any).²³ ²⁴

What to discuss with your doctor before starting ondansetron

Ondansetron is available by prescription only, so you’ll need to meet with your doctor before you start taking it. At your appointment, be sure to bring up any concerns you have about your condition or the medication. Additionally, you should discuss the following topics:²⁵ ²⁶

  • Any prescribed and over-the-counter medications you are taking or occasionally take, including herbs, supplements, and vitamins

  • If you have (or have a family history of) long QT syndrome — a condition characterized by an abnormal heart rhythm, or other rhythm disturbances

  • If you’ve ever had low potassium or magnesium levels 

  • If you have liver disease or chronic heart failure, as this may necessitate a dose adjustment

  • If you have had an allergic reaction to ondansetron or any of the medication’s inactive ingredients

  • If you are pregnant or planning a pregnancy, breastfeeding, or planning to breastfeed 

  • If you have phenylketonuria, as the ODT form of ondansetron contains phenylalanine and may be unsuitable for you 

Stopping ondansetron

Ondansetron must be taken precisely as directed by a doctor. If you suddenly stop taking the drug when you need it, you might experience episodes of nausea or vomiting.

Drug approval history

1991: Ondansetron (in its branded form) gains US FDA approval²⁷

2005: The generic form of ondansetron becomes available²⁸

Tips for taking ondansetron

The following tips can help you maximize the safety and effectiveness of ondansetron:

  • You can take your medication with or without food.

  • Do not chew the oral soluble film.

  • If your medication is in a blister pack, don’t open the package until you’re ready to take your dose.

  • Let the ODT or soluble film dissolve in your mouth. Don’t chew it, swallow it whole, or take it with water.

  • Store your medicine at room temperature, and keep it away from heat, light, and moisture.

Frequently asked questions

When should I take ondansetron?

Your doctor will provide you with a detailed dosing schedule. Always follow your doctor’s instructions precisely, and if there’s something you’re unsure about, don’t hesitate to ask.

Can I take ondansetron on an empty stomach?

This drug may be taken with or without food.

Will ondansetron make me sleepy?

Drowsiness and fatigue are common side effects of ondansetron. You should be careful about driving or operating machinery, particularly if you are not used to taking the drug or have recently increased your dose.

If you’re worried about the impact these side effects may have on your daily functioning, discuss your concerns with your doctor.

  1. Zofran (ondansetron hydrochloride) label | U.S. Food and Drug Administration

  2. Ondansetron | MedlinePlus

  3. Zofran (ondansetron hydrochloride) label | U.S. Food and Drug Administration

  4. Ondansetron (2022)

  5. Zofran (ondansetron hydrochloride) label | U.S. Food and Drug Administration

  6. (As above)

  7. (As above)

  8. Ondansetron (Rx) | Medscape

  9. (As above)

  10. Zofran (ondansetron hydrochloride) label | U.S. Food and Drug Administration

  11. Ondansetron (Rx) | Medscape

  12. Zofran - ondansetron | Epocrates

  13. Ondansetron | MedlinePlus

  14. Ondansetron (Rx) | Medscape

  15. Zofran (ondansetron hydrochloride) label | U.S. Food and Drug Administration

  16. Get online poison help now | Poison Help

  17. Zofran (ondansetron hydrochloride) label | U.S. Food and Drug Administration

  18. Anaphylaxis | MedlinePlus

  19. Conclusions and implications for decision or policy making (2014)

  20. Zofran (ondansetron hydrochloride) label | U.S. Food and Drug Administration

  21. Ondansetron (2006)

  22. Zofran (ondansetron hydrochloride) label | U.S. Food and Drug Administration

  23. Ondansetron for reduction of drinking among biologically predisposed alcoholic patients (2000)

  24. Ondansetron (Rx) | Medscape

  25. Ondansetron | MedlinePlus

  26. Zofran (ondansetron hydrochloride) label | U.S. Food and Drug Administration

  27. (As above)

  28. First-time generic approvals: Metaglip, zofran ODT, floxin otic | Medscape

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Here at HealthMatch, we’ve done our best to ensure that the information provided in this article is helpful, up to date, and, most importantly, accurate.

However, we can’t replace the one-to-one advice of a qualified medical practitioner or outline all of the possible risks associated with this particular drug and your circumstances.

It is therefore important for you to note that the information contained in this article does not constitute professional medical or healthcare advice, diagnosis or recommendation of treatment and is not intended to, nor should be used to, replace professional medical advice. This article may not always be up to date and is not exhaustive of all of the risks and considerations relevant to this particular drug. In no circumstances should this article be relied upon without independent consideration and confirmation by a qualified medical practitioner.

Your doctor will be able to explain all possible uses, dosages, precautions, interactions with other drugs, and other potential adverse effects, and you should always talk to them about any kind of medication you are taking, thinking about taking or wanting to stop taking.

Curious about clinical trials?

Access the latest treatments and medications. unavailable elsewhere - entirely free of charge. We make it easy to take part.