Ondansetron

What it is

Ondansetron is a selective serotonin 5-HT3 receptor antagonist used to prevent and treat nausea and vomiting. It comes as tablets, orally disintegrating tablets, oral film, oral solution and injection. It was developed for chemotherapy-induced nausea and vomiting and is now also approved for radiotherapy-induced and postoperative nausea and vomiting, and used very widely off-label in emergency departments and in pregnancy.

What the research says

Ondansetron blocks the serotonin signal that vomiting is triggered by. Its approved uses rest on the chemotherapy, radiotherapy and postoperative trial programmes. Off-label, the best-quantified use is childhood gastroenteritis, where a meta-analysis found more children stopped vomiting in the early hours (RR 1.33, number needed to treat about 5, with an internally inconsistent confidence interval in the source) and fewer needed intravenous fluids (RR 0.42, NNT about 7), but with no significant difference in vomiting by 24 hours and significantly more diarrhoea in the ondansetron groups, and the reviewers concluded the evidence did not support routine use. It prolongs the QT interval in a dose-related way. In pregnancy, a 1.8-million-pregnancy cohort found no association with cardiac malformations but a small increase in oral clefts.

Evidence grade: Well established.

How it works

Drug class: Selective serotonin 5-HT3 receptor antagonist (antiemetic)

When the gut lining is irritated or damaged, for example by chemotherapy or infection, it releases serotonin, which switches on 5-HT3 receptors on the vagus nerve and in the vomiting centres of the brainstem. Ondansetron sits on those 5-HT3 receptors and blocks that signal, so the message to be sick is not sent. The same receptors in the gut are involved in moving the bowel, which is why constipation is a common effect. (Source 1)

What it is used for

• A 2023 systematic review of registered clinical trials identified 13 trials of ondansetron prophylaxis in adult cancer patients and reported effective management of chemotherapy-induced nausea and vomiting; it did not pool absolute effects, which is a real limitation of the evidence we could reach. The label sets different regimens for the two levels of emetogenicity: a single 24 mg dose before single-day HIGHLY emetogenic chemotherapy, and 8 mg before MODERATELY emetogenic chemotherapy with a second 8 mg dose 8 hours later and then 8 mg twice daily for 1 to 2 days. Evidence: established. (Source 2)
• An approved indication covering total body irradiation and abdominal radiotherapy. We could not reach a systematic review or outcome trial for this indication, so it is recorded here as the regulator's position with its date and not as evidence. Evidence: unknown. (Source 3)
• An approved indication. The Cochrane network meta-analysis that quantifies it (Weibel 2020) would not load for us despite repeated attempts, so we have not recorded an effect size; this is the weakest-sourced use in this entry. Evidence: unknown. (Source 3)
• A meta-analysis of 4 randomised trials (n=490) found more children stopped vomiting in the early hours (RR 1.33, NNT 5) and fewer needed intravenous rehydration (RR 0.42, 95% CI 0.27 to 0.67, NNT 7), but there was no significant difference in vomiting by 24 hours, two studies reported significantly more diarrhoea at 24 hours with no numbers given, and the reviewers said the evidence was insufficient to recommend routine use. The confidence interval printed for the vomiting result (1.33 to 1.50 around an estimate of 1.33) is internally inconsistent in the source. Evidence: limited. (Source 4)
• Very widely used off-label. A cohort of 1,816,414 pregnancies found no association with cardiac malformations or overall malformations but a small increase in oral clefts (adjusted RR 1.24, 95% CI 1.03 to 1.48; 2.7 extra cases per 10,000 births). This is observational, so it shows association and not cause. Evidence: disputed. (Source 5)

Interactions

• Apomorphine (label): The combination is contraindicated: it can cause profound low blood pressure and loss of consciousness. (Source 6)
• SSRIs, SNRIs and other serotonergic drugs (case reports): Serotonin syndrome has been described when 5-HT3 blockers are combined with other serotonergic drugs, and the label names both SSRIs and SNRIs; a pharmacology review says the evidence is not strong enough to call it causal. The label tells prescribers to monitor for it and to stop ondansetron if it appears. (Source 7)
• Phenytoin, carbamazepine and rifampin (pharmacokinetic study): These strong CYP3A4 inducers speed up the breakdown of ondansetron, so blood levels fall. The label adds that on the data available it does not recommend any dosage adjustment for people taking them. (Source 8)
• St John's wort (theoretical): St John's wort is a well-known CYP3A4 inducer, so on the same reasoning as phenytoin and rifampin it would be expected to lower ondansetron levels. We found no study testing this specific pair, so this is an extrapolation from the labelled CYP3A4 class effect, not a measured interaction. (Source 8)

