Ondansetron is used to prevent nausea and vomiting caused by cancer chemotherapy, radiation therapy, and surgery. Ondansetron is in a class of medications called serotonin 5-HT₃ receptor antagonists. It works by blocking the action of serotonin, a natural substance that may cause nausea and vomiting.

Ondansetron comes as a tablet, a rapidly disintegrating (dissolving) tablet, film, and an oral solution (liquid) to take by mouth. The first dose of ondansetron is usually taken 30 minutes before the start of chemotherapy, 1 to 2 hours before the start of radiation therapy, or 1 hour before surgery. Additional doses are sometimes taken one to three times a day during chemotherapy or radiation therapy and for 1 to 2 days after the end of treatment. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. Take ondansetron exactly as directed. Do not take more or less of it or take it more often than prescribed by your doctor.

Do not chew the film.

If you are taking the rapidly disintegrating tablet, remove the tablet from the package just before you take your dose. To open the package, do not try to push the tablet through the foil backing of the blister. Instead, use dry hands to peel back the foil backing. Gently remove the tablet and immediately place the tablet on the top of your tongue. The tablet will dissolve in a few seconds and can be swallowed with saliva.

This medication may be prescribed for other uses. Ask your doctor or pharmacist for more information.

Before taking ondansetron,

• tell your doctor and pharmacist if you are allergic to ondansetron, alosetron (Lotronex), dolasetron (Anzemet), granisetron (Kytril), palonosetron (Aloxi, in Akynzeo), any other medications, or any of the ingredients in ondansetron products. Ask your pharmacist for a list of the ingredients.
• tell your doctor if you are receiving apomorphine (Apokyn). Your doctor will probably tell you not to take ondansetron if you are receiving this medication.
• tell your doctor and pharmacist what prescription and nonprescription medications, vitamins, nutritional supplements, and herbal products you are taking or plan to take. Be sure to mention any of the following: certain medications for seizures such as carbamazepine (Carbatrol, Epitol, Equetro, Tegretol) or phenytoin (Dilantin); clarithromycin (Biaxin, in Prevpac); erythromycin (E.E.S., Erythrocin, others); fentanyl (Abstral, Actiq, Duragesic, Fentora, Lazanda, Onsolis, Subsys); lithium (Lithobid); medications for irregular heart beat; medications for mental illness; medications to treat migraines such as almotriptan (Axert), eletriptan (Relpax), frovatriptan (Frova), naratriptan (Amerge), rizatriptan (Maxalt), sumatriptan (Imitrex), and zolmitriptan (Zomig); methylene blue; mirtazapine (Remeron); monoamine oxidase (MAO) inhibitors including isocarboxazid (Marplan), linezolid (Zyvox), phenelzine (Nardil), selegiline (Eldepryl, Emsam, Zelapar), and tranylcypromine (Parnate); moxifloxacin (Avelox); selective serotonin reuptake inhibitors (SSRIs) such as citalopram (Celexa), escitalopram (Lexapro), fluoxetine (Prozac, Sarafem, in Symbyax), fluvoxamine (Luvox), paroxetine (Brisdelle, Paxil, Pexeva), and sertraline (Zoloft); and tramadol (Conzip, Ultram, in Ultracet). Your doctor may need to change the doses of your medications or monitor you more carefully for side effects. Many other medications may also interact with ondansetron, so be sure to tell your doctor about all the medications you are taking, even those that do not appear on this list.
• tell your doctor if you or anyone in your family has or has ever had long QT syndrome (condition that increases the risk of developing an irregular heartbeat that may cause fainting or sudden death), or another type of irregular heart beat or heart rhythm problem, or if you have or have ever had low blood levels of magnesium or potassium in your blood, heart failure (HF; condition in which the heart cannot pump enough blood to other parts of the body), or liver disease.
• tell your doctor if you are pregnant, plan to become pregnant, or are breast-feeding. If you become pregnant while taking ondansetron, call your doctor.
• if you have phenylketonuria (PKU, an inherited condition in which a special diet must be followed to prevent damage to your brain that can cause severe intellectual disability), you should know that the orally disintegrating tablets contain aspartame that forms phenylalanine.

Unless your doctor tells you otherwise, continue your usual diet.

Take the missed dose as soon as you remember it. However, if it is almost time for the next dose, skip the missed dose and continue your regular dosing schedule. Do not take a double dose to make up for a missed one.

Ondansetron may cause side effects. Tell your doctor if any of these symptoms are severe or do not go away:

• headache
• constipation
• weakness
• tiredness
• chills
• drowsiness

Some side effects can be serious. If you experience any of the following symptoms, call your doctor immediately or seek emergency medical treatment:

• blurred vision or vision loss
• rash
• hives
• itching
• swelling of the eyes, face, lips, tongue, throat, hands, feet, ankles, or lower legs
• hoarseness
• difficulty breathing or swallowing
• chest pain
• shortness of breath
• dizziness, light-headedness, or fainting
• fast, slow or irregular heartbeat
• agitation
• hallucinations (seeing things or hearing voices that do not exist)
• fever
• excessive sweating
• confusion
• nausea, vomiting, or diarrhea
• loss of coordination
• stiff or twitching muscles
• seizures
• coma (loss of consciousness)

Ondansetron may cause other side effects. Call your doctor if you have any unusual problems while you are taking this medication.

Keep this medication in the container it came in, tightly closed, and out of reach of children. Store the tablets and rapidly disintegrating tablets away from light, at room temperature or in the refrigerator. Store the solution in the bottle upright at room temperature and away from light, excess heat, and moisture (not in the bathroom).

It is important to keep all medication out of sight and reach of children as many containers (such as weekly pill minders and those for eye drops, creams, patches, and inhalers) are not child-resistant and young children can open them easily. To protect young children from poisoning, always lock safety caps and immediately place the medication in a safe location – one that is up and away and out of their sight and reach. http://www.upandaway.org

Unneeded medications should be disposed of in special ways to ensure that pets, children, and other people cannot consume them. However, you should not flush this medication down the toilet. Instead, the best way to dispose of your medication is through a medicine take-back program. Talk to your pharmacist or contact your local garbage/recycling department to learn about take-back programs in your community. See the FDA’s Safe Disposal of Medicines website (http://goo.gl/c4Rm4p) for more information if you do not have access to a take-back program.

In case of overdose, call the poison control helpline at 1-800-222-1222. Information is also available online at https://www.poisonhelp.org/help. If the victim has collapsed, had a seizure, has trouble breathing, or can’t be awakened, immediately call emergency services at 911.

Symptoms of overdose may include:

• sudden loss of vision for a short time
• dizziness or lightheadedness
• fainting
• constipation
• irregular heart beat

Keep all appointments with your doctor.

Do not let anyone else take your medication. Ask your pharmacist any questions you have about refilling your prescription.

It is important for you to keep a written list of all of the prescription and nonprescription (over-the-counter) medicines you are taking, as well as any products such as vitamins, minerals, or other dietary supplements. You should bring this list with you each time you visit a doctor or if you are admitted to a hospital. It is also important information to carry with you in case of emergencies.

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