Abemaciclib and Letermovir: Interaction Details
AI-assisted, pharmacist-reviewed · AI content regenerated Jul 11, 2026 · Source data updated Jul 10, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Letermovir
Abemaciclib
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.
These two medicines can interact. Letermovir (Prevymis) slows down a liver enzyme that normally helps clear abemaciclib (Verzenio) from your body. When that enzyme is slowed, more abemaciclib can build up in your blood.
Because abemaciclib can already cause side effects like diarrhea, low blood counts, tiredness, and nausea, higher levels may make those side effects stronger. The good news is this is very manageable. Your care team may watch you a bit more closely and can adjust your abemaciclib dose if needed. Keep taking both as prescribed, and let your pharmacist or doctor know if you notice new or worsening side effects.
Effect: Increased abemaciclib exposure. Mechanism: Letermovir is a moderate CYP3A4 inhibitor; abemaciclib is a CYP3A4 substrate (active drug, not a prodrug), so inhibition reduces metabolism and raises abemaciclib and active-metabolite plasma levels, increasing toxicity risk.
- Direction: Increased abemaciclib effect/toxicity.
- Evidence: Theoretical; magnitude unquantified