How many mg of Zofran is too much?

Zofran is the brand name for the generic drug ondansetron, which is used to prevent nausea and vomiting caused by surgery, chemotherapy, or radiation treatments for cancer. It works by blocking serotonin receptors in the brain and gut which can trigger nausea and vomiting.

What is Zofran?

Zofran was originally developed by GlaxoSmithKline and approved by the FDA in 1991 for preventing nausea and vomiting caused by chemotherapy. It has since also been approved for preventing and treating nausea and vomiting after surgery.

Zofran comes as a tablet, orally disintegrating tablet, liquid solution, and as an injection given intravenously. The injection form is given in hospitals right before and after chemotherapy to help prevent acute nausea and vomiting.

The orally disintegrating tablets or liquid solution can be taken at home to help control nausea and vomiting between chemotherapy treatments. Zofran tablets and liquid typically need to be taken multiple times per day.

How Does Zofran Work?

Zofran is a serotonin 5-HT3 receptor antagonist. It works by blocking serotonin receptors called 5-HT3 receptors in the brain and gut.

Activation of 5-HT3 receptors in the brain can trigger nausea and vomiting. The receptors send signals to the vomiting center in the brain via the vagus nerve, which can induce vomiting.

5-HT3 receptors in the gut can also detect toxic substances and trigger nausea signals to the brain. By blocking these receptors, Zofran helps interrupt the nausea and vomiting process.

Uses of Zofran

The main uses of Zofran are:

  • Preventing nausea and vomiting caused by cancer chemotherapy
  • Preventing nausea and vomiting after surgery
  • Treating breakthrough nausea and vomiting that occurs despite preventive medications

Zofran can help control nausea and vomiting in over 50% of people receiving chemotherapy or radiation therapy for cancer. It is considered highly effective for controlling acute and delayed nausea from these treatments.

For surgery, Zofran is given intravenously right before an operation to help prevent post-operative nausea and vomiting (PONV). Around 30% of surgical patients experience PONV, which Zofran can significantly reduce.

Dosage Information

The dose of Zofran depends on the form given (oral, injection, or liquid) and the condition it is treating. Some general dosage guidelines include:

  • Injection dose: 0.15 mg/kg given 30 minutes before chemotherapy, then every 4 hours as needed. Maximum daily dose is 32 mg.
  • Tablet dose: Starting dose is typically 8 mg taken 2-3 times per day. Maximum daily dose is 24-32 mg.
  • Liquid dose: Starting dose is usually 4 mg (5 mL), taken 3 times a day. Maximum daily dose is 20 mg (25 mL).

Higher doses may be used in certain situations under medical supervision. Your doctor will determine the appropriate dose based on your treatment plan and response.

Side Effects

Zofran is generally well tolerated, but some potential side effects can occur. The most common side effects include:

  • Headache
  • Fatigue
  • Constipation
  • Diarrhea
  • Drowsiness

More serious side effects are rare but can include irregular heart rhythms, muscle spasms, and serotonin syndrome (a dangerous reaction caused by too much serotonin).


There are some precautions to be aware of with Zofran:

  • May prolong the QT interval on an EKG which can increase the risk of irregular heartbeats.
  • Use cautiously in people with heart conditions, electrolyte abnormalities, or congenital long QT syndrome.
  • Can interact with other medications that prolong the QT interval.
  • May cause drowsiness, so caution is needed when driving or operating machinery.
  • Should be avoided in the first trimester of pregnancy due to potential risks to the fetus.

Talk to your doctor about any medical conditions you may have and all medications you are taking before using Zofran. Dosage adjustments or extra monitoring may be needed in some situations.

Drug Interactions

Some medications that can interact with Zofran and may require dosage adjustments include:

  • Antibiotics like erythromycin and ketoconazole
  • Antidepressants like fluoxetine, paroxetine, sertraline
  • Antiarrhythmics like amiodarone and dronedarone
  • Antipsychotics like chlorpromazine, haloperidol, ziprasidone
  • Opioid pain medications like methadone

This is not a complete list. Let your doctor know about all medications you are taking before starting Zofran.


Some important warnings for Zofran use include:

  • Can cause severe constipation which may require treatment.
  • Rarely, may cause serotonin syndrome when taken with other serotonergic drugs.
  • Has been linked to a small increased risk of birth defects when taken in early pregnancy.
  • Use caution if you have phenylketonuria (PKU), as Zofran contains phenylalanine.
  • Intravenous form contains sodium bisulfite which can cause allergic reactions.

Talk to your doctor about any concerns or allergies you have before taking Zofran.

Who Should Not Take Zofran?

You should not take Zofran if you:

  • Have congenital long QT syndrome
  • Have an allergy or hypersensitivity to ondansetron
  • Are taking apomorphine (a Parkinson’s drug)
  • Are taking any medications that prolong the QT interval

Women who are pregnant or breastfeeding should use caution and speak to their doctor before taking Zofran.


Zofran overdose can be serious and lead to cardiac arrhythmias, seizures, and death in severe cases. Seek immediate medical attention if you suspect an overdose.

Symptoms of overdose may include:

  • Blurred vision
  • Severe drowsiness
  • Fainting or loss of consciousness
  • Seizures
  • Irregular heartbeat and EKG changes

To prevent overdose, carefully follow dosage directions and never take more Zofran than prescribed. Do not mix with alcohol or other medications that can cause drowsiness or slowed breathing.

Alternative Medications

Some alternatives to Zofran for nausea and vomiting include:

  • Phenergan (promethazine)
  • Kytril (granisetron)
  • Aloxi (palonosetron)
  • Emend (aprepitant)
  • Compazine (prochlorperazine)
  • Tigan (trimethobenzamide)
  • Anzemet (dolasetron)

Talk to your doctor about whether an alternative medication may be appropriate for your situation.

When to See a Doctor

See your doctor or seek medical help if you experience:

  • Severe or ongoing nausea, vomiting, or diarrhea
  • Dizziness, weakness, or irregular heartbeat
  • Signs of allergic reaction such as rash, itching, or swelling
  • Serotonin syndrome symptoms like high fever, confusion, or rigid muscles
  • Suicidal thoughts or depression

Routine monitoring with your doctor is important while taking Zofran to watch for any side effects or complications.


In summary, Zofran is an effective medication used to prevent and treat nausea and vomiting from chemotherapy, radiation, or surgery. It works by blocking serotonin receptors that can trigger the vomiting reflex.

Zofran dosage varies depending on the form and condition being treated. Side effects are usually mild but more serious reactions are possible in some cases.

Use caution if you have heart issues, electrolyte disturbances, or take other QT-prolonging drugs. Never take more Zofran than prescribed and speak to your doctor about any concerns.

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