Letermovir and Cyclosporine Injection: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Letermovir
Cyclosporine 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 cycloSPORINE exposure and an increased letermovir exposure
Interaction Deep Dive
Coadministration of cycloSPORINE and letermovir led to a 66% increase in cycloSPORINE AUC and 111% increase in letermovir AUC. If concomitant use is necessary, the dose of letermovir should be reduced to 240 mg once daily in adult and pediatric patients 12 years of age and older. Dose adjustment may be required in pediatric patients less than 12 years of age. Frequently monitor cycloSPORINE whole blood concentrations during treatment and following discontinuation with subsequent dose adjustment as necessary1.
Why it happens (mechanism)
Inhibition of CYP3A4-mediated metabolism of cycloSPORINE; inhibition of OATP1B1-mediated efflux transport of letermovir; inhibition of OATP1B3-mediated efflux transport of letermovir
Literature reports
4 reports — tap to read
a) Mean cycloSPORINE AUC, Cmax and C24hr increased by 66%, 8% and 119%, respectively following a single 50 mg oral dose given concomitantly with letermovir 240 mg. Additionally, mean letermovir AUC, Cmax and C24hr increased by 111%, 48% and 106%, respectively following a single 240 mg oral dose given concomitantly with cycloSPORINE 200 mg 1.
b) Mean sirolimus AUC and Cmax increased by 66% and 176% following a single 2 mg oral dose given concomitantly with letermovir 480 mg 1.
c) Mean tacrolimus AUC and Cmax increased by 142% and 57% respectively, following a single 5 mg oral dose given concomitantly with letermovir 480 mg 1.
d) In a drug interaction study, coadministration of letermovir (oral 240 mg once daily) with a CYP3A substrate, midazolam (1 mg single dose IV) led to increased AUC, Cmax and C24hr of midazolam by 47%, 5% and 174%, respectively. And, coadministration of letermovir (oral 240 mg once daily) with midazolam (2 mg single dose orally) led to increased AUC and Cmax of midazolam by 125% and 72%, respectively 1.
Common questions
Can I take Letermovir and Cyclosporine Injection together?
Increased cycloSPORINE exposure and an increased letermovir exposure Always confirm with your pharmacist or prescriber before making any change.
How serious is the Letermovir and Cyclosporine 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 "established". Well documented — supported by controlled studies or strong clinical data.
Questions for your pharmacist
- Does my dose of Letermovir or Cyclosporine 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: PREVYMIS(R) oral tablets, oral pellets, intravenous injection, letermovir oral tablets, oral pellets, intravenous injection. Merck Sharp & Dohme Corp (per FDA), Rahway, NJ, 2024. DailyMed
Keep reading about Letermovir
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