Phenytoin and Posaconazole Interaction
Drug interaction information between Phenytoin and Posaconazole.
Phenytoin and Posaconazole have a documented major interaction in FDA labeling.
FDA drug labeling documents a major-severity interaction between Phenytoin and Posaconazole. 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 causes the body to break down posaconazole much faster than usual, which can make the antifungal medicine less effective.
What To Do
Avoid using these medicines at the same time unless your doctor decides it is necessary and monitors you closely.
FDA Label Information
Interaction Drug Interaction Rifabutin, phenytoin, efavirenz, cimetidine, esomeprazole* Avoid coadministration unless the benefit outweighs the risks ( 7.6 , 7.7 , 7.8 , 7.9 ) Other drugs metabolized by CYP3A4 Consider dosage adjustment and monitor for adverse effects and toxicity ( 7.1 , 7.10 , 7.11 ) Digoxin Monitor digoxin plasma concentrations ( 7.12 ) Fosamprenavir, metoclopramide* Monitor for breakthrough fungal infections ( 7.6 , 7.13 ) *The drug interactions with esomeprazole and metoclopramide do not apply to posaconazole tablets. 7.8 Phenytoin Phenytoin induces UDP-glucuronidase...
Phenytoin Also Interacts With
- Nirmatrelvir/Ritonavir major
- Ranolazine major
- Voriconazole major
- Apixaban moderate
- Aprepitant moderate
Posaconazole Also Interacts With
- Esomeprazole major
- Digoxin major
- Cimetidine major
- Metoclopramide major
- Efavirenz/Emtricitabine/Tenofovir moderate
Frequently Asked Questions
Can I take Phenytoin and Posaconazole together?
This is a major interaction. Avoid using these medicines at the same time unless your doctor decides it is necessary and monitors you closely.
How serious is the interaction between Phenytoin and Posaconazole?
This interaction is classified as "major" severity by the FDA. Major interactions may be life-threatening or cause serious side effects.
Why do Phenytoin and Posaconazole interact?
Phenytoin causes the body to break down posaconazole much faster than usual, which can make the antifungal medicine less effective.
Understanding the Phenytoin and Posaconazole Interaction
FDA-approved prescribing information for these two drugs flags their combination as a major-severity interaction. Phenytoin belongs to the Anticonvulsant (Hydantoin) class and Posaconazole belongs to the Azole Antifungal class - two categories that can collide when co-prescribed. The mechanism described in FDA labeling is: Phenytoin causes the body to break down posaconazole much faster than usual, which can make the antifungal medicine less effective. 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 Posaconazole has 27. 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: Avoid using these medicines at the same time unless your doctor decides it is necessary and monitors you closely. 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 Posaconazole 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.