This medication is used alone or with other medications to prevent nausea and vomiting caused by cancer drug treatment (chemotherapy) and radiation therapy. It is also used to prevent and treat nausea and vomiting after surgery. It works by blocking one of the body's natural substances (serotonin) that causes vomiting.

Zofran may be used for purposes not listed in this medication guide.

To prevent nausea from chemotherapy, take this medication by mouth usually within 30 minutes before treatment begins. To prevent nausea from radiation treatment, take this medication by mouth 1 to 2 hours before the start of your treatment. To prevent nausea after surgery, take ondansetron by mouth 1 hour before the start of surgery. This medication may be taken with or without food. However, your doctor may tell you not to eat before chemotherapy, radiation, or surgery.

If you are using the liquid form of this medication, use a special measuring spoon or device to measure out your prescribed dose. Do not use a household spoon because you may not get the correct dose.

Take any other doses as directed by your doctor. Ondansetron may be taken up to 3 times a day for 1 to 2 days after your chemotherapy or radiation treatment is finished. If you are taking this medication on a prescribed schedule, take it regularly in order to get the most benefit from it. To help you remember, take it at the same times each day.

Dosage is based on your medical condition and response to therapy. The dosage for children may also be based on age and weight. The usual maximum dose in patients with severe liver problems is 8 milligrams in 24 hours. Take this medication exactly as directed. Do not take more medication or take it more often than prescribed. Ask your doctor or pharmacist if you have questions.

Inform your doctor if your condition does not improve or if it worsens.

You should not use Zofran if you are also using apomorphine (Apokyn).

You should not use Zofran if you are allergic to ondansetron or to similar medicines such as dolasetron (Anzemet), granisetron (Kytril), or palonosetron (Aloxi).

Serious side effects include blurred vision or temporary vision loss (lasting from only a few minutes to several hours), slow heart rate, trouble breathing, anxiety, agitation, shivering, feeling like you might pass out, and urinating less than usual or not at all. Stop taking Zofran and call your doctor at once if you have any of these side effects. Ondansetron may impair your thinking or reactions. Be careful if you drive or do anything that requires you to be alert.

You should not use Zofran if:

• you are also using apomorphine (Apokyn); or
• you are allergic to ondansetron or similar medicines (dolasetron, granisetron, palonosetron).

To make sure Zofran is safe for you, tell your doctor if you have:

• liver disease;
• an electrolyte imbalance (such as low levels of potassium or magnesium in your blood);
• congestive heart failure, slow heartbeats;
• a personal or family history of long QT syndrome; or
• a blockage in your digestive tract (stomach or intestines).

Zofran is not expected to harm an unborn baby. Tell your doctor if you are pregnant.

It is not known whether ondansetron passes into breast milk or if it could harm a nursing baby. Tell your doctor if you are breast-feeding a baby.

Ondansetron is not approved for use by anyone younger than 4 years old.

Headache, lightheadedness, dizziness, drowsiness, tiredness, or constipation may occur. If these effects persist or worsen, notify your doctor promptly.

Remember that your doctor has prescribed this medication because he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.

Tell your doctor right away if any of these unlikely but serious side effects occur: stomach pain, muscle spasm/stiffness, vision changes (e.g., temporary loss of vision, blurred vision).

Get medical help right away if any of these rare but very serious side effects occur: chest pain, slow/fast/irregular heartbeat, severe dizziness, fainting.

This medication may increase serotonin and rarely cause a very serious condition called serotonin syndrome/toxicity. The risk increases if you are also taking other drugs that increase serotonin, so tell your doctor or pharmacist of all the drugs you take (see Drug Interactions section). Get medical help right away if you develop some of the following symptoms: fast heartbeat, hallucinations, loss of coordination, severe dizziness, severe nausea/vomiting/diarrhea, twitching muscles, unexplained fever, unusual agitation/restlessness.

A very serious allergic reaction to this drug is unlikely, but stop taking this medication and seek immediate medical attention if it occurs. Symptoms of a serious allergic reaction include: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing.

This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.

What should I tell my health care provider before I take this medicine?

They need to know if you have any of these conditions:

• heart disease
• history of irregular heartbeat
• liver disease
• low levels of magnesium or potassium in the blood
• an unusual or allergic reaction to ondansetron, granisetron, other medicines, foods, dyes, or preservatives
• pregnant or trying to get pregnant
• breast-feeding

How should I use this medicine?

This medicine is taken by mouth. Follow the directions on your prescription label. Use a specially marked spoon or container to measure your medicine. Ask your pharmacist if you do not have one. Household spoons are not accurate. Take your doses at regular intervals. Do not take your medicine more often than directed.

Talk to your pediatrician regarding the use of this medicine in children. Special care may be needed.

What if I miss a dose?

If you miss a dose, take it as soon as you can. If it is almost time for your next dose, take only that dose. Do not take double or extra doses.

What may interact with this medicine?

Do not take this medicine with any of the following medications:

• apomorphine
• certain medicines for fungal infections like fluconazole, itraconazole, ketoconazole, posaconazole, voriconazole
• cisapride
• dofetilide
• dronedarone
• pimozide
• thioridazine
• ziprasidone

This medicine may also interact with the following medications:

• carbamazepine
• certain medicines for depression, anxiety, or psychotic disturbances
• fentanyl
• linezolid
• MAOIs like Carbex, Eldepryl, Marplan, Nardil, and Parnate
• methylene blue (injected into a vein)
• other medicines that prolong the QT interval (cause an abnormal heart rhythm)
• phenytoin
• rifampicin
• tramadol