How Often Can I Take Zofran (Ondansetron) When Nauseous?

Written by
Juhi Modi

Medically reviewed by
HaVy Ngo-Hamilton, Pharm.D.
| Updated Jun 13, 2023

If you are experiencing nausea and vomiting after surgery or cancer treatment, such as chemotherapy and radiation, your doctor may prescribe a medication called ondansetron (brand name: Zofran). Please continue reading to learn more about this medicine, including its uses, dosage, frequency of use, drug interactions, and tips on safe use. 

What is ondansetron (Zofran)?

Ondansetron (Zofran, Zofran ODT) is a prescription medication used to prevent nausea and vomiting for cancer patients after chemotherapy and radiation treatment. This medication is also used to treat nausea after surgery. It is available in various dosage forms, including oral tablets (4 mg, 8 mg, and 24 mg), orally disintegrating tablets and oral films (4 mg and 8 mg), oral solution (4 mg/5 mL), and injection vials (2 mg/mL).

These different dosage forms, such as the orally disintegrating tablets, the oral liquid, and the oral film, help lessen the nauseating feeling that may be caused by swallowing a tablet. Some of the uses of ondansetron (Zofran) may not be listed in this article.

How does Zofran work?

Zofran is an antiemetic that belongs to a group of drugs called 5-HT3 receptor antagonists. It works by blocking the action of serotonin, a natural substance in the body that can trigger nausea and initiate a vomiting reflex.

What is the usual dose of Zofran?

The usual dose of ondansetron (Zofran) in adults and children is as follows: 

Adults

• Severe nausea and vomiting due to chemotherapy: a single 24-mg dose to be taken 30 minutes before the start of a single day of highly emetogenic (vomiting-inducing) chemotherapy.  
• Moderate nausea and vomiting due to chemotherapy: An 8-mg dose is administered 30 minutes before the start of chemotherapy, followed by another 8-mg dose 8 hours later. After the completion of chemotherapy, 8 mg of Zofran should be taken every 12 hours for 1 to 2 days.
• Radiation therapy: An 8-mg ondansetron is taken 1-2 hours before radiation therapy. After the first dose, the regimen of 8 mg every 8 hours is recommended depending on the type and the location of radiation.
• Post-operative (after-surgery) nausea and vomiting: 16 mg of ondansetron is to be administered 1 hour before anesthesia.

  Children:

• Age 4 to 11: For moderate nausea and vomiting due to chemotherapy, take 4 mg of ondansetron (Zofran) 30 minutes before the start of chemotherapy, followed by another 4 mg dose given at 4 and 8 hours after the first dose. After the completion of chemotherapy, take 4 mg of ondansetron (Zofran) every 8 hours for 1 to 2 days.
   
• Age 12 to 17: For moderate nausea and vomiting due to chemotherapy, take 8 mg of ondansetron (Zofran) 30 minutes before the start of chemotherapy, followed by another 8 mg dose given 8 hours after the first dose. After the completion of chemotherapy, take 8 mg of ondansetron (Zofran) every 12 hours for 1 to 2 days.

Precautions:

• People with severe liver disease should not take more than 8 mg of ondansetron in a day.
• Zofran is not approved by the FDA to be used during pregnancy. However, there are studies that show Zofran’s safety in treating morning sickness during the first trimester. You should discuss with your doctor if you are pregnant or breastfeeding. 
• Do not give Zofran to children younger than 4 years old. 

Can I take Zofran again after 4 hours?

In some cases, you may take Zofran again after 4 hours. For example, in children, 4 mg or 8 mg of Zofran is given 30 minutes before the start of chemotherapy. (The dose is 4 mg in children between 4 and 11 years of age and 8 mg in children between 12 and 17 years of age.) This is followed by another dose 4 hours later and then one more dose 8 hours after the first dose. The child is then given one dose every 8 hours for 1-2 days after completion of chemotherapy.

Can you take Zofran every 6 hours?

The usual frequency of Zofran dosing in adults is an 8 mg tablet every 8-12 hours or 2-3 times a day. A scenario where you could take it more frequently would be if the dose taken was less than 8mg. Nevertheless, the sum of the doses taken cannot exceed 8 mg within that time frame. You should not take this medicine more often than prescribed, so leave it to your provider to determine your dose frequency. Talk to your doctor or pharmacist if your nausea and vomiting are not controlled with the prescribed dose and frequency of Zofran. 

