Phenytoin and Aprepitant Interaction
Drug interaction information between Phenytoin and Aprepitant.
Phenytoin and Aprepitant have a documented moderate interaction in FDA labeling.
FDA drug labeling documents a moderate-severity interaction between Phenytoin and Aprepitant. Major interactions are generally avoided, moderate ones may need monitoring or a dose adjustment, and minor ones are usually low-risk. This page shows the documented mechanism and guidance. Label-documented interactions are not a complete safety review, so always confirm your own medications with a pharmacist or doctor. Educational information, not medical advice.
How They Interact
Phenytoin can speed up how your body processes aprepitant, which may make the aprepitant not work as well.
What To Do
You should avoid taking these two medications together. Ask your doctor for an alternative treatment.
FDA Label Information
Intervention Avoid concomitant use of aprepitant Examples rifampin, carbamazepine, phenytoin 7.1 Effect of Aprepitant on the Pharmacokinetics of Other Drugs Aprepitant is a substrate, a weak-to-moderate (dose-dependent) inhibitor, and an inducer of CYP3A4. Intervention Avoid concomitant use of aprepitant Examples rifampin, carbamazepine, phenytoin
Phenytoin Also Interacts With
- Nirmatrelvir/Ritonavir major
- Posaconazole major
- Ranolazine major
- Voriconazole major
- Apixaban moderate
Aprepitant Also Interacts With
- Pimozide major
- Alprazolam moderate
- Diltiazem moderate
- Methylprednisolone moderate
- Ketoconazole moderate
Frequently Asked Questions
Can I take Phenytoin and Aprepitant together?
This is a moderate interaction. You should avoid taking these two medications together. Ask your doctor for an alternative treatment.
How serious is the interaction between Phenytoin and Aprepitant?
This interaction is classified as "moderate" severity by the FDA. Moderate interactions may worsen your condition or change how your medications work.
Why do Phenytoin and Aprepitant interact?
Phenytoin can speed up how your body processes aprepitant, which may make the aprepitant not work as well.
Understanding the Phenytoin and Aprepitant Interaction
FDA-approved prescribing information for these two drugs flags their combination as a moderate-severity interaction. Phenytoin belongs to the Anticonvulsant (Hydantoin) class and Aprepitant belongs to the NK1 Receptor Antagonist (Antiemetic) class - two categories that can collide when co-prescribed. The mechanism described in FDA labeling is: Phenytoin can speed up how your body processes aprepitant, which may make the aprepitant not work as well. Severity tiers matter: major flags generally advise avoidance, moderate flags often require monitoring or dose adjustment, and minor flags may only call for awareness.
Context around a specific patient determines real-world impact. Phenytoin has 147 total documented interactions on file in this dataset, and Aprepitant has 22. Each additional medication compounds the interaction surface, which is why pharmacists run full-profile checks rather than evaluating one pair at a time. FDA-derived guidance for this pair is: You should avoid taking these two medications together. Timing of doses, renal and hepatic function, age, and other concurrent prescriptions all shape whether a labeled interaction matters clinically.
An interaction flag is not a verdict. A large share of labeled interactions are managed routinely in clinical practice, the fix may be as simple as spacing doses or adding a monitoring test. Others require the prescriber to choose a different medication entirely. This page surfaces FDA-sourced labeling and openFDA data for educational purposes only; it is not medical advice and cannot account for your full clinical picture. Never start, stop, or adjust either Phenytoin or Aprepitant based on a web page, speak with your prescriber or pharmacist before making any change.
Sources: FDA Drug Labels (SPL) via openFDA (2026). This is for informational purposes only and is not medical advice. Always consult your healthcare provider about drug interactions.
Read our methodology - how this data is sourced, computed, and verified.