Drug Monograph
Ondansetron
Brand name: Zofran
Pharmacokinetic Profile
Half-Life
3.5–5.5 h (age-dependent)
Bioavailability
~60% (oral); 85–87% in cancer patients
Protein Binding
70–76%
Volume of Distribution
~1.8 L/kg
Metabolism
Hepatic (CYP3A4, CYP2D6, CYP1A2)
Clinical Information
Drug Class
5-HT3 Receptor Antagonist
Available Doses
4 mg, 8 mg tablets; 4 mg, 8 mg ODT; 4 mg/5 mL soln; 2 mg/mL injection
Route
PO, IV, IM
Renal Adjustment
None required
Hepatic Adjustment
Severe impairment: max 8 mg/day
Pregnancy
No clear evidence of fetal risk; inconsistent epidemiologic data
Lactation
Excreted in rat milk; human data limited
QT Prolongation Risk
Yes — dose-dependent; Torsade de Pointes reported
Schedule
Rx Only
Generic Available
Yes
Indications
| Indication | Approved Population | Therapy Type | Status |
|---|---|---|---|
| Highly emetogenic chemotherapy-induced N/V (cisplatin ≥50 mg/m²) | Adults | Prophylaxis | FDA Approved |
| Moderately emetogenic chemotherapy-induced N/V | Adults & pediatrics ≥4 years | Prophylaxis | FDA Approved |
| Radiation-induced N/V (TBI, single high-dose fraction, daily fractions to abdomen) | Adults | Prophylaxis | FDA Approved |
| Postoperative nausea and/or vomiting (PONV) | Adults & pediatrics ≥1 month (IV) | Prophylaxis & treatment | FDA Approved |
Ondansetron is one of the most widely used antiemetics worldwide and is included on the WHO Model List of Essential Medicines. It is effective across multiple emetogenic triggers through selective blockade of serotonin 5-HT3 receptors. Routine prophylaxis for PONV is not recommended when the expectation of postoperative nausea is low; rather, ondansetron should be reserved for patients at moderate-to-high risk based on validated risk scores.
Off-Label Uses
Nausea and vomiting of pregnancy / hyperemesis gravidarum — Evidence quality: Moderate. ACOG considers ondansetron an option when first-line agents (pyridoxine, doxylamine) fail. Electrolyte and ECG monitoring suggested. Inconsistent epidemiologic data regarding a possible association with cleft palate.
Acute gastroenteritis (pediatric and adult) — Evidence quality: High. Extensive ED-based trial evidence supports a single oral dose to facilitate oral rehydration and reduce hospital admissions.
IBS with diarrhea (IBS-D) — Evidence quality: Moderate. Studies show improvement in stool consistency, frequency, and urgency, but not pain.
Cyclic vomiting syndrome — Evidence quality: Low. Used as one of several antiemetics during the emetic phase.
Dosing
Adult Dosing by Clinical Scenario
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Highly emetogenic chemo (cisplatin ≥50 mg/m²) — oral | 24 mg PO × 1 | Single dose | 24 mg | Give 30 min before chemo Single-day therapy only |
| Highly emetogenic chemo — IV | 0.15 mg/kg IV × 3 doses | Doses at 0, 4, and 8 h | 16 mg per single IV dose | Infuse over 15 min; first dose 30 min before chemo 32 mg single IV dose withdrawn (QT risk) |
| Moderately emetogenic chemo — oral | 8 mg PO 30 min before chemo | 8 mg 8 h later, then 8 mg BID × 1–2 days | 8 mg per dose | Continue BID for 1–2 days after chemo completion |
| Radiation-induced N/V — TBI | 8 mg PO 1–2 h before each fraction | 8 mg before each daily fraction | 8 mg per dose | Administer each day of radiotherapy |
| Radiation-induced N/V — high-dose abdominal | 8 mg PO 1–2 h before RT | 8 mg q8h × 1–2 days after RT | 8 mg per dose | Single fraction; continue after completion |
| Radiation-induced N/V — daily fractionated abdominal | 8 mg PO 1–2 h before RT | 8 mg q8h after first dose each RT day | 8 mg per dose | Give for each day of radiotherapy |
| PONV prophylaxis — oral | 16 mg PO × 1 | Single dose | 16 mg | 1 h before induction of anesthesia |
| PONV prophylaxis — IV | 4 mg IV × 1 | Single dose | 4 mg | Over 2–5 min immediately before or at induction A second 4 mg IV dose post-op does not add benefit |
Pediatric Dosing
