Ondansetron works by selectively blocking 5-HT3 receptors in the chemoreceptor trigger zone (CTZ) and the gastrointestinal (GI) tract. By inhibiting these receptors, ondansetron:

1. Prevents the stimulation of the vomiting center in the brain, reducing nausea and vomiting.
2. Blocks serotonin release from enterochromaffin cells in the GI tract, which is often triggered by chemotherapy or radiation (Lippincott, Lange).

Ondansetron does not have significant effects on dopamine or muscarinic receptors, making it less likely to cause extrapyramidal side effects or sedation compared to other antiemetics (BNF).

1. Chemotherapy-Induced Nausea and Vomiting (CINV):
   • Used to prevent acute and delayed nausea and vomiting associated with highly emetogenic chemotherapy (BNF).
2. Radiotherapy-Induced Nausea and Vomiting:
   • Effective in managing nausea and vomiting caused by radiation therapy (Lippincott).
3. Postoperative Nausea and Vomiting (PONV):
   • Used to prevent and treat nausea and vomiting after surgery (Lange).
4. Hyperemesis Gravidarum:
   • Occasionally used off-label for severe nausea and vomiting during pregnancy (BNF).

• Chemotherapy-Induced Nausea and Vomiting:
  • 8 mg orally or intravenously 30 minutes before chemotherapy, followed by 8 mg twice daily for 1–2 days (BNF).
• Radiotherapy-Induced Nausea and Vomiting:
  • 8 mg orally 1–2 hours before radiotherapy, repeated every 8 hours as needed (Lippincott).
• Postoperative Nausea and Vomiting:
  • 4–8 mg intravenously or orally before or after surgery (Lange).
• Hyperemesis Gravidarum:
  • 4–8 mg orally or intravenously every 8 hours as needed (BNF).

Administration:

• Oral tablets can be taken with or without food.
• Intravenous administration should be done slowly over 2–5 minutes to avoid side effects (Lippincott).

• Common: Headache, constipation, and dizziness (Lippincott).
• Serious:
  • QT Prolongation: Risk of arrhythmias, particularly with high doses or in patients with underlying heart conditions (Lange).
  • Hypersensitivity Reactions: Rare cases of anaphylaxis or rash (BNF).

• QT Prolongation: Avoid in patients with congenital long QT syndrome or those taking other QT-prolonging drugs (BNF).
• Pregnancy: Generally considered safe but should be used under medical supervision (Lippincott).
• Liver Impairment: Dose reduction may be necessary in patients with severe hepatic impairment (Lange).

• QT-Prolonging Drugs (e.g., amiodarone, sotalol): Increased risk of arrhythmias (BNF).
• Apomorphine: Contraindicated due to risk of profound hypotension and loss of consciousness (Lippincott).
• Tramadol: Ondansetron may reduce the analgesic effect of tramadol (Lange).

Ondansetron is the generic name for this medication. It is marketed under the brand name Zofran® in some regions. Generic versions are widely available and cost-effective.