Stopping it

• We found no trial, case series or review describing dependence, a withdrawal syndrome or rebound nausea after stopping ondansetron. It is studied and labelled as short prophylactic courses tied to a chemotherapy cycle, a course of radiotherapy or an operation, rather than as continuous long-term therapy, which is part of why a discontinuation literature does not exist. (Source 3)
• In overdose there is no reversal agent, so stopping the drug and supportive care is the whole of the management described. (Source 9)

What goes wrong

Intravenous ondansetron measurably prolongs the QT interval, more so at 8 mg than 4 mg. (Source 10)

• Cohort study, Low certainty.
• Size: 106 emergency department patients.
• Who: adult emergency department patients given 4 mg or 8 mg intravenous ondansetron.
• How long: QT measured at baseline and 60 minutes after the dose.
• Result: mean QTc rise 54.7 msec (SD 25.1) at 60 minutes; 28 patients (26.4%) exceeded QTc 480 msec despite normal baseline; no adverse cardiac events observed during the emergency department stay. Prospective observational, single centre, no control group.
• Funding: not stated.

Moreover, 8 mg doses were associated with higher rates of QTc60 prolongation, while 4 mg doses favored maintaining QTc60 within normal limits.

In a US Medicaid population, first-trimester ondansetron was associated with a small increase in oral clefts but not with cardiac or overall malformations. (Source 5)

• Cohort study, Low certainty.
• Size: 1,816,414 pregnancies, of which 88,467 (4.9%) exposed during organogenesis.
• Who: US Medicaid-covered pregnancies, 2000 to 2013.
• How long: first-trimester exposure, outcomes at birth.
• Result: cardiac malformations adjusted RR 0.99 (95% CI 0.93 to 1.06), risk difference -0.8 per 10,000 births; oral clefts adjusted RR 1.24 (95% CI 1.03 to 1.48), risk difference 2.7 per 10,000 births; any congenital malformation RR 1.01 (0.98 to 1.05), risk difference 5.4 per 10,000. Observational: residual confounding by indication cannot be excluded, so this is association, not cause.
• Funding: not stated in the abstract text we read.

Among offspring of mothers enrolled in Medicaid, first-trimester exposure to ondansetron was not associated with cardiac malformations or congenital malformations overall after accounting for measured confounders, but was associated with a small increased risk of oral clefts.

Common adverse effects of 5-HT3 antagonists including ondansetron are headache, fatigue, malaise and constipation, at rates in the high single to mid double digits. (Source 11)

• Expert review, not systematic, Low certainty.
• Size: not stated; a summary of class data rather than a pooled analysis.
• Who: patients receiving 5-HT3 receptor antagonists.
• How long: not stated.
• Result: headache 9% to 27%, fatigue 9% to 13%, malaise 9% to 13%, constipation 6% to 11%. These are incidence ranges as reported, not drug-minus-placebo differences, so they overstate drug-attributable harm.
• Funding: not stated.

Headache (9% to 27%), Fatigue (9% to 13%), Malaise (9% to 13%), Constipation (6% to 11%).

The label's position is that ondansetron must be avoided in congenital long QT syndrome, and that ECG monitoring is recommended where electrolytes are abnormal, in congestive heart failure, in bradyarrhythmias, and where other QT-prolonging medicines are being taken. (Source 12)

• Official position, Certainty not rated.
• Size: not applicable.
• Who: not applicable.
• How long: label listing updated January 30, 2024.
• Result: a regulatory warning, not an effect size.
• Funding: not applicable.

Avoid ondansetron in patients with congenital long QT syndrome. ECG monitoring is recommended in patients with electrolyte abnormalities (e.g., hypokalemia or hypomagnesemia), congestive heart failure, bradyarrhythmias, or patients taking other medicinal products that lead to QT prolongation.

What the evidence supports

Trials of ondansetron prophylaxis in adult cancer patients report effective control of chemotherapy-induced nausea and vomiting. (Source 2)

• Systematic review, Low certainty.
• Size: 13 of 23 identified registered clinical trials in adult cancer patients.
• Who: adults receiving chemotherapy.
• How long: varied by trial; not pooled.
• Result: no pooled effect size, confidence interval or absolute benefit is reported. The review searched ClinicalTrials.gov registry records rather than the published trial literature, which limits what can be concluded.
• Funding: not stated.

