Granisetron is a potent and highly selective serotonin 5-HT3 receptor antagonist used primarily for the prevention and treatment of chemotherapy-induced nausea and vomiting (CINV) and postoperative nausea and vomiting (PONV). It is a first-line antiemetic agent in modern oncology protocols. The 3mg tablet is a convenient, once-daily oral formulation, particularly used for delayed CINV.
Adult: Chemotherapy-induced: A single 2mg oral dose once daily OR two 1mg doses (the first given 1 hour before chemotherapy and the second 12 hours later). For delayed CINV, a single 3mg tablet is often used 1 hour before chemotherapy on day 1, and then once daily for up to 5 days post-chemotherapy as per protocol. Radiotherapy-induced: 2mg once daily within 1 hour of radiotherapy. Postoperative: 1-3mg single dose, usually 1 hour before induction of anesthesia.
Note: Tablet can be taken with or without food, swallowed whole with a glass of water. For chemotherapy, the first dose is usually administered 1 hour before the start of chemotherapy. Do not crush or chew.
Granisetron selectively and competitively blocks serotonin (5-hydroxytryptamine or 5-HT) at the 5-HT3 receptors located peripherally on vagal nerve terminals in the gastrointestinal tract and centrally in the chemoreceptor trigger zone (CTZ) of the area postrema in the medulla oblongata. Chemotherapy and radiotherapy cause the release of serotonin from enterochromaffin cells in the gut, which activates 5-HT3 receptors on vagal afferents, initiating the vomiting reflex. By blocking these receptors, granisetron interrupts the afferent signals to the vomiting center.
Pregnancy: Category B: Animal studies have shown no risk, but adequate and well-controlled studies in pregnant women are lacking. Use only if clearly needed and potential benefit justifies potential risk to the fetus.
Driving: May cause dizziness or somnolence. Patients should not drive or operate machinery until they know how the drug affects them.
| Apomorphine | Granisetron may cause profound hypotension and loss of consciousness when used with apomorphine. Contraindicated. | Major |
| Drugs that prolong QT interval (e.g., Class Ia/III antiarrhythmics, macrolides, fluoroquinolones, antipsychotics) | Additive risk of QT prolongation and cardiac arrhythmias. Monitor ECG. | Major |
| Strong CYP3A4 inducers (e.g., Rifampicin, Phenytoin, Carbamazepine, St. John's Wort) | May decrease granisetron plasma concentrations, potentially reducing efficacy. | Moderate |
| Strong CYP3A4 inhibitors (e.g., Ketoconazole, Clarithromycin, Ritonavir) | May increase granisetron plasma concentrations. Clinical significance is likely low due to wide therapeutic index. | Minor |
| Other Serotonergic Drugs (e.g., SSRIs, SNRIs, TCAs, Tramadol, Fentanyl) | Theoretical increased risk of serotonin syndrome. Monitor for symptoms. | Moderate |