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| MEC — ages 12–17 years (oral) | 8 mg PO 30 min before chemo | 8 mg at 4 & 8 h, then 8 mg TID × 1–2 days | 8 mg per dose | Three doses on day 1 (0, 4, 8 h), then TID maintenance Differs from adult: adults get 2 doses on day 1 and BID maintenance |
| MEC — ages 4–11 years (oral) | 4 mg PO 30 min before chemo | 4 mg at 4 & 8 h, then 4 mg TID × 1–2 days | 4 mg per dose | Not established for HEC or patients <4 y (oral) |
| CINV — ages 6 months–18 years (IV) | 0.15 mg/kg IV × 3 doses | Doses at 0, 4, and 8 h | 16 mg per single dose | Infuse over 15 min; first dose 30 min before chemo |
| PONV — ages 1 month–12 years (IV) | 0.1 mg/kg IV × 1 | Single dose | 4 mg | For patients ≤40 kg; patients >40 kg receive 4 mg IV |
Special Population Adjustments
| Population | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Severe hepatic impairment (Child-Pugh ≥10) | 8 mg/day maximum | 8 mg/day | 8 mg/day | Clearance substantially reduced; t½ prolonged No experience beyond first-day administration in these patients |
| Elderly (≥65 years) | No routine adjustment | Clearance decreased, t½ increased (~5.5 h in >75 y vs 3.5 h in young adults) No differences in safety observed | ||
| Renal impairment | No adjustment required | <5% excreted unchanged; no clinically significant PK change | ||
Clinical Pearl: The 32 mg IV Dose Is Withdrawn
The FDA withdrew the single 32 mg IV dose in 2012 due to dose-dependent QT prolongation risk. The maximum recommended single IV dose is now 16 mg, infused over at least 15 minutes. For PONV, 4 mg IV is the standard. Always prefer oral dosing when feasible, as QT risk is lower with the oral route.
Pharmacology
Mechanism of Action
Ondansetron is a selective serotonin 5-HT3 receptor antagonist. It blocks 5-HT3 receptors located both peripherally on vagal nerve terminals in the gastrointestinal tract and centrally in the chemoreceptor trigger zone of the area postrema. During chemotherapy, radiation, or surgical insult, enterochromaffin cells in the small intestinal mucosa release serotonin, which activates vagal afferents via 5-HT3 receptors, triggering the vomiting reflex. By competitively antagonizing these receptors, ondansetron interrupts this emetic signal at both peripheral and central sites. Importantly, ondansetron is not a dopamine receptor antagonist, which accounts for its low incidence of extrapyramidal reactions compared to older antiemetics. It does not stimulate gastric or intestinal peristalsis and should not be used as a substitute for nasogastric suction. Ondansetron has no clinically significant effects on plasma prolactin, gastrin, or other hormones, and multiday administration has been shown to slow colonic transit time in healthy volunteers.
ADME Profile
| Parameter | Value | Clinical Implication |
|---|---|---|
| Absorption | Rapidly absorbed orally; Tmax ~1.5 h; bioavailability ~60% (first-pass metabolism); nonlinear increase at higher doses due to saturation of first-pass | Administer at least 30 min before emetogenic stimulus; cancer patients achieve ~85% bioavailability |
| Distribution | Vd ~1.8 L/kg (~160 L); protein binding 70–76%; CSF levels only 10–15% of plasma | Extensive tissue distribution; limited CNS penetration sufficient for CTZ activity |
| Metabolism | Hepatic oxidative metabolism (>95%) via CYP3A4, CYP2D6, CYP1A2; major metabolites: 7-OH and 8-OH ondansetron (inactive, excreted as conjugates) | CYP3A4 inducers (rifampin, phenytoin, carbamazepine) increase clearance; no dosage adjustment currently recommended |
| Elimination | t½ 3.5 h (young adults), 5.5 h (>75 y), 2.5 h (children 7–12 y); <5% excreted unchanged in urine; ~80% appears in urine as metabolites | No renal adjustment needed; reduce dose in severe hepatic impairment (max 8 mg/day) |
Side Effects
Ondansetron is generally well tolerated. The adverse effect profile varies by indication and dosing regimen. Data below are from controlled clinical trials.