The collective findings from these trials showed an effective management of CINV using ondansetron.

In children with acute gastroenteritis, a meta-analysis found vomiting stopped in more children in the early hours and fewer needed intravenous fluids, with absolute benefits of roughly one in five and one in seven treated; by 24 hours the difference in vomiting was no longer significant. (Source 4)

• Meta-analysis, Low certainty.
• Size: up to 490 children across 4 randomised controlled trials; 466 for vomiting cessation, 359 for intravenous rehydration.
• Who: children in emergency departments with vomiting from acute gastroenteritis.
• How long: early hours after dosing, with 24-hour follow-up.
• Result: cessation of vomiting in the first hours RR 1.33 (reported 95% CI 1.33 to 1.50, p<0.00001; 3 studies, n=466), NNT 5 (95% CI 4 to 8); no significant difference for cessation of vomiting within 24 hours, RR 1.22 (95% CI 0.89 to 1.67; 2 studies, n=144); intravenous rehydration RR 0.42 (95% CI 0.27 to 0.67, p=0.0003; 2 studies, n=359), NNT 7 (95% CI 5 to 14)
• Funding: not stated.

Limit of this finding: The review's own numbers do not hold together here. It reports the relative risk of vomiting stopping as 1.33 and then gives its 95% confidence interval as 1.33 to 1.50, so the estimate sits exactly on the edge of its own interval, which cannot be right; the lower bound has almost certainly been mistyped. Read this as: the review found more children stopped vomiting in the early hours, and treat the precision of that figure as unreliable. The review also found no significant difference in vomiting stopping by 24 hours, and the diarrhoea results it reports are given without any numbers behind them.

There was a greater likelihood of cessation of vomiting during the first hours in the ondansetron group compared with the placebo group (RR 1.33, 95% CI: 1.33, 1.50, p<0.00001; 3 studies, n=466); the NNT was 5 (95% CI: 4, 8). There was no evidence of statistical heterogeneity in this group. However, there were no statistically significant differences between the ondansetron and control groups for cessation of vomiting within 24 hours (RR 1.22, 95% CI: 0.89, 1.67; 2 studies, n=144), but these studies demonstrated significant statistical heterogeneity (I-squared 55.5%). Intravenous rehydration. The ondansetron group showed a reduction in the risk of intravenous rehydration in comparison with those in the placebo group (RR 0.42, 95% CI: 0.27, 0.67, p=0.0003; 2 studies, n=359); the NNT was 7 (95% CI: 5, 14).

What the evidence does not support

The reviewers of that same evidence concluded it was not enough to justify routine use in childhood gastroenteritis. (Source 13)

• Meta-analysis, Low certainty.
• Size: 4 randomised controlled trials, n=490.
• Who: children with acute gastroenteritis.
• How long: hours to 24 hours.
• Result: a conclusion about sufficiency of evidence rather than an effect size; the authors called for further well-designed trials with validated outcome measures.
• Funding: not stated.

There is insufficient evidence to recommend the routine use of ondansetron for vomiting during acute gastroenteritis in children, despite some clinical benefits.

Where the evidence is mixed

The serotonin syndrome signal for 5-HT3 antagonists is a regulatory precaution rather than an established causal finding. (Source 9)

• Expert review, not systematic, Very low certainty.
• Size: not stated; based on case reports.
• Who: patients on 5-HT3 antagonists, usually alongside other serotonergic drugs.
• How long: not applicable.
• Result: no rate given; the review states the evidence is insufficient to conclude these drugs contribute to serotonin syndrome, while agencies still advise monitoring.
• Funding: not stated.

Evidence is insufficient to conclude that 5-HT3 receptor antagonists contribute to the development of serotonin syndrome.

Where the research disagrees

Whether ondansetron should be used routinely for vomiting in childhood gastroenteritis

• The meta-analysis results, meta-analysis of 4 randomised trials, n=490: Two studies showed significantly more episodes of diarrhoea at the 24-hour follow-up in the ondansetron group compared with the placebo group (data not reported). (Source 4)
• The same reviewers' conclusion, authors' conclusion from the same meta-analysis: There is insufficient evidence to recommend the routine use of ondansetron for vomiting during acute gastroenteritis in children, despite some clinical benefits. (Source 13)

