Phenobarbital 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
Phenobarbital
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.
What's going on: Phenobarbital speeds up a liver enzyme (called CYP3A) that breaks down many medicines. Belumosudil (Rezurock) is cleared by that same enzyme. So when you take them together, phenobarbital can cause your body to clear belumosudil faster, leaving less of it in your blood. That could make the belumosudil work less well against your graft-versus-host disease.
The good news: this concern is based mostly on how these drugs behave in theory, not on strong real-world reports. Your care team can manage it, sometimes by adjusting your belumosudil dose or watching your response more closely. Please don't change either medicine on your own. Just talk with your pharmacist or doctor.
Mechanism: Phenobarbital is a broad enzyme inducer (CYP3A among others). Belumosudil is a CYP3A substrate, so co-administration is expected to increase its metabolism, lowering belumosudil AUC/Cmax and potentially reducing efficacy.
- Direction: reduced belumosudil exposure and effect (neither drug is a prodrug here).
- Evidence: theoretical; magnitude not quantified.
- Onset: unspecified (enzyme induction typically develops over days to weeks).
- Severity on file: major.
Management: Avoid the combination if possible. If unavoidable, monitor for reduced belumosudil response and consider dose adjustment individualized by the care team. Belumosudil labeling advises against strong CYP3A inducers.
What happens
Reduced CYP3A substrate exposure and reduced efficacy of CYP3A substrate
Interaction Deep Dive
When PHENobarbital, which acts as a CYP3A inducer, is taken together with a CYP3A substrate, exposure to that substrate may be lowered, potentially diminishing the substrate's therapeutic effect. In situations where combined use is unavoidable, monitor neonates carefully and think about raising the CYP3A substrate dose as appropriate1.
Why it happens (mechanism)
Induction of CYP3A-mediated metabolism of CYP3A substrates
How to manage this interaction
Keep taking both medicines as prescribed unless your care team tells you otherwise. Here is what teams usually do with this combination:
- Try to avoid pairing belumosudil with a strong inducer like phenobarbital when a suitable alternative exists.
- If both are needed, your belumosudil dose may need to be adjusted and individualized by your care team.
- They may monitor you more closely for signs your belumosudil is working less well.
What to do: ask your pharmacist or prescriber whether an alternative to phenobarbital is possible, and report any worsening of your GVHD symptoms.
Management is individual — confirm any change with your pharmacist or prescriber.
Common questions
Can I take Phenobarbital and Belumosudil together?
Phenobarbital can lower belumosudil levels and may make it less effective, so ask your care team whether to avoid the combination or adjust and monitor your belumosudil dose. Do not change either medicine on your own. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Phenobarbital 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 Phenobarbital and Belumosudil interaction managed?
Keep taking both medicines as prescribed unless your care team tells you otherwise. Here is what teams usually do with this combination: Try to avoid pairing belumosudil with a strong inducer like phenobarbital when a suitable alternative exists. If both are needed, your belumosudil dose may need to be adjusted and individualized by your care team. They may monitor you more closely for signs your… Management is individual — always follow your own care team's guidance.
How strong is the evidence for this interaction?
The evidence is graded "theoretical". Predicted from the drugs' pharmacology; not yet confirmed in people.
Questions for your pharmacist
- Does my dose of Phenobarbital 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: SEZABY(TM) intravenous injection, phenobarbital sodium intravenous injection. Sun Pharmaceutical Industries Inc (per manufacturer), Cranbury, NJ, 2022. DailyMed
Keep reading about Phenobarbital
Keep reading about Belumosudil
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