≥10%Very Common
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Headache | 11% (HEC 24 mg); 24% (MEC 8 mg BID) | Most common AE across all indications; often not significantly different from placebo |
| Malaise / fatigue | 13% (MEC) | Reported in MEC trials; significantly higher than placebo (2%) |
| Constipation | 11% (multiday chemo); 9% (MEC) | Related to 5-HT3 blockade slowing colonic transit; manage with laxatives if needed |
1–10%Common
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Diarrhea | 4–6% | Reported in 4% (HEC) and 6% (MEC) of patients |
| Dizziness | 7% (PONV) | Not consistently different from placebo in PONV trials |
| Urinary retention | 5% (PONV) | Reported in PONV trials; patients receiving multiple perioperative medications |
| Pruritus | 5% (PONV) | Not significantly different from placebo |
| Transaminase elevation (AST/ALT >2× ULN) | 1–2% | Transient; seen with cyclophosphamide-based regimens; role of chemo unclear |
| Rash | ~1% | Non-specific; usually mild |
SeriousSerious (Regardless of Frequency)
| Adverse Effect | Estimated Frequency | Typical Onset | Required Action |
|---|---|---|---|
| QT prolongation / Torsade de Pointes | Rare; dose-dependent | Minutes to hours after IV; predominantly with IV route | Avoid in congenital long QT; monitor ECG with electrolyte abnormalities, CHF, or concurrent QT-prolonging drugs; max single IV dose 16 mg |
| Serotonin syndrome | Rare; higher risk with serotonergic co-medications | Hours; often in PACU or infusion centers | Discontinue ondansetron and all serotonergic drugs; supportive care; some fatal cases reported |
| Anaphylaxis / severe hypersensitivity | Rare | Minutes after administration | Discontinue immediately; epinephrine and supportive care; cross-sensitivity with other 5-HT3 antagonists reported |
| Stevens-Johnson syndrome / toxic epidermal necrolysis | Very rare | Days to weeks | Permanent discontinuation; dermatology consultation; supportive care |
| Transient blindness (predominantly IV) | Very rare | During or shortly after IV infusion | Self-resolving within minutes to 48 hours; reassure patient; document |
| Masking of progressive ileus / gastric distension | Unknown | Post-abdominal surgery | Monitor bowel function; do not substitute for nasogastric suction |
DiscontinuationDiscontinuation Rates
Chemotherapy Trials
Low
Ondansetron was well tolerated in trials of over 300 patients (HEC) and over 240 patients (MEC). Adverse events were generally mild and rarely led to discontinuation. Headache was the most common complaint but was often not different from placebo.
PONV Trials
Low
In 550 patients receiving 16 mg oral prophylaxis, adverse events were confounded by concomitant perioperative medications. No specific discontinuation rate was attributable solely to ondansetron.
Managing Constipation
Constipation occurs in up to 11% of chemotherapy patients on multiday ondansetron regimens. This results from 5-HT3 receptor blockade reducing intestinal secretion and slowing colonic transit. Prophylactic stool softeners or osmotic laxatives (polyethylene glycol) are advisable, particularly in patients already predisposed to constipation from opioid co-administration or reduced mobility.
Drug Interactions
Ondansetron is metabolized by CYP3A4, CYP2D6, and CYP1A2, but does not itself induce or inhibit these enzymes. Its interaction profile is dominated by pharmacodynamic concerns (QT prolongation, serotonin syndrome) rather than pharmacokinetic interactions.