Why can you only take Zofran every 8 hours? 

You can only take Zofran every 8 hours because clinical trials have shown that ondansetron (Zofran) increases the risk of QT prolongation. This is an abnormality in the heart’s electrical system. The risk of QT prolongation appears to be dose related. For this reason, doctors do not give intravenous (IV) doses of more than 16 mg or advise taking Zofran more often than every 8 hours when at home and without proper monitoring of a patient’s vitals.

What are the risks associated with Zofran use?

Severe allergic reactions

In a small number of people, Zofran can cause severe allergic reactions with signs and symptoms such as difficulty breathing, swelling of the throat, skin rash, and other effects. Stop taking ondansetron (Zofran) and seek immediate medical attention if you experience symptoms while on this medication to control nausea and vomiting after cancer treatment or surgery.

Serotonin syndrome

Patients taking too much ondansetron (Zofran) can develop a serious and potentially life-threatening condition called serotonin syndrome. This occurs due to high serotonin levels in the body. The risk is higher in people taking other drugs that affect serotonin levels, such as lithium, antidepressants, and migraine medications. Symptoms of serotonin syndrome may include sweating, fast heartbeat, nausea and vomiting, muscle spasms, muscle stiffness, blurred vision, confusion, and fever. Seek emergency medical attention immediately if you develop any of these symptoms while on Zofran.

Heart rhythm problems

High doses of ondansetron (Zofran) can increase the risk of heart rhythm problems, also known as arrhythmias. The risk is higher in people with congenital long QT syndrome or a history of QT prolongation. Taking other medications that cause heart rhythm problems, congestive heart failure, and low potassium or magnesium can also lead to an increased risk. Tell your healthcare provider if you have any heart rhythm abnormalities before starting ondansetron. Contact your doctor if you notice signs and symptoms such as chest pain, palpitations, slow heartbeat, or irregular heartbeat.

Masking of intestinal blockage symptoms

When you take ondansetron to treat nausea and vomiting after abdominal surgery or chemotherapy, the medicine can mask some of the symptoms of intestinal blockage and abdominal bloating, which happen to be nausea and vomiting. Tell your healthcare provider if you are not passing gas or stools. 

Risk to people with phenylketonuria

People with a rare disorder called phenylketonuria cannot break down an amino acid called phenylalanine. High levels of phenylalanine in the body can cause serious health problems. The orally disintegrating tablet of ondansetron (Zofran ODT) contains phenylalanine. Needless to say, consult your doctor before starting Zofran ODT if you have phenylketonuria.

Tips on the safe use of ondansetron (Zofran)

• Give your doctor or pharmacist a complete list of your medications, including prescription drugs, over-the-counter medicines, dietary supplements, and herbal remedies. This can help avoid dangerous drug interactions between ondansetron and another OTC product or prescription medication.
• You can take ondansetron with or without food.
• If you are taking the disintegrating tablet of Zofran, remove the foil backing with dry hands and immediately place the tablet under your tongue.
• Besides common side effects of ondansetron, like headache, tiredness, diarrhea, and constipation, this medicine can also cause dizziness. Do not drive or operate machinery until you know how ondansetron affects you. Drinking alcohol can make these side effects worse.
• In case of a missed dose, take your Zofran dose as soon as you remember. If it is time for the next dose, however, skip the missed dose and take the next dose according to the prescribed schedule. Do not take two doses to make up for a missed dose.

References:

1. https://medlineplus.gov/druginfo/meds/a601209.html
2. https://www.accessdata.fda.gov/drugsatfda_docs/label/2016/020103s035_020605s019_020781s019lbl.pdf
3. https://journals.lww.com/ajnonline/Abstract/2012/10000/The_FDA_Limits_Maximum_IV_Dose_of_Ondansetron. 27.aspx

Ondansetron (Zofran)

How does this medicine work?

Ondansetron (on-dan-se-tron) is a medicine used to treat and prevent nausea and vomiting.

How should I give it?

Ondansetron may be given by mouth as a liquid, pill, or tablet; or in a vein (IV). It may be given in one of these ways:

• as needed
• at regular times
• continuously through an IV

Your child should be awake and alert when taking any medicine by mouth. Follow the checked instructions below:

___ If using the liquid form, draw up the correct amount in the medicine dropper or oral syringe. Give a small squirt of the medicine inside the cheek. To avoid choking, let your child swallow each squirt before giving more.

