Ondansetron, sold under the brand name Zofran among others, is a medication used to prevent nausea and vomiting caused by chemotherapy, radiation therapy, migraines, or surgery. It is also effective for treating gastroenteritis. It can be given orally (by mouth), intramuscularly (injection into a muscle), or intravenously (injection into a vein). Common side effects include diarrhea, constipation, headache, sleepiness, and itchiness. Serious side effects include QT prolongation and severe allergic reaction. It appears to be safe during pregnancy but has not been well studied in this group. It is a serotonin 5-HT3 receptor antagonist. It does not have any effect on dopamine receptors or muscarinic acetylcholine receptor and therefore does not cause akathisia. Ondansetron was patented in 1984 and approved for medical use in 1990. It is on the World Health Organization's List of Essential Medicines. It is available as a generic medication. In 2023, it was the 53rd most commonly prescribed medication in the United States, with more than 12 million prescriptions.
Medical uses Ondansetron is indicated for the prevention of chemotherapy-induced nausea and vomiting and postoperative nausea and vomiting.
Pregnancy Ondansetron is used off-label to treat morning sickness and hyperemesis gravidarum of pregnancy. It is typically used after other antiemetic drugs have failed. A large multi-center cohort study found no association between ondansetron exposure and fetal risk compared to other antiemetics.
Cyclic vomiting syndrome Ondansetron is one of several antiemetics used during the vomiting phase of cyclic vomiting syndrome.
Gastroenteritis Trials in emergency department settings support the use of ondansetron to abort vomiting episodes associated with gastroenteritis and dehydration. A randomized controlled trial using a single dose of oral ondansetron in children with presumably viral gastroenteritis found it to be highly effective in stopping vomiting and increasing the effectiveness of oral rehydration therapy, thereby significantly increasing patient satisfaction. Only 16 of the 123 children treated with ondansetron vomited in the following 6 hours. A retrospective review found that ondansetron was used commonly for vomiting due to gastroenteritis, being administered in over 58% of cases. Its use reduced hospital admissions, but was also associated with higher rates of return visits to the emergency department. Furthermore, people who had initially received ondansetron were more likely to be admitted on the return visit than people who had not received the drug. However, this effect may simply be due to the agent being used more frequently in people who present with more severe illness. Its use was not found to mask serious diagnoses.
Irritable bowel syndrome (IBS) In a study of patients diagnosed as having IBS with diarrhea (IBS-D), ondansetron showed statistically significant effects on stool consistency, frequency, urgency and bloating, but not on pain scores. This was confirmed in a later trial and meta-analysis and is included in international guidelines.
Special populations
Children Ondansetron has rarely been studied in people under 4 years of age. As such, little data is available to guide dosage recommendations. Three open non-comparative studies have been conducted to assess the safety and efficacy of ondansetron in children receiving a variety of chemotherapy regimens. Ondansetron was well tolerated and none of the patients experienced extrapyramidal symptoms.
Veterinary Ondansetron is sometimes prescribed "off label" to relieve vomiting in cats and in dogs, at least in the United States as of 2026.
Adverse effects Headaches are the most common adverse effect. A review of use for post-operative nausea and vomiting found that for every 36 people treated, one would experience headache, which could be severe. Constipation, diarrhea, and dizziness are other commonly reported side effects. It is broken down by the hepatic cytochrome P450 system and it has little effect on the metabolism of other drugs broken down by this system.
QT prolongation Use of ondansetron has been associated with prolongation of the QT interval, which can lead to a potentially fatal heart rhythm known as torsades de pointes. Although this may happen in any person with any formulation, the risk is most salient with the injectable (intravenous) form of the drug and increases with dose. The risk is also higher in people taking other medicines that prolong the QT interval, as well as in people with congenital long QT syndrome, congestive heart failure, and/or bradyarrhythmias. As such, single doses of injectable ondansetron should not exceed 16 mg at one time. (Oral dosing recommendations remain intact, including the recommendation of a single 24 mg oral dose when indicated.) Electrolyte imbalances should be corrected before the use of injectable ondansetron. People are cautioned to seek immediate medical care if symptoms such as irregular heartbeat or palpitations, shortness of breath, dizziness, or fainting occur while taking ondansetron.
Overdose No specific treatment is available for ondansetron overdose; people are managed with supportive measures. An antidote to ondansetron is not known.
Serotonin syndrome In 2012, the World Health Organization and Food and Drug Administration, and Health Canada in 2014, released reports and added a boxed warning, that stated use of ondansetron in conjunction with serotonergic drugs (SSRIs, SNRIs, MAOIs and TCAs) had been linked to rare cases of serotonin syndrome. Although some members of the scientific and medical community believe these reports were released without substantive evidential foundation, and that the studies cited by the FDA, WHO and HC were flawed. They claim there is no clear and convincing evidence that ondansetron is capable of causing serotonin syndrome, nor any well documented cases where serotonin syndrome could be definitively linked to ondansetron use with a serotonergic drug, leading to needless avoidance of ondansetron for patients taking SSRIs, SNRIs and tricyclics.
Pharmacology
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