Ondansetron (Zofran) for Nausea: Patient Information
For nausea and vomiting, ondansetron is typically given at a dose of 4-8 mg by mouth, IV, or intramuscular injection every 8 hours as needed, with the option to switch to scheduled dosing (8 mg twice daily) if nausea persists. 1, 2, 3
How to Take Ondansetron
Standard Dosing Options
- Oral tablets or dissolving tablets (ODT): 4-8 mg every 8 hours as needed 1, 2
- Intramuscular injection: 4-8 mg when oral medication cannot be taken due to vomiting 1
- Intravenous (IV): 4-8 mg every 8 hours 2, 3
For Persistent Nausea
- If nausea continues despite taking ondansetron as needed, switch to scheduled dosing of 8 mg twice daily for at least one week rather than waiting until nausea occurs 2, 3
- The oral dissolving tablet (ODT) form can be particularly helpful when swallowing pills is difficult 2
Special Situations
For chemotherapy-related nausea:
- Take 16-24 mg as a single dose OR 8 mg every 8 hours on the first day of chemotherapy 2
- Continue with 8 mg twice daily for 2-3 days after chemotherapy 2
For radiation therapy-related nausea:
When Ondansetron Alone Is Not Enough
If Nausea Persists After Taking Ondansetron
Do not simply take more ondansetron—instead, your doctor should add a different medication that works through a different mechanism. 2, 3
- Dexamethasone 4-8 mg can be added to enhance the anti-nausea effect 2, 3
- Metoclopramide 10-20 mg may be added if ondansetron alone is insufficient 2
- Lorazepam 0.5-2 mg can help if anxiety or anticipatory nausea is present 2
Before Adding More Medications
Your doctor should check for other treatable causes of nausea: 2, 3
- Constipation (which ondansetron can worsen)
- Electrolyte imbalances
- Dehydration
- Other medications causing nausea
Important Safety Information
Common Side Effects
- Constipation is the most common side effect and can paradoxically worsen nausea if not addressed 2, 3
- Headache may occur 5
- Mild dizziness 6
Serious but Rare Side Effects
- Heart rhythm changes (QT prolongation): This is primarily a concern with high IV doses (32 mg), which are no longer recommended 7
- The standard doses of 4-8 mg are considered safe for most patients 7, 8
Who Should Use Caution
- Elderly patients: May need to start with the lower dose of 4 mg rather than 8 mg 3
- Patients with severe liver disease: Ondansetron is cleared more slowly, so dosing may need adjustment 9
- Patients prone to constipation: Monitor bowel movements closely, as ondansetron can worsen constipation 2, 3
How Quickly Does It Work?
- Oral tablets: Peak effect occurs within 1-2 hours 6
- IV or IM injection: Works within 30 minutes 8, 10
- Oral dissolving tablets (ODT): Similar to regular oral tablets 8
The medication lasts approximately 3.5-4 hours in the body, which is why it's typically dosed every 8 hours 9, 6
What to Avoid
- Do not rely solely on ondansetron if it's not working—contact your doctor to add a different medication rather than just taking more ondansetron 2, 3
- Avoid first-generation antihistamines like Benadryl (diphenhydramine) for nausea, as they can cause low blood pressure, rapid heart rate, and excessive sedation without effectively treating nausea 2, 3
- Monitor for constipation and use stool softeners or laxatives if needed, as constipation from ondansetron can make nausea worse 2, 3
When to Contact Your Doctor
- Nausea persists despite taking ondansetron as directed for 24-48 hours 2
- You develop severe constipation 2, 3
- You experience chest pain, rapid heartbeat, or fainting 7
- You cannot keep down any fluids or medications 2
Evidence-Based Effectiveness
Ondansetron has been proven effective in multiple settings: 5, 8, 10