___ For babies, you may want to mix the medicine with a small amount of formula or breast milk and give it with a bottle nipple before feeding. Do not add the medicine to a whole bottle because if your baby does not finish it, you will not know how much of the medicine was taken.

___ For older children who cannot swallow pills, a special tablet called Zofran ODT (oral disintegrating tablet) is available. Place it under the tongue and it will dissolve.

Other instructions:

Are there any precautions about food or other medicines?

This medicine may be given with or without food.

Check with the doctor, nurse practitioner, or pharmacist before giving any other prescription or non-prescription medicines, herbs, or vitamins.

What should I do if a dose is missed?

If a dose is missed, give it as soon as you remember. Never give a double dose.

If your child vomits a regular dose (not of the ODT type) within 30 minutes after receiving it, give it again. If your child vomits the second dose, do not repeat it again. If your child vomits an ODT dose, do not repeat it. Ondansetron ODT is absorbed into the body right away.

If your child misses or vomits two doses in a row, please call the clinic.

What are the side effects?

Common

• headache
• constipation

Occasional

• fatigue
• change in liver tests (with long-term use)
• diarrhea
• feeling dizzy

When should I call the clinic?

• continued vomiting after 2 doses
• signs of allergic reaction:
  • fever or chills
  • rash or hives
  • wheezing
  • trouble breathing – call 911

What else do I need to know?

You and your child should know the names and doses of all medicines he or she is taking. Share this information with anyone involved in your child’s care. Please remember to bring the medicine container when your child comes to the clinic or emergency department.

Always make sure you have enough medicine on hand. Each time you refill the prescription, check to see how many refills are left. If no refills are left, the pharmacy will need 2 or 3 days to contact the clinic to renew the prescription.

Check the label and the expiration date before giving each dose. Ask your pharmacist what to do with outdated or unused medications. Empty them into the trash if there is no “take-back” program.

Store all medicines in their original containers and away from direct sunlight or heat. Do not store in humid places such as the bathroom. Keep them out of children’s reach; lock up if possible.

If too much or the wrong kind of medicine is taken, call the Poison Control Center toll-free at 1-800-222-1222. If your child is unconscious or has a seizure, call 911.

Questions?

This sheet is not specific to your child but provides general information. If you have any questions, please call the clinic or pharmacy.

Children’s Hospitals and Clinics of Minnesota
Patient/Family Education
2525 Chicago Avenue South
Minneapolis, MN 55404
Last reviewed 8/2015 ©Copyright

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Use of ondansetron in the treatment of children with cancer

Supportive care

Trademarks:

Zofran®

Other names:

Ondansetron Hydrochloride

Often used for:

treatment of nausea and vomiting

Ondansetron is a drug that is used to prevent and treat nausea and vomiting during chemotherapy and radiation therapy, as well as in the postoperative period.

The rate of breakdown of ondansetron in the body may vary. This is determined by the activity of an enzyme called P450 2D6 (CYP2D6). Genetic analysis may be required to determine the quality of the enzymes in a particular patient. The drug may not be effective enough if the rate of breakdown of ondansetron in the patient’s body is higher than normal.

This drug is usually taken 30 minutes before chemotherapy starts.

Oral administration in the form of tablets or absorbable films or tablets

Oral liquid form

Administered intravenously (by drip or injection) in liquid form

• Headache
• Abdominal pain
• Dizziness
• Diarrhea
• Constipation
• Rash
• Increased fatigue or general weakness
• Blurred vision
• Alarm
• Disorders of the heart
• Liver disorders

The listed side effects are not observed in all patients who are prescribed ondansetron. The most common side effects are highlighted in bold, but others are not excluded. Report all possible side effects to your doctor or pharmacist.

Be sure to discuss these and other recommendations with your doctor or pharmacist.

Home use of ondansetron:

• Non-absorbable tablets should be swallowed whole. Do not crush or chew before taking.
• Resorbable tablets or films hold on tongue until completely dissolved. They should not be chewed or swallowed with water or other liquid. Hands must be dry when handling the preparation.
• When taking ondansetron in liquid form, measure the dosage using the measuring device included in the kit.
• Take your dose as soon as possible if you miss it. Do not do this only if there is little time left until the next appointment. In no case do not double the dose at the next dose!
• Store at room temperature. Ondansetron for intravenous administration should be stored in the refrigerator.
• Do not use an expired drug.
• Follow the instructions for disposal of the drug.

