Ondansetron Should Generally Be Avoided During Gastric Emptying Studies
Ondansetron (Zofran) should not be administered immediately before or during a gastric emptying study, as it may interfere with test accuracy, though the evidence suggests it likely does not significantly alter gastric emptying of solids in most patients.
Key Considerations for Test Accuracy
Direct Effects on Gastric Emptying
Ondansetron does not significantly affect gastric emptying of solid meals in healthy volunteers, as demonstrated in a randomized, placebo-controlled crossover study where gastric half-emptying times were essentially identical (86 min vs 85.5 min, p=0.082) 1
Minor effects on antral motility may occur during nutrient infusion, with ondansetron causing slightly higher antral wave numbers during the first 30 minutes of duodenal nutrient infusion, but this did not translate to clinically significant changes in overall gastric emptying 2
Critical FDA Warning About Test Interference
The FDA label explicitly warns that ondansetron can mask gastric distension and ileus, stating: "The use of ondansetron tablets in patients following abdominal surgery or in patients with chemotherapy-induced nausea and vomiting may mask a progressive ileus and/or gastric distension" 3
Ondansetron does not stimulate gastric peristalsis, and the FDA specifically notes "Ondansetron tablets are not a drug that stimulates gastric or intestinal peristalsis" 3
Practical Clinical Algorithm
Timing Recommendations
Hold ondansetron for at least 24-48 hours before the gastric emptying study to ensure no residual antiemetic effects that could confound symptom assessment during the test
If nausea/vomiting is severe and medication cannot be held:
- Consider alternative antiemetics that have been studied with gastric emptying (e.g., antihistamines like meclizine 12.5-25 mg) 4
- Document ondansetron use clearly in the test report
- Interpret results with caution, understanding that symptom masking rather than motility changes is the primary concern
When Ondansetron Use Is Unavoidable
The test can still be performed since ondansetron does not appear to significantly delay solid gastric emptying based on scintigraphic studies 1
Document the medication use and note that while gastric emptying measurements may remain valid, the patient's symptomatic response during the test may be blunted 3
Important Caveats
Why This Matters Clinically
Gastric emptying studies assess both objective emptying rates AND symptom correlation - ondansetron's antiemetic properties may dissociate symptoms from actual gastric retention, reducing diagnostic accuracy for symptom-motility correlation
The concern is symptom masking, not test invalidation - the actual scintigraphic measurement of gastric emptying appears unaffected by ondansetron, but the clinical interpretation of whether symptoms correlate with delayed emptying becomes problematic 1, 2
Comparison with Other Medications That DO Affect Results
Proton pump inhibitors (omeprazole, rabeprazole) significantly delay gastric emptying and should definitely be held before testing, with delays of 30+ minutes documented 5, 6, 7
GLP-1 receptor agonists (semaglutide, liraglutide) markedly delay gastric emptying through vagal mechanisms and can persist for weeks, making interpretation extremely difficult 4
Prokinetic agents (metoclopramide, domperidone) accelerate gastric emptying and must be discontinued before testing 4, 6