Ondansetron Side Effects and Contraindications
Ondansetron is contraindicated in patients with known hypersensitivity to the drug and in those receiving concomitant apomorphine, with key warnings including QT prolongation, serotonin syndrome, and myocardial ischemia. 1
Absolute Contraindications
- Known hypersensitivity (including anaphylaxis) to ondansetron or any formulation components 1
- Concomitant use with apomorphine due to risk of profound hypotension and loss of consciousness 1
Critical Warnings and Precautions
QT Prolongation and Cardiac Effects
- ECG changes including QT interval prolongation have been documented in patients receiving ondansetron 1
- Avoid in patients with congenital long QT syndrome 1
- ECG monitoring is recommended in patients with:
- Postmarketing cases of Torsade de Pointes have been reported 1
Myocardial Ischemia
- Myocardial ischemia has been reported, with symptoms sometimes appearing immediately after administration 1
- Coronary artery spasm appears to be the most common underlying mechanism 1
- Monitor patients for signs/symptoms of myocardial ischemia after oral administration 1
Serotonin Syndrome
- Development of serotonin syndrome has been reported with 5-HT3 antagonists, including fatal cases 1
- Highest risk with concomitant serotonergic drugs including SSRIs, SNRIs, MAO inhibitors, mirtazapine, fentanyl, lithium, tramadol, and intravenous methylene blue 1
- Symptoms include: mental status changes (agitation, hallucinations, delirium, coma), autonomic instability (tachycardia, labile blood pressure, hyperthermia), neuromuscular symptoms (tremor, rigidity, myoclonus, hyperreflexia), seizures, and GI symptoms 1
- Discontinue ondansetron immediately if serotonin syndrome occurs 1
Gastrointestinal Masking Effects
- May mask progressive ileus and gastric distension in post-abdominal surgery patients or those with chemotherapy-induced nausea 1
- Ondansetron does not stimulate gastric or intestinal peristalsis and should not replace nasogastric suction 1
- Monitor for decreased bowel activity, particularly in patients with risk factors for gastrointestinal obstruction 1
Common Side Effects
Most Frequent Adverse Reactions (≥4%)
- Headache: 11-24% of patients 1, 2
- Diarrhea: 4-6% of patients 1
- Constipation: 9% of patients 1, 2, 3
- Malaise/Fatigue: 13% of patients 1
Less Common but Clinically Significant (1-2%)
- Hepatic effects: AST/ALT elevations exceeding twice the upper limit of normal in 1-2% of patients receiving cyclophosphamide-based chemotherapy 1
- Transient transaminase elevations that do not appear dose or duration-related 1
- Liver failure and death reported in cancer patients receiving concurrent hepatotoxic chemotherapy and antibiotics (etiology unclear) 1
- Rash: approximately 1% 1
Rare but Serious (<2%)
- Anaphylaxis 1
- Bronchospasm 1
- Tachycardia 1
- Angina 1
- Hypokalemia 1
- Vascular occlusive events 1
- Grand mal seizures 1
- Extrapyramidal reactions (<1%) 1
Clinical Pitfalls and Caveats
Drug Interactions
- No significant interactions documented with temazepam, atracurium, alfentanil, alcohol, pentobarbitone, morphine, neostigmine, prednisolone, or diazepam 3
- Exercise caution with all serotonergic medications due to serotonin syndrome risk 1
Special Populations
- Elderly patients: 31% reduction in plasma clearance (not clinically significant for dosing) 3
- Severe hepatic failure: 80% reduction in clearance 3
- Pediatric patients: increased clearance compared to adults 4
- General anesthesia: slight prolongation of terminal half-life without clinical significance 3
Tolerability Profile
- Overall incidence of adverse events: 36% in chemotherapy studies 2
- Adverse events are typically mild and do not require discontinuation 2, 5, 3
- No evidence of respiratory depression potentiation during general anesthesia 3
- No effects on cardiac output, blood pressure, heart rate, or haemostatic function in volunteers 3