Additional information about ondansetron

• Cytochrome P450 2D6 (CYP2D6) and drugs

More about ondansetron

Antiemetics – articles on veterinary medicine from Svoi Doktor specialists

Vomiting is a common clinical sign in small animal veterinary practice. It requires a rational approach to treatment based primarily on suspected or established pathology. A complete and detailed history is required to examine an animal with vomiting. It is most important to distinguish vomiting from regurgitation, since their etiology, on which specific diagnostic tests depend, is different. Regurgitation is a passive process in which undigested food or saliva is ejected spontaneously under the influence of gravity, while vomiting is a reflex accompanied by signs of nausea, increased salivation and contractions of the abdominal muscles.

Maropitant. Trade name Serenia/ Cerenia

This drug acts as an antiemetic by blocking the neurokinin-1 (NK1) receptor. Neurokinin-1 is a neurotransmitter that initiates vomiting from the vomiting center. Although NK1 receptors are involved in other physiological and behavioral responses, no side effects associated with blockade of other receptors have been reported at doses used to control vomiting. Maropitant can inhibit vomiting that is stimulated by both central and peripheral sources mediated by acetylcholine, histamine, dopamine, and serotonin. The NK1 receptor is also involved in pain transmission (via substance P). Blockade of the NK1 receptor may have potential as an adjunctive treatment for some types of pain (eg, visceral pain), but there are currently no clinical studies demonstrating an analgesic effect from maropitant. [1], [2], [3], [7]

The drug effectively suppresses vomiting from chemotherapy, gastrointestinal diseases, intoxication, kidney disease, vestibular irritants (sickness). [4]

Adverse reactions: pain reaction during injection. Very high doses in cats can cause hemolysis. The injections should not be mixed with any other agent as there are no compatibility studies.

Maropitant is available as 16, 24, 60, or 160 mg tablets and 10 mg/mL injection. However, in the Russian Federation the tablet form is not available.

Dosage: dogs, cats: 1 mg/kg daily or 2 mg/kg orally every 24 hours. To prevent motion sickness in foreign literature, tablets are used at a dose of 8 mg / kg per day for a maximum of 2 days.

Metoclopramide. Trade name Cerucal/Reglan/Maxolon

Antiemetic and prokinetic drug. Metoclopramide stimulates upper gastrointestinal motility and is a centrally acting antiemetic. Among the proposed mechanisms are stimulation of 5-HT (serotonin) receptors or increased release of acetylcholine in the gastrointestinal tract. Affinity for 5-HT4 receptors is low compared to other more effective motility-modifying drugs. It inhibits dopamine-induced relaxation of the stomach, thus enhancing the cholinergic responses of gastric smooth muscle to increase motility. It also increases the tone of the lower esophageal sphincter. Metoclopramide acts centrally by inhibiting dopamine, which is responsible for the antiemetic effects. The antiemetic effect is achieved through its antidopamine (D2) action.

The main effect in dogs is its antiemetic properties. Because this drug transiently increases prolactin secretion, there has been interest in its use for the treatment of agalactia in animals, but its efficacy has not been determined. [5]

Adverse effects are associated with blockade of central dopaminergic receptors. Use in patients with epilepsy or diseases caused by obstruction of the gastrointestinal tract is unacceptable. [6]

Most commonly used for general antiemetic purposes, but doses up to 2 mg/kg have been used to prevent vomiting during cancer chemotherapy (higher doses may cause antiserotonin effects). [3]

Metoclopramide is available as 5 and 10 mg tablets, 1 mg/ml oral solution and 5 mg/ml injection in 2, 10 and 30 ml vials.

Dosage: 0.2-0.5 mg/kg every 6-8 hours IV, IM or orally. For antiemetic treatment in cancer chemotherapy, the dose used is up to 2 mg/kg per day.

Ondansetron. Trade name Latran/Emetron/Zofran/ Zofran

Ondansetron is an antiemetic drug in a class of drugs called serotonin antagonists. Like other drugs in this class, ondansetron works by inhibiting serotonin type 3 (5-HT3) receptors.

In cats, oral absorption is 32% from oral administration and 75% from subcutaneous administration. In dogs, it is much less bioavailable (less than 10%) after oral administration and has a shorter half-life of 30 minutes, raising questions about the clinical efficacy of ondansetron in dogs.

Only limited information is available on the efficacy of ondansetron in animals, but ondansetron has been found to be effective for treating chemotherapy vomiting in animals.