What the FDA labels say about taking them together
Ondansetron and phenytoin
Ondansetron's FDA label of January 8, 2026 names phenytoin in its drug interactions section; the phenytoin labels PlainMeds reads (2) do not name ondansetron. The ondansetron label: “In patients treated with potent inducers of CYP3A4, the clearance of ondansetron was significantly increased and ondansetron blood concentrations were decreased.”
Check it at the source: the ondansetron FDA label on DailyMed (effective January 8, 2026, Glenmark Pharmaceuticals Inc., USA); the phenytoin FDA label on DailyMed (effective October 15, 2024, Mylan Pharmaceuticals Inc.).
What does the ondansetron label say about phenytoin?
Ondansetron label, drug interactionsIn patients treated with potent inducers of CYP3A4 (i.e., phenytoin, carbamazepine, and rifampin), the clearance of ondansetron was significantly increased and ondansetron blood concentrations were decreased.
Source: FDA drug label (Ondansetron), Glenmark Pharmaceuticals Inc., USA, effective January 8, 2026 (SPL set eee5217a-eba8-463a-812d-bf3c66f0934f), via openFDA; the full label on DailyMed
What does the phenytoin label say about ondansetron?
Phenytoin's label (effective October 15, 2024) and its 1 other brand-application label do not name ondansetron in these sections. A label lists what its studies and reports found; it is not a complete list.
Source: FDA drug label (Phenytek), Mylan Pharmaceuticals Inc., effective October 15, 2024 (SPL set 0f94c791-8b07-4b41-927e-c9f28bec5f60), via openFDA; the full label on DailyMed
These are the labels' own sentences. PlainMeds does not grade interactions; a pharmacist or prescriber can say what they mean for one person's medicines.
Both medicines on the record
| Ondansetron | Phenytoin | |
|---|---|---|
| Labeled for | Nausea and vomiting | Seizures and epilepsy |
| Boxed warning | No | No |
| Pharmacy cost (NADAC) | $0.0582 per tablet 4 mg tablet | $0.145 per capsule 100 mg capsule |
| Shortage now | No current entry | No current entry |
Source: FDA NDC Directory, CMS NADAC of October 7, 2026 and the FDA Drug Shortage Database (FDA files October 8, 2026)
What next?
- Check more medicines together
- up to five at once