How much

• Reference intake: There is no reader dose here: ondansetron dosing is set by the prescriber, and the FDA tablet label (revised 12/2025) sets a SEPARATE regimen for each indication in Table 1, so a dose only means something alongside the condition it belongs to. For HIGHLY emetogenic cancer chemotherapy the label's position is a single 24 mg dose 30 minutes before single-day treatment. For MODERATELY emetogenic chemotherapy it is 8 mg before chemotherapy, a second 8 mg eight hours later, then 8 mg twice a day for 1 to 2 days. For radiotherapy it is 8 mg one to two hours beforehand, and for postoperative nausea and vomiting 16 mg one hour before induction of anaesthesia. The 8 mg schedule is not an alternative to the 24 mg one. (Source 14)
• Upper limit: The oral tablet label we read states no numerical maximum daily dose; the highest single oral dose it describes is the 24 mg given before highly emetogenic chemotherapy. Intravenous maximum single-dose restrictions are covered in the injection labelling, which we did not read. (Source 14)
• Studied: An emergency department study gave adults 4 mg or 8 mg intravenously and measured QT at baseline and 60 minutes. (Source 10)
• Studied: Trials pooled for childhood gastroenteritis used oral ondansetron in emergency departments, with outcomes of vomiting cessation and need for intravenous rehydration. (Source 4)

A common belief, and what the research shows

The belief: Ondansetron is a harmless anti-sickness pill with no real downside.

What the research shows: It has two documented downsides worth knowing. It measurably stretches the heart's QT interval, dose-dependently: "Moreover, 8 mg doses were associated with higher rates of QTc60 prolongation, while 4 mg doses favored maintaining QTc60 within normal limits." And in a 1.8-million-pregnancy cohort, "Among offspring of mothers enrolled in Medicaid, first-trimester exposure to ondansetron was not associated with cardiac malformations or congenital malformations overall after accounting for measured confounders, but was associated with a small increased risk of oral clefts." That was 2.7 extra cases per 10,000 births, from observational data.

Questions and answers

What is it?

Ondansetron is a prescription anti-sickness medicine of the 5-HT3 blocker class, available as tablets, dissolving tablets, oral film, liquid and injection. It was developed to make chemotherapy tolerable and is now used far more widely than that. Its approved uses are chemotherapy, radiotherapy and postoperative nausea and vomiting. (Source 3)

What does it do in the body?

Irritation or damage to the gut lining releases serotonin, which activates 5-HT3 receptors on nerves running to the brain's vomiting centres. Ondansetron blocks those receptors so the vomiting signal does not get through. The same receptors help drive bowel movement, which is why constipation is one of its more common effects. (Source 1)

Is it good or bad for you?

Good where the trials were done and more debatable where it is used most casually. It works for chemotherapy-related sickness. In children with gastroenteritis it helps measurably but causes more diarrhoea and the reviewers stopped short of recommending routine use. It stretches the QT interval, and in pregnancy it carries a small oral cleft association. (Source 13)

How do you get more of it?

Not applicable. Ondansetron is a manufactured prescription medicine; there is no dietary or bodily source of it, and the only way anyone is exposed to it is by being prescribed or given it. (Source 1)

We searched: We searched the FDA label, a 5-HT3 pharmacology review, Cochrane-linked meta-analyses and pregnancy cohort studies; none describes any natural source of ondansetron.

If it is harmful, what reduces it?

It is cleared by the liver, and drugs that speed up the CYP3A4 enzyme (phenytoin, carbamazepine, rifampin) clear it faster and lower its levels. In overdose there is no antidote and management is supportive. Effects that matter, such as QT stretching, resolve as the drug clears. (Source 8)

Why might someone be low in it or missing it?

Nobody is deficient in ondansetron. The relevant version of this question is why someone might not be given it: it is contraindicated with apomorphine because of the risk of profound low blood pressure and loss of consciousness, and it is avoided in congenital long QT syndrome. (Source 6)

We searched: We searched the FDA label and reviews; the literature frames absence of ondansetron as a prescribing decision, not a deficiency state.

Which whole foods contain it or feed it?

No food contains ondansetron. The food-and-drink question that does have an answer is what to avoid alongside it: the documented interactions in the label are with other medicines rather than foods, and no alcohol or grapefruit interaction appears in the label we read. (Source 8)

We searched: We searched the ondansetron tablet label's drug interactions section and a 5-HT3 pharmacology review for food, alcohol and supplement interactions; only drug interactions and the CYP3A4 inducer effect were documented.

What happens if you do not have it?