MajorApomorphine
MechanismUnknown; profound hypotension observed
EffectProfound hypotension and loss of consciousness
ManagementConcomitant use is contraindicated
FDA PIMajorSerotonergic drugs (SSRIs, SNRIs, MAOIs, fentanyl, tramadol, lithium)
MechanismAdditive serotonergic activity; 5-HT3 blockade may redistribute serotonin to other receptor subtypes
EffectRisk of serotonin syndrome (agitation, diaphoresis, tremor, rigidity, hyperthermia, seizures); fatal cases reported
ManagementMonitor for serotonin syndrome; discontinue ondansetron if symptoms occur; inform patients of increased risk
FDA PIMajorQT-prolonging drugs (class III antiarrhythmics, fluoroquinolones, antipsychotics, macrolides)
MechanismAdditive QT interval prolongation
EffectIncreased risk of Torsade de Pointes and fatal arrhythmia
ManagementECG monitoring recommended; correct electrolyte abnormalities; avoid in congenital long QT syndrome
FDA PIModerateTramadol
MechanismOndansetron may reduce analgesic efficacy of tramadol via shared serotonergic pathway
EffectIncreased patient-controlled tramadol use observed in two small trials
ManagementMonitor pain control; consider alternative antiemetic or analgesic
FDA PIModerateCYP3A4 inducers (rifampin, phenytoin, carbamazepine)
MechanismIncreased hepatic clearance of ondansetron
EffectSignificantly increased clearance and decreased ondansetron blood concentrations; potential loss of antiemetic efficacy
ManagementNo dosage adjustment currently recommended per FDA PI; monitor antiemetic response
FDA PIMinorChemotherapy agents (cisplatin, carmustine, etoposide)
MechanismNo pharmacokinetic interaction
EffectThese agents do not alter ondansetron PK; ondansetron does not alter methotrexate levels in pediatric patients
ManagementNo adjustment needed; safe for co-administration
FDA PIMonitoring
- ECGBaseline and as needed
Trigger-basedRecommended in patients with electrolyte abnormalities (hypokalemia, hypomagnesemia), CHF, bradyarrhythmias, congenital long QT syndrome, or concurrent use of other QT-prolonging medications. QT prolongation is dose-dependent and predominantly reported with IV administration. - ElectrolytesBefore and during therapy
RoutineCorrect hypokalemia and hypomagnesemia before administering ondansetron. Chemotherapy and vomiting can deplete electrolytes, compounding QT prolongation risk. - Serotonin Syndrome SignsOngoing when co-prescribed with serotonergic drugs
Trigger-basedMonitor for agitation, hallucinations, tachycardia, labile BP, diaphoresis, hyperthermia, tremor, rigidity, myoclonus, hyperreflexia, seizures. Most reports occurred in post-anesthesia care or infusion settings. - Hepatic FunctionBaseline in patients with known liver disease
RoutineTransient AST/ALT elevations (>2× ULN) reported in 1–2% of patients on chemo regimens. Dose cap at 8 mg/day for severe hepatic impairment (Child-Pugh ≥10). Liver failure reported in cancer patients receiving concurrent hepatotoxic chemotherapy. - Bowel FunctionPost-operatively; during multiday chemo
Trigger-basedOndansetron slows colonic transit and may mask progressive ileus or gastric distension after abdominal surgery. Monitor for decreasing bowel sounds, distension, or obstruction. - Antiemetic EfficacyEach chemotherapy cycle
RoutineAssess breakthrough emesis. Consider adding dexamethasone or NK1 antagonist (aprepitant/fosaprepitant) if ondansetron alone is insufficient, per ASCO/MASCC guidelines.
