Asciminib and Fosphenytoin: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Jul 11, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Fosphenytoin
Asciminib
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
Reduced asciminib exposure, reduced efficacy of asciminib, an increased phenytoin or fosphenytoin (prodrug of phenytoin) exposure and an increased risk of phenytoin or fosphenytoin-related adverse reactions
Interaction Deep Dive
Concomitant use of asciminib (CYP3A4 substrate and CYP2C9 inhibitor) with phenytoin or fosphenytoin (prodrug of phenytoin) (strong CYP3A4 inducer and CYP2C9 substrate with NTI) may reduce asciminib exposure, which may reduce the efficacy of asciminib and may also increase phenytoin or fosphenytoin exposure, which may increase the risk of adverse reactions. Avoid coadministration of total doses of asciminib 80 mg/day with phenytoin or fosphenytoin where minimal concentration changes may lead to serious adverse reactions. If coadministration is unavoidable, reduce phenytoin or fosphenytoin dosage as recommended. Avoid coadministration of asciminib 200 mg twice daily with phenytoin or fosphenytoin where minimal concentration changes may lead to serious adverse reactions. If coadministration is unavoidable, consider alternative therapy with a non-CYP2C9 substrate1.
Why it happens (mechanism)
Induction of CYP3A4-mediated metabolism of asciminib; inhibition of CYP2C9-mediated metabolism of phenytoin or fosphenytoin (prodrug of phenytoin)
Literature reports
1 report — tap to read
a) In a drug interaction study, coadministration of asciminib dose of 200 mg with a strong CYP3A4 inducer (phenytoin) 100 mg three times daily decreased asciminib AUC (inf) by 34% and C max decreased by 22%. The asciminib AUC inf decreased by 15% while C max increased by 9%, following coadministration of a single asciminib dose of 40 mg with a strong CYP3A4 inducer (rifampicin) 1.
Common questions
Can I take Asciminib and Fosphenytoin together?
Reduced asciminib exposure, reduced efficacy of asciminib, an increased phenytoin or fosphenytoin (prodrug of phenytoin) exposure and an increased risk of phenytoin or fosphenytoin-related adverse reactions Always confirm with your pharmacist or prescriber before making any change.
How serious is the Asciminib and Fosphenytoin 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 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 Asciminib or Fosphenytoin 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: SCEMBLIX® oral tablets, asciminib oral tablets. Novartis Pharmaceuticals Corporation (per FDA), East Hanover, NJ, 2025. DailyMed
Keep reading about Fosphenytoin
Keep reading about Asciminib
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