This medication is used alone or with other medications to prevent nausea and vomiting caused by cancer chemotherapy. 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.

To prevent nausea from chemotherapy, ondansetron is given by injection into a vein as directed by your doctor, usually over 15 minutes. This drug is usually started 30 minutes before your chemotherapy treatment begins. Do not give this medication any faster than recommended or take more than the recommended dose because this may increase the chance of serious side effects such as blurred vision or slowed/irregular heartbeat. The usual maximum single dose is 16 milligrams. Older adults are usually prescribed a lower dose to decrease the risk of side effects. Depending on the type of chemotherapy treatment you are receiving, a second and third dose may be given 4 and 8 hours after your first dose. You may also be switched to a medication you can take by mouth.

Your doctor may direct you to continue this medication for 1 or 2 days after your chemotherapy treatment. If you are using this medication on a prescribed schedule, use it regularly to get the most benefit from it. To help you remember, use it at the same times each day. Use this medication exactly as directed. Do not use more medication or use it more often than prescribed. Ask your doctor or pharmacist if you have questions.

If you are using this medication at home, learn all preparation and usage instructions from your health care professional. Vials may sometimes have particles near the top of the vial. If particles are present, shake the vial to try to dissolve the particles. Before using, check this product for particles or discoloration. If either is present, do not use the liquid. Learn how to store and discard medical supplies safely.

To prevent or treat nausea after surgery, a single dose of ondansetron is given by injection into a muscle or into a vein shortly before the start of surgery, during surgery, or after surgery by a health care professional.

The dosage is based on your weight, medical condition, age, and response to treatment.

Tell your doctor if your condition does not get better or if it gets worse.

Headache, fever, lightheadedness, dizziness, drowsiness, tiredness, constipation, or redness/pain/burning at the injection site may occur. If these effects last or get worse, tell your doctor promptly.

Remember that this medication has been prescribed because your doctor 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 you have any serious side effects, including:

• stomach pain
• muscle stiffness/spasm
• vision changes (such as temporary loss of vision, blurred vision, uncontrollable eye movements)

Get medical help right away if you have any very serious side effects, including:

• 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 rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including:

• 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.

In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch.

In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

Before using ondansetron, tell your doctor or pharmacist if you are allergic to it; or to other serotonin blockers (such as granisetron); or if you have any other allergies. This product may contain inactive ingredients, which can cause allergic reactions or other problems. Talk to your pharmacist for more details.

Before using this medication, tell your doctor or pharmacist your medical history, especially of:

• irregular heartbeat
• liver disease
• stomach/intestinal problems (such as recent abdominal surgery, ileus, swelling)

Ondansetron may cause a condition that affects the heart rhythm (QT prolongation). QT prolongation can rarely cause serious (rarely fatal) fast/irregular heartbeat and other symptoms (such as severe dizziness, fainting) that need medical attention right away.

The risk of QT prolongation may be increased if you have certain medical conditions or are taking other drugs that may cause QT prolongation. Before using ondansetron, tell your doctor or pharmacist of all the drugs you take and if you have any of the following conditions:

• certain heart problems (heart failure, slow heartbeat, QT prolongation in the EKG)
• family history of certain heart problems (QT prolongation in the EKG, sudden cardiac death)

Low levels of potassium or magnesium in the blood may also increase your risk of QT prolongation. This risk may increase if you use certain drugs (such as diuretics/"water pills") or if you have conditions such as severe sweating, diarrhea, or vomiting. Talk to your doctor about using ondansetron safely.

This drug may make you dizzy or drowsy or blur your vision. Alcohol or marijuana (cannabis) can make you more dizzy or drowsy. Do not drive, use machinery, or do anything that needs alertness or clear vision until you can do it safely. Limit alcoholic beverages. Talk to your doctor if you are using marijuana (cannabis).

Infants younger than 5 months may be more sensitive to the effects of this drug, especially diarrhea.

Older adults may be more sensitive to the side effects of this drug, especially QT prolongation (see also How To Use section).

During pregnancy, this medication should be used only when clearly needed. It may harm an unborn baby. Discuss the risks and benefits with your doctor.

It is unknown if this drug passes into breast milk. Consult your doctor before breastfeeding.

Drug interactions may change how your medications work or increase your risk for serious side effects. This document does not contain all possible drug interactions. Keep a list of all the products you use (including prescription/nonprescription drugs and herbal products) and share it with your doctor and pharmacist. Do not start, stop, or change the dosage of any medicines without your doctor's approval.

Some products that may interact with this drug include:

• apomorphine
• tramadol

The risk of serotonin syndrome/toxicity increases if you are also taking other drugs that increase serotonin. Examples include street drugs such as MDMA/"ecstasy," St. John's wort, certain antidepressants (including SSRIs such as fluoxetine/paroxetine, SNRIs such as duloxetine/venlafaxine), among others. The risk of serotonin syndrome/toxicity may be more likely when you start or increase the dose of these drugs.

If someone has overdosed and has serious symptoms such as passing out or trouble breathing, call 911. Otherwise, call a poison control center right away. US residents can call 1-800-222-1222. Canada residents can call 1-844-764-7669. Symptoms of overdose may include: change in vision, severe dizziness, irregular heartbeat.

Lab and/or medical tests (such as EKG) should be done while you are using this medication. Keep all medical and lab appointments. Consult your doctor for more details.

It is important that the dose be given before cancer chemotherapy as directed. If you miss a dose, ask your doctor or pharmacist for a new dose schedule. If you are using this medication on a regular schedule and you miss a dose, use it as soon as you remember. If it is near the time of the next dose, skip the missed dose. Use your next dose at the regular time. Do not double the dose to catch up.

Consult the product instructions and your pharmacist for storage details. Keep all medications away from children and pets.

Do not flush medications down the toilet or pour them into a drain unless instructed to do so. Properly discard this product when it is expired or no longer needed. Consult your pharmacist or local waste disposal company.