Contraindications & Cautions
Absolute Contraindications
- Known hypersensitivity to ondansetron or any component of the formulation; anaphylaxis and bronchospasm reported; cross-sensitivity with other 5-HT3 antagonists possible
- Concomitant apomorphine — reports of profound hypotension and loss of consciousness
Relative Contraindications (Specialist Input Recommended)
- Congenital long QT syndrome — ondansetron prolongs QT in a dose-dependent manner; Torsade de Pointes reported post-marketing
- Severe hepatic impairment without dose reduction — clearance is substantially decreased; max 8 mg/day required
Use with Caution
- Electrolyte abnormalities (hypokalemia, hypomagnesemia) — correct before administering; increases QT prolongation risk
- Congestive heart failure or bradyarrhythmias — ECG monitoring recommended
- Concurrent serotonergic drugs — risk of serotonin syndrome; most commonly seen with SSRIs, SNRIs, fentanyl, tramadol in perioperative or infusion settings
- Post-abdominal surgery patients — antiemetic effect may mask progressive ileus
- Phenylketonuria (ODT formulation) — contains aspartame (phenylalanine component); each 4 mg and 8 mg ODT tablet contains <0.03 mg phenylalanine
FDA Safety Communication
QT Prolongation and Torsade de Pointes
ECG changes including QT interval prolongation have been reported in patients receiving ondansetron, predominantly with intravenous administration. Postmarketing cases of Torsade de Pointes have been reported. The single 32 mg IV dose was withdrawn in 2012 due to this risk. The maximum single IV dose is now 16 mg. Avoid ondansetron in patients with congenital long QT syndrome. ECG monitoring is recommended when ondansetron is used in patients with electrolyte abnormalities, CHF, bradyarrhythmias, or concurrent use of other QT-prolonging drugs.
Patient Counselling
Purpose of Therapy
Ondansetron is prescribed to prevent nausea and vomiting caused by your chemotherapy, radiation treatment, or surgery. It works by blocking a specific chemical signal (serotonin) that triggers the vomiting reflex. It does not treat nausea from motion sickness.
How to Take
Take ondansetron exactly as prescribed, usually 30 minutes to 1 hour before your treatment. If you are using the orally disintegrating tablet (ODT), peel back the foil with dry hands, place the tablet on your tongue and let it dissolve, then swallow with saliva — no water is needed. For the liquid form, use the measuring device provided.
Constipation
Tell patientConstipation is common, especially if you are receiving ondansetron for several days with chemotherapy. Drink plenty of fluids, eat fiber-rich foods, and ask about a stool softener if your bowels become sluggish.
Call prescriberIf you have no bowel movement for 3 or more days, or if you develop severe bloating, abdominal pain, or vomiting.
Headache
Tell patientHeadache is the most commonly reported side effect. It is usually mild and can be treated with paracetamol (acetaminophen). Ensure you stay well hydrated.
Call prescriberIf headache is severe, persistent, or accompanied by visual disturbances, confusion, or stiff neck.
Heart Rhythm Warning
Tell patientOndansetron can rarely affect your heart rhythm, especially at higher doses or if given intravenously. This risk is higher if you have a heart condition or take other medications that affect heart rhythm.
Call prescriberImmediately if you experience a racing or irregular heartbeat, dizziness, lightheadedness, or fainting.
Serotonin Syndrome
Tell patientIf you take antidepressants (SSRIs like sertraline or fluoxetine, or SNRIs like venlafaxine), pain medications (tramadol, fentanyl), or other serotonin-affecting drugs, tell your doctor. There is a small risk of a condition called serotonin syndrome.
Call prescriberIf you develop agitation, confusion, rapid heartbeat, high temperature, muscle stiffness or twitching, loss of coordination, or diarrhea while taking ondansetron with other medications.
ODT Administration
Tell patientDo not push the ODT tablet through the foil backing. Peel the foil with dry hands and gently place the tablet on your tongue. It dissolves in seconds. Swallowing with water is not necessary but may increase the chance of headache.
Call prescriberIf you have difficulty with the ODT formulation or if you have phenylketonuria (PKU), as the ODT contains aspartame.
Sources
Regulatory (PI / SmPC)
- Zofran (ondansetron) Tablets, ODT, Oral Solution — FDA-Approved Prescribing Information, revised October 2016. GlaxoSmithKline. FDA Label (Oral)Current FDA-approved label for oral ondansetron formulations; source for indications, dosing, adverse reactions, and pharmacokinetic data.
- Zofran (ondansetron HCl) Injection — FDA-Approved Prescribing Information, revised January 2025. GlaxoSmithKline. FDA Label (Injection)Current FDA-approved label for ondansetron injection; includes IV dosing for CINV and PONV, pediatric IV dosing, and QT prolongation data.
