Zofran dosage, forms, and strengths

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The dosages listed are general guidelines; your healthcare provider may adjust your dose based on individual health factors.

Ondansetron is a prescription drug FDA-approved to prevent nausea and vomiting in people undergoing chemotherapy, radiation therapy, or surgery. It works as a 5-HT3 receptor antagonist, blocking serotonin in the brain and gut to calm the vomiting reflex.

Doses are typically taken by mouth before chemotherapy or radiation and may continue for one to two days following the procedure. Oral doses can be taken with or without food.

Ondansetron forms and strengths

Ondansetron is available in various dosage forms, including regular tablets, orally disintegrating tablets (ODTs), and oral solutions. In a hospital or outpatient clinic, ondansetron may be given as an injectable solution either intravenously (IV) or intramuscularly (IM). 

• Tablet: 4 mg, 8 mg, and 24 mg

• Orally disintegrating tablet: 4 mg and 8 mg

• Oral solution: 4 mg per 5 mL

• Injectable solution: 2 mg per mL

Brand names of ondansetron include Zofran and Zofran ODT. However, it is currently available as a generic drug only.

Ondansetron dosage for chemotherapy-induced nausea and vomiting

Ondansetron can be taken by mouth or given as an IV injection before chemotherapy for the prevention of nausea and vomiting. Some chemotherapy drugs, such as cisplatin, are considered highly emetogenic, meaning they often cause severe nausea and vomiting. In these cases, a single oral dose of 24 mg is administered 30 minutes before treatment. For chemotherapy that causes moderate nausea and vomiting, the typical oral regimen is 8 mg before chemotherapy, followed by 8 mg every eight hours for one to two days afterward.

Ondansetron dosage for radiotherapy-induced nausea and vomiting

Ondansetron is also FDA-approved for nausea and vomiting caused by radiation therapy, particularly when radiation is directed to the abdomen. Because much of the digestive system is located in this area, abdominal radiation often leads to nausea and vomiting. Radiation treatment to the abdomen can involve a single high dose of radiation, lower-dose fractions of radiation administered over several sessions, or total body irradiation before procedures like a bone marrow transplant. 

For abdomen-only radiotherapy, a dose of 8 mg is given before the treatment. Ondansetron can then be taken in 8 mg doses every eight hours for one or two days after the procedure. 

Total body irradiation is different. It typically requires two to three daily radiotherapy sessions, spread over three to four days. In such cases, ondansetron is administered as an 8 mg dose one to two hours before each radiotherapy session. 

Ondansetron dosage for postoperative nausea and vomiting

Ondansetron may be given if a surgeon expects nausea and vomiting as an adverse reaction to surgery. For prevention, the standard oral regimen consists of a single 16 mg dose administered one hour before anesthesia. Alternatively, an intravenous dose of 4 mg may be administered immediately before or after anesthesia, or at the conclusion of surgery.

Ondansetron is not used before all surgeries. It’s only used for those who have an increased risk of causing nausea and vomiting. For other surgeries, a person might wake up feeling nauseated or begin vomiting. In such cases, a healthcare professional may administer an ondansetron injection to alleviate symptoms quickly. 

Personalized dosing considerations

People with severe liver disease (hepatic impairment) may be restricted to a lower dose of 8 mg of ondansetron per day. People with kidney disease (renal impairment) or other medical conditions generally do not require a dosage adjustment. 

Ondansetron safety info

Ondansetron is an effective antiemetic but may not be right for everyone. 

Ondansetron has contraindications for people with known allergies to any of the drug’s ingredients. It should also be avoided in anyone receiving Apokyn (apomorphine), as this combination can cause sudden low blood pressure (hypotension) and loss of consciousness.

Other possible drug interactions include:

• Antidepressants and other medicines that affect serotonin levels, such as SSRIs, SNRIs, and MAOIs

• Medications that affect heart rhythm or can cause irregular heartbeats

• Drugs that change how the liver breaks down ondansetron, such as phenytoin, carbamazepine, or rifampin

• Ultram (tramadol), which may not work as well when taken with ondansetronOndansetron may cause serious problems, including:

• Heart rhythm changes (QT interval prolongation), especially in individuals with heart failure, electrolyte imbalances, long QT syndrome, and those taking other medications that affect the heart. 

