Ketoconazole Topical and Phenytoin: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Phenytoin
Ketoconazole Topical
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
Increased phenytoin exposure, reduced ketoconazole exposure and reduced efficacy of ketoconazole
Interaction Deep Dive
Coadministration of oral ketoconazole with strong CYP3A4 inducers, such as phenytoin is not recommended as it may reduce ketoconazole exposure to such an extent that efficacy may be reduced. Avoid the use of these drugs from 2 weeks before and during treatment with oral ketoconazole, unless the benefits outweigh the risk of potentially reduced ketoconazole efficacy. Upon coadministration, the antifungal activity should be monitored and the oral ketoconazole dose increased as deemed necessary1. Ketoconazole may increase phenytoin serum levels, increasing the risk for phenytoin toxicity 2.
Why it happens (mechanism)
Induction of CYP3A4-mediated metabolism of ketoconazole; altered metabolism of one or both drugs
Common questions
Can I take Ketoconazole Topical and Phenytoin together?
Increased phenytoin exposure, reduced ketoconazole exposure and reduced efficacy of ketoconazole Always confirm with your pharmacist or prescriber before making any change.
How serious is the Ketoconazole Topical and Phenytoin interaction?
It is rated major. Potentially serious — often needs a change or close monitoring.
How quickly could this interaction happen?
The documented onset is "delayed". Effects tend to build up gradually over days to weeks.
How strong is the evidence for this interaction?
The evidence is graded "probable". Good supporting evidence, though not definitively proven.
Questions for your pharmacist
- Does my dose of Ketoconazole Topical or Phenytoin need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there a safer alternative to one of these medications for me?
References (2)
Keep reading about Phenytoin
Keep reading about Ketoconazole Topical
These medications also interact with supplements
Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:
major · moderate · minor — check everything you take with our drug–supplement interaction checker.
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