Fostemsavir and Fosphenytoin: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Fosphenytoin
Fostemsavir
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 temsavir exposure, possible loss of virologic response and an increased risk of QT interval prolongation
Interaction Deep Dive
Concomitant use of fostemsavir with strong CYP3A4 inducers including fosphenytoin is contraindicated. Significant decrease in temsavir (the active moiety of fostemsavir) exposure may occur, which may result in loss of virologic response. Coadministration of fostemsavir with a drug with a known risk of torsade de pointes including fosphenytoin may increase the risk of torsade de pointes; use caution with coadministration of these drugs. In a drug interaction study, coadministration of fostemsavir 1200 mg and rifAMPin (strong CYP3A4 inducer) 600 mg among 15 subjects resulted in decreased Cmax and AUC by 0.24-fold and by 0.18-fold, respectively1.
Why it happens (mechanism)
Induction of CYP3A4-mediated metabolism of fostemsavir; additive QT interval prolongation
Literature reports
1 report — tap to read
a) In a drug interaction study, coadministration of fostemsavir 1200 mg and rifAMPin (strong CYP3A4 inducer) 600 mg among 15 subjects resulted in decreased Cmax and AUC by 0.24-fold and by 0.18-fold, respectively 1.
Common questions
Can I take Fostemsavir and Fosphenytoin together?
Reduced temsavir exposure, possible loss of virologic response and an increased risk of QT interval prolongation Always confirm with your pharmacist or prescriber before making any change.
How serious is the Fostemsavir and Fosphenytoin interaction?
It is rated contraindicated. These should generally not be used together.
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 Fostemsavir 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: RUKOBIA oral extended-release tablets, fostemsavir oral extended-release tablets. ViiV Healthcare (per FDA), Durham, NC, 2024. DailyMed
Keep reading about Fosphenytoin
Keep reading about Fostemsavir
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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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