Larotrectinib and Letermovir Injection: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Larotrectinib
No brand names on recordLetermovir Injection
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 larotrectinib exposure and increase in the incidence or severity of adverse reactions
Interaction Deep Dive
Monitor for adverse reactions more frequently in patients coadministered with larotrectinib and a moderate CYP3A4 inhibitor, and reduce the larotrectinib dosage based on the severity of emergent adverse reactions. Concomitant use of larotrectinib with a moderate CYP3A4 inhibitor may increase larotrectinib plasma concentrations, which may result in a higher incidence of adverse reactions. For Grade 3 or 4 adverse reactions, withhold larotrectinib until adverse reaction resolves or improves to baseline or Grade 1. If resolution occurs within 4 weeks, resume at the next dosage modification (first dosage modification, 75 mg orally twice daily; second dosage modification, 50 mg twice daily; third dosage modification, 100 mg once daily). Permanently discontinue larotrectinib if an adverse reaction does not resolve within 4 weeks and in patients who are unable to tolerate larotrectinib after 3 dose modifications. Coadministration of larotrectinib with fluconazole (a moderate CYP3A4 inhibitor) is predicted to increase larotrectinib steady state AUC by 2.7-fold and Cmax by 1.9-fold1.
Why it happens (mechanism)
Inhibition of CYP3A4-mediated metabolism of larotrectinib
Literature reports
1 report — tap to read
a) Coadministration of larotrectinib with fluconazole (a moderate CYP3A4 inhibitor) is predicted to increase larotrectinib steady state AUC by 2.7-fold and Cmax by 1.9-fold 1.
Common questions
Can I take Larotrectinib and Letermovir Injection together?
Increased larotrectinib exposure and increase in the incidence or severity of adverse reactions Always confirm with your pharmacist or prescriber before making any change.
How serious is the Larotrectinib and Letermovir Injection 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 "theoretical". Predicted from the drugs' pharmacology; not yet confirmed in people.
Questions for your pharmacist
- Does my dose of Larotrectinib or Letermovir Injection 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: VITRAKVI(R) oral capsules, oral solution, larotrectinib oral capsules, oral solution. Bayer HealthCare Pharmaceuticals Inc (per FDA), Whippany, NJ, 2022. DailyMed
Keep reading about Larotrectinib
Keep reading about Letermovir Injection
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