Selpercatinib and Letermovir Injection: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Letermovir Injection
Selpercatinib
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 selpercatinib exposure and an increased risk of QT interval prolongation
Interaction Deep Dive
Avoid concomitant use of selpercatinib with a moderate CYP3A inhibitor as it leads to increased selpercatinib exposure which may increase the risk of adverse effects, including QTc interval prolongation. Simulated coadministration of multiple doses of dilTIAZem, fluconazole, or verapamil (moderate CYP3A inhibitors) is predicted to increase selpercatinib AUC (0 to inf) by 60% to 99% and Cmax by 46% to 76%. If coadministration is required, reduce selpercatinib dosage and frequently monitor QT interval and ECG. Reduce selpercatinib 160 mg twice daily to 120 mg twice daily, 120 mg twice daily to 80 mg twice daily, 80 mg twice daily to 40 mg twice daily and 40 mg three times daily to 40 mg once daily. After the inhibitor has been discontinued for 3 to 5 half-lives, resume selpercatinib at the dose taken prior to the CYP3A inhibitor1.
Why it happens (mechanism)
Inhibition of CYP3A-mediated metabolism of selpercatinib
Literature reports
1 report — tap to read
a) Simulated coadministration of multiple doses of dilTIAZem, fluconazole, or verapamil (moderate CYP3A inhibitors) is predicted to increase selpercatinib AUC (0 to inf) by 60% to 99% and Cmax by 46% to 76% 1.
Common questions
Can I take Selpercatinib and Letermovir Injection together?
Increased selpercatinib exposure and an increased risk of QT interval prolongation Always confirm with your pharmacist or prescriber before making any change.
How serious is the Selpercatinib 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 Selpercatinib 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: RETEVMO(R) oral capsules, tablets, selpercatinib oral capsules, tablets. Lilly USA LLC (per FDA), Indianapolis, IN, 2024. DailyMed
Keep reading about Letermovir Injection
Keep reading about Selpercatinib
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