Fostemsavir and Grazoprevir: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Fostemsavir
Grazoprevir
No brand names on record
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 grazoprevir exposure and increased risk of ALT elevations
Interaction Deep Dive
Concomitant use of grazoprevir with OATP1B1/3 inhibitors, such as fostemsavir is contraindicated2. Concomitant administration of fostemsavir and grazoprevir may increase exposures of grazoprevir; however, the magnitude of increase in exposure is unknown. Increased exposures of grazoprevir may increase the risk of ALT elevations. Use an alternative hepatitis C virus regimen if possible 1. In single-dose studies, coadministration of rifAMPin (OATP1B1/3 inhibitor) 600 mg IV with grazoprevir 200 mg in 12 healthy participants resulted in an increased grazoprevir AUC by 10.21-fold and Cmax by 10.94-fold. Similarly, when rifAMPin 600 mg orally was coadministered with grazoprevir 200 mg once daily in 12 healthy participants, grazoprevir AUC increased 8.35-fold and Cmax 6.52-fold. And when cycloSPORINE (OATP1B1/3 inhibitor) 400 mg single dose was coadministered with grazoprevir 200 mg once daily, the AUC of grazoprevir increased by 15.21-fold and the Cmax of grazoprevir increased by 17-fold 2.
Why it happens (mechanism)
Inhibition of OATP1B1/3-mediated efflux transport of grazoprevir
Literature reports
2 reports — tap to read
a) When cycloSPORINE (OATP1B1/3 inhibitor) 400 mg single dose was coadministered with grazoprevir 200 mg once daily, the AUC of grazoprevir increased by 15.21-fold and the Cmax of grazoprevir increased by 17-fold 2.
b) In single-dose studies, coadministration of rifAMPin (OATP1B1/3 inhibitor) 600 mg IV with grazoprevir 200 mg in 12 healthy participants resulted in an increased grazoprevir AUC by 10.21-fold and Cmax by 10.94-fold. Similarly, when rifAMPin 600 mg orally was coadministered with grazoprevir 200 mg once daily in 12 healthy participants, grazoprevir AUC increased 8.35-fold and Cmax 6.52-fold 2.
Common questions
Can I take Fostemsavir and Grazoprevir together?
Increased grazoprevir exposure and increased risk of ALT elevations Always confirm with your pharmacist or prescriber before making any change.
How serious is the Fostemsavir and Grazoprevir 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 Grazoprevir 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)
- Product Information: RUKOBIA oral extended-release tablets, fostemsavir oral extended-release tablets. ViiV Healthcare (per FDA), Research Triangle Park, NC, 2020. DailyMed
- Product Information: ZEPATIER(R) oral tablets, elbasvir grazoprevir oral tablets. Merck Sharp & Dohme Corp (per FDA), Whitehouse Station, NJ, 2021. DailyMed
Keep reading about Fostemsavir
Keep reading about Grazoprevir
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