Without an antiemetic, nausea and vomiting run their course. The measurable consequences in trials are the outcomes it improves: in childhood gastroenteritis, roughly one extra child in five stopped vomiting in the early hours and one in seven avoided an intravenous drip, though with more diarrhoea in the treated group and no significant difference in vomiting by 24 hours. (Source 4)

How can you test for it?

There is no blood test for ondansetron in routine care. The test that matters with this drug is an electrocardiogram, because it stretches the QT interval in a dose-related way; the label recommends ECG monitoring where electrolytes are abnormal, in congestive heart failure, in bradyarrhythmias and where other QT-prolonging medicines are being taken, and an emergency department study found a quarter of patients crossed QTc 480 msec an hour after a dose. (Source 12)

References

1. StatPearls, NCBI Bookshelf. Antiemetics, Selective 5-HT3 Antagonists (Mechanism of Action). 2024. Read the source
2. Frontiers in Pharmacology. The preventive effects of ondansetron on chemotherapy-induced nausea and vomiting in adult cancer patients: systematic review from ClinicalTrials.gov. 2023. PMID 38074143, DOI 10.3389/fphar.2023.1310455. Read the source
3. DailyMed, US National Library of Medicine (FDA-approved labelling; a POSITION, not evidence; DailyMed shows this listing updated January 30, 2024). ONDANSETRON tablet, film coated - INDICATIONS AND USAGE. 2024. Read the source
4. Database of Abstracts of Reviews of Effects (DARE) quality-assessed abstract, NCBI Bookshelf (DARE record dated 16/05/2008); original review: Szajewska H, Gieruszczak-Bialek D, Dylag M. Meta-analysis: ondansetron for vomiting in acute gastroenteritis in children. Alimentary Pharmacology and Therapeutics 2007; 25(4): 393-400. Meta-analysis: ondansetron for vomiting in acute gastroenteritis in children (Results, DARE quality-assessed abstract). 2007. Read the source
5. JAMA. Association of Maternal First-Trimester Ondansetron Use With Cardiac Malformations and Oral Clefts in Offspring. 2018. PMID 30561479, DOI 10.1001/jama.2018.18307. Read the source
6. DailyMed, US National Library of Medicine (FDA-approved labelling; a POSITION, not evidence; DailyMed shows this listing updated January 30, 2024). ONDANSETRON tablet, film coated - CONTRAINDICATIONS. 2024. Read the source
7. DailyMed, US National Library of Medicine (FDA-approved labelling; a POSITION, not evidence; DailyMed shows this listing updated January 30, 2024). ONDANSETRON tablet, film coated - DRUG INTERACTIONS (serotonergic drugs). 2024. Read the source
8. DailyMed, US National Library of Medicine (FDA-approved labelling; a POSITION, not evidence; DailyMed shows this listing updated January 30, 2024). ONDANSETRON tablet, film coated - DRUG INTERACTIONS (CYP3A4 inducers). 2024. Read the source
9. StatPearls, NCBI Bookshelf. Antiemetics, Selective 5-HT3 Antagonists (serotonin syndrome and toxicity). 2024. Read the source
10. International Journal of Emergency Medicine (Springer). Single intravenous dose ondansetron induces QT prolongation in adult emergency department patients: a prospective observational study. 2024. DOI 10.1186/s12245-024-00621-5. Read the source
11. StatPearls, NCBI Bookshelf. Antiemetics, Selective 5-HT3 Antagonists (Adverse Effects). 2024. Read the source
12. DailyMed, US National Library of Medicine (FDA-approved labelling; a POSITION, not evidence; DailyMed shows this listing updated January 30, 2024). ONDANSETRON tablet, film coated - WARNINGS AND PRECAUTIONS (QT prolongation). 2024. Read the source
13. Database of Abstracts of Reviews of Effects (DARE) quality-assessed abstract, NCBI Bookshelf (DARE record dated 16/05/2008); original review: Szajewska H, Gieruszczak-Bialek D, Dylag M. Meta-analysis: ondansetron for vomiting in acute gastroenteritis in children. Alimentary Pharmacology and Therapeutics 2007; 25(4): 393-400. Meta-analysis: ondansetron for vomiting in acute gastroenteritis in children (Authors' conclusions, DARE quality-assessed abstract). 2007. Read the source
14. DailyMed, US National Library of Medicine (FDA-approved labelling; a POSITION, not evidence; label page shows Revised: 12/2025). ONDANSETRON tablet - DOSAGE AND ADMINISTRATION, 2.1 Recommended Dosage, Table 1 row: Highly Emetogenic Cancer Chemotherapy. 2025. Read the source