- FDA Drug Safety Communication: Updated information on 32 mg intravenous ondansetron (Zofran) dose. FDA, 2012. FDA Safety CommunicationFDA communication withdrawing the 32 mg single IV dose due to dose-dependent QT prolongation and Torsade de Pointes risk.
Key Clinical Trials
- Hesketh PJ, Kris MG, Basch E, et al. Antiemetics: ASCO Guideline Update. J Clin Oncol. 2020;38(24):2782–2797. PubMed: 32658626Current ASCO antiemetic guideline positioning ondansetron within multi-agent regimens for CINV prevention across emetogenic risk categories.
- Freedman SB, Adler M, Seshadri R, Powell EC. Oral ondansetron for gastroenteritis in a pediatric emergency department. N Engl J Med. 2006;354(16):1698–1705. PubMed: 16625009Landmark RCT demonstrating ondansetron’s efficacy in reducing vomiting and facilitating oral rehydration in pediatric gastroenteritis.
- Pasternak B, Svanström H, Hviid A. Ondansetron in pregnancy and risk of adverse fetal outcomes. N Engl J Med. 2013;368(9):814–823. PubMed: 23445092Large Danish cohort study (1,970 exposed pregnancies) finding no increased risk of major congenital malformations, miscarriage, or stillbirth with ondansetron.
Guidelines
- ACOG Practice Bulletin No. 189: Nausea and Vomiting of Pregnancy. Obstet Gynecol. 2018;131(1):e15–e30. PubMed: 29266076ACOG guideline positioning ondansetron as a second-line option for nausea and vomiting of pregnancy after pyridoxine/doxylamine failure.
- Gan TJ, Belani KG, Bergese S, et al. Fourth Consensus Guidelines for the Management of Postoperative Nausea and Vomiting. Anesth Analg. 2020;131(2):411–448. PubMed: 32467512Consensus guideline for PONV management; ondansetron recommended for moderate-to-high-risk patients based on validated risk scoring (Apfel).
Mechanistic / Basic Science
- Gregory RE, Ettinger DS. 5-HT3 receptor antagonists for the prevention of chemotherapy-induced nausea and vomiting. A comparison of their pharmacology and clinical efficacy. Drugs. 1998;55(2):173–189. PubMed: 9506239Comparative pharmacology review of 5-HT3 antagonists including ondansetron, granisetron, and dolasetron; covers receptor binding and selectivity.
- Rojas C, Raje M, Tsukamoto T, Slusher BS. Molecular mechanisms of 5-HT3 and NK1 receptor antagonists in prevention of emesis. Eur J Pharmacol. 2014;722:26–37. PubMed: 24184670Review of molecular mechanisms explaining peripheral vagal and central CTZ pathways underlying 5-HT3 antagonist antiemetic activity.
Pharmacokinetics / Special Populations
- Roila F, Del Favero A. Ondansetron clinical pharmacokinetics. Clin Pharmacokinet. 1995;29(2):95–109. PubMed: 7586904Comprehensive PK review: bioavailability ~60%, t½ 3.8±1 h, Vd ~160 L, protein binding 70–76%, hepatic metabolism >95%.
- Pritchard JF. Ondansetron metabolism and pharmacokinetics. Semin Oncol. 1992;19(4 Suppl 10):9–15. PubMed: 1387254Seminal PK study documenting age-dependent half-life (3.5 h young adults, 5.5 h >75 y, 2.5 h children), CYP-mediated metabolism, and no renal adjustment requirement.
- Christofaki M, Papaioannou A. Ondansetron: a review of pharmacokinetics and clinical experience in postoperative nausea and vomiting. Expert Opin Drug Metab Toxicol. 2014;10(3):437–444. PubMed: 24471415Review focused on ondansetron PK in the PONV setting, including gender differences in absorption and clearance.
- Villikka K, Kivisto KT, Neuvonen PJ. The effect of rifampin on the pharmacokinetics of oral and intravenous ondansetron. Clin Pharmacol Ther. 1999;65(4):377–381. PubMed: 10223773PK interaction study showing rifampin significantly increases ondansetron clearance and reduces bioavailability by 33%, supporting CYP3A4 induction concern.