• Severe allergic reactions like rash, trouble breathing, or swelling of the face or throat

• Serotonin syndrome, a reaction that can cause confusion, restlessness, fever, sweating, muscle stiffness, or seizures, especially if combined with certain antidepressants or pain medicines

• Chest pain or tightness caused by reduced blood flow to the heart (rare, usually with IV use)

• Masking of bowel blockage, since ondansetron may cover up symptoms like severe bloating, stomach pain, or not being able to pass gas

People with phenylketonuria (PKU) should avoid the orally disintegrating tablets because they contain aspartame.

Ondansetron is sometimes prescribed during pregnancy to treat severe morning sickness. While many healthcare providers consider it safe, some uncertainty remains due to the limited available clinical trials. Its safety during lactation or breastfeeding is not fully known.

How to take ondansetron

Ondansetron can be taken orally as a regular tablet, an orally disintegrating tablet, or an oral solution. All oral formulations can be taken with or without food. 

Tablet

• Take ondansetron tablets with a full glass of water.

• Store tablets at room temperature (36°F to 86°F) in a light-resistant container.

Orally disintegrating tablet 

• Do not open the blister pack containing the tablet until you are ready to take it.

• Do not push the tablet through the foil backing to remove it.

• With dry hands, peel back the foil backing of one blister and gently remove the tablet.

• Place the tablet on the tongue. Allow the tablet to dissolve fully. It will dissolve in seconds.

• Swallow what’s left of the tablet with saliva.

• Store ondansetron orally-disintegrating tablets in their original package at room temperature (36°F to 86°F).

Oral solution

• Always use the calibrated oral syringe when measuring doses.

• Ensure the bottles are kept upright and stored between 59°F and 86°F.

• To protect the oral liquid form from light exposure, keep the bottles inside their cartons.

Ondansetron dosage FAQs

How long does it take ondansetron to work?

Ondansetron typically begins to take effect within 30 minutes to an hour. It reaches peak levels in the blood at around one and a half hours after oral administration. 

How long does ondansetron stay in your system?

The half-life of an 8-mg tablet or IV dose of ondansetron is around three to four hours in adults. That means it can take the body up to 20 hours to completely clear a dose. It may take longer in older adults with decreased liver function. 

Why do you push ondansetron slowly?

When administered by IV, ondansetron is injected slowly to minimize the risk of adverse effects, such as dizziness, flushing, or headache. In adults, smaller doses (such as 4–8 mg) are typically administered over a period of two to five minutes. Higher doses, if used, are given by infusion over about 15 minutes. 

What happens if I miss a dose of ondansetron?

If you miss a dose of ondansetron, take it as soon as you remember unless it's almost time for the next dose. In that case, skip the missed pill and continue the regular dosing schedule. Do not take a double dose to make up for the missed one. If you're unsure what to do, consult your doctor or pharmacist for medical advice on managing additional doses.

Can you overdose on ondansetron?

Yes, it is possible to overdose on ondansetron. An ondansetron overdose can be a serious medical emergency that can cause blurred vision, constipation, low blood pressure, and fainting. Children who overdose on ondansetron are especially vulnerable to serotonin syndrome, a potentially dangerous condition that can cause drowsiness, rapid heart rate, increased sweating, and seizures. Seek immediate medical help if you suspect an overdose. 

Sources

• Zofran highlights of prescribing information, U.S. Food and Drug Administration (2025)

• Common side effects from radiation therapy, Fox Chase Cancer Center (n.d.)

• Total body irradiation for hematopoietic stem cell transplantation: what can we agree on?, Current Oncology (2021)

• Ondansetron injection solution, DailyMed (2025)

• Ondansetron use in pregnancy, Obstetrics & Gynecology (2021)

• Ondansetron, StatPearls (2023)

• Ondansetron clinical pharmacokinetics, Clinical Pharmacokinetics (1995)

• Ondansetron, MedlinePlus (2024)