Ondansetron blocks the actions of chemicals in the body that can trigger nausea and vomiting.

Ondansetron is used to prevent nausea and vomiting that may be caused by surgery or by medicine to treat cancer (chemotherapy).

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

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

Ondansetron is injected into a vein through an IV. A healthcare provider will give you this injection.

Ondansetron is usually given just before your surgery begins, or within 2 hours after surgery.

To prevent nausea and vomiting caused by chemotherapy, ondansetron is given 30 minutes before the start of chemotherapy. A second and third dose of ondansetron are sometimes given 4 hours and 8 hours after the first dose.

You should not use this medicine if you are allergic to ondansetron, or if you are also using apomorphine (Apokyn).

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

• liver disease;

• heart disease, congestive heart failure, a heart rhythm disorder;

• an electrolyte imbalance (such as low levels of potassium or magnesium in your blood);

• a personal or family history of Long QT syndrome; or

• a condition for which you take a diuretic or "water pill."

Ondansetron 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.

Because you will receive ondansetron in a clinical setting, you are not likely to miss a dose.

Since this medicine is given by a healthcare professional in a medical setting, an overdose is unlikely to occur.

This medicine may cause blurred vision and may impair your thinking or reactions. Be careful if you drive or do anything that requires you to be alert and able to see clearly.

Get emergency medical help if you have any of these signs of an allergic reaction: rash, hives; fever, chills, difficult breathing; swelling of your face, lips, tongue, or throat.

Zofran may cause serious side effects. Call your doctor at once if you have:

• headache with chest pain and severe dizziness, fainting, fast or pounding heartbeats;

• blurred vision or temporary vision loss (lasting from only a few minutes to several hours);

• severe constipation, stomach pain, or bloating;

• a light-headed feeling, like you might pass out;

• little or no urination; or

• high levels of serotonin in the body--agitation, hallucinations, fever, fast heart rate, overactive reflexes, nausea, vomiting, diarrhea, loss of coordination, fainting.

Common side effects of Zofran may include:

• diarrhea or constipation;

• drowsiness;

• fever; or

• headache.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Ondansetron can cause a serious heart problem, especially if you use certain medicines at the same time, including antibiotics, antidepressants, heart rhythm medicine, antipsychotic medicines, and medicines to treat cancer, malaria, HIV or AIDS. Tell your doctor about all medicines you use, and those you start or stop using during your treatment with ondansetron.

Receiving ondansetron while you are using certain other medicines can cause high levels of serotonin to build up in your body, a condition called "serotonin syndrome," which can be fatal. Tell your doctor if you also use:

• medicine to treat depression;

• medicine to treat a psychiatric disorder;

• a narcotic (opioid) medication; or

• medicine to prevent nausea and vomiting.

This list is not complete and many other drugs can interact with ondansetron. This includes prescription and over-the-counter medicines, vitamins, and herbal products. Give a list of all your medicines to any healthcare provider who treats you.