1. Clinical Overview
Ondansetron is a potent, highly selective competitive antagonist of the serotonin 5-HT3 receptor. It is a cornerstone antiemetic agent, primarily used to prevent and treat nausea and vomiting induced by chemotherapy, radiotherapy, and post-operative recovery. In the Indian context, it is widely available, cost-effective, and a first-line agent in its class.
| Onset | Duration | Bioavailability |
|---|---|---|
| Oral: 30-60 minutes; IV: Within minutes. | Oral: 8-12 hours; IV: Variable, typically 4-8 hours. | Approximately 60% for oral tablets. |
2. Mechanism of Action
Ondansetron selectively blocks serotonin (5-HT3) receptors both peripherally (on vagal nerve terminals in the gastrointestinal tract) and centrally (in the chemoreceptor trigger zone of the area postrema). This blockade inhibits the vomiting reflex initiated by serotonin release, which is a key mediator in chemotherapy-induced and post-operative nausea and vomiting (PONV).
3. Indications & Uses
- Prevention of nausea and vomiting associated with highly and moderately emetogenic cancer chemotherapy.
- Prevention of post-operative nausea and vomiting (PONV).
- Prevention of nausea and vomiting associated with radiotherapy (total body irradiation or single high-dose fraction to abdomen).
4. Dosage & Administration
Adult Dosage: Chemotherapy: 8 mg orally twice daily (first dose 30 min before chemo). PONV: 16 mg orally 1 hour before anesthesia or 4 mg IV just before induction. Radiotherapy: 8 mg orally three times daily.
Administration: Tablets can be taken with or without food. Orally Disintegrating Tablets (ODTs): Place on tongue, allow to dissolve, then swallow with or without water. Do not chew. For PONV prophylaxis, administer at the specified time before the event.
5. Side Effects
Common side effects may include:
- Headache
- Constipation
- Diarrhea
- Malaise/Fatigue
- Dizziness
- Transient elevation in liver transaminases (AST/ALT)
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Apomorphine | Profound hypotension and loss of consciousness. | Contraindicated |
| Drugs that prolong QT interval (e.g., Class Ia/III antiarrhythmics, macrolides, fluoroquinolones, antipsychotics) | Additive risk of QT prolongation and Torsades de Pointes. | Major |
| Serotonergic drugs (e.g., SSRIs, SNRIs, tramadol, fentanyl, lithium, MAOIs, mirtazapine) | Increased risk of serotonin syndrome. | Moderate |
| Phenytoin, Carbamazepine, Rifampicin | Induce CYP3A4, potentially reducing ondansetron plasma levels and efficacy. | Moderate |
| CYP3A4 inhibitors (e.g., Ketoconazole, Erythromycin, Clarithromycin) | May increase ondansetron plasma levels. | Minor |
7. Patient Counselling
- DO take the tablet 30-60 minutes before chemotherapy or as directed by your doctor for PONV.
- DO inform your doctor if you are pregnant, planning pregnancy, or breastfeeding.
- DO inform your doctor about all other medicines you are taking, including herbs and supplements.
- DONT take with apomorphine.
- DONT crush or chew the ODT; let it dissolve on your tongue.
8. Toxicology & Storage
Overdose: Sudden transient blindness (amaurosis), severe constipation, hypotension, dizziness, and syncope. At very high doses, ECG changes including QT prolongation may occur.
Storage: Store below 30°C. Protect from light and moisture. Keep out of reach of children. For ODTs, keep in the original blister pack until use to protect from moisture.