Phenytoin and Belumosudil: Interaction Details
AI-assisted, pharmacist-reviewed · AI content regenerated Jul 15, 2026 · Source data updated Jul 11, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Phenytoin
Belumosudil
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.
Here's what's going on. Phenytoin (Dilantin) is a strong "speed-up" drug for the liver. It makes your body clear out certain other medicines faster than normal. Belumosudil (Rezurock) is broken down by one of those liver pathways, so taking phenytoin alongside it can cause your body to burn through the belumosudil too quickly.
The concern here isn't a dangerous buildup, it's the opposite: the belumosudil may not work as well because there's less of it in your body. In a study, a similar strong "speed-up" drug cut belumosudil levels by about two-thirds. The good news is your care team can manage this, often by adjusting your belumosudil dose. Please don't change anything on your own, just talk with them.
Effect: Reduced belumosudil exposure and potential loss of efficacy.
Mechanism: Phenytoin is a strong CYP3A inducer; belumosudil is a CYP3A substrate (not a prodrug), so induction accelerates its metabolism and lowers systemic exposure.
- Magnitude: With rifampin (strong inducer), belumosudil Cmax fell 59% and AUC fell 72%.
- Direction: Reduced drug effect.
- Evidence: Probable; severity major.
- Management: If coadministered, increase belumosudil to 200 mg twice daily. Monitor for reduced GVHD response.
What happens
Reduced belumosudil exposure and reduced efficacy of belumosudil
Interaction Deep Dive
When belumosudil, a CYP3A substrate, is given together with a potent CYP3A inducer, belumosudil exposure can fall, potentially lowering the drug's effectiveness. A drug interaction trial conducted in healthy volunteers showed that combining belumosudil with rifAMPin, a strong CYP3A inducer, lowered belumosudil Cmax by 59% and AUC by 72%. Should belumosudil be used alongside a strong CYP3A inducer, the belumosudil dose should be raised to 200 mg twice daily1.
Why it happens (mechanism)
Induction of CYP3A-mediated belumosudil metabolism
How to manage this interaction
The main point: phenytoin can lower belumosudil levels, so belumosudil may not control your chronic GVHD as well. This is manageable.
- Keep taking both medicines exactly as prescribed unless your care team tells you otherwise.
- When these are used together, the labeled approach is to increase belumosudil to 200 mg twice daily, so your prescriber may adjust and individualize your dose.
- Your team may also watch your response more closely.
- Tell your pharmacist or doctor that you take both, and ask before starting or stopping either one.
Management is individual — confirm any change with your pharmacist or prescriber.
Literature reports
1 report — tap to read
a) In a drug interaction study conducted in healthy subjects, giving belumosudil together with rifAMPin (a strong CYP3A inducer) reduced belumosudil Cmax by 59% and AUC by 72% 1.
Common questions
Can I take Phenytoin and Belumosudil together?
Phenytoin can substantially lower belumosudil levels and weaken its effect, but your care team can manage this, often by increasing the belumosudil dose to 200 mg twice daily. Don't change anything on your own; check with your prescriber or pharmacist. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Phenytoin and Belumosudil interaction?
It is rated major. Potentially serious — often needs a change or close monitoring.
How quickly could this interaction happen?
The documented onset is "unspecified". The timing of this interaction is not well characterized.
How is the Phenytoin and Belumosudil interaction managed?
The main point: phenytoin can lower belumosudil levels, so belumosudil may not control your chronic GVHD as well. This is manageable. Keep taking both medicines exactly as prescribed unless your care team tells you otherwise. When these are used together, the labeled approach is to increase belumosudil to 200 mg twice daily, so your prescriber may adjust and individualize your dose. Your team may… Management is individual — always follow your own care team's guidance.
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 Phenytoin or Belumosudil 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 (1)
- Product Information: REZUROCK(R) oral tablets, belumosudil oral tablets. Kadmon Pharmaceuticals LLC (per FDA), Bridgewater, NJ, 2024. DailyMed
Keep reading about Phenytoin
Keep reading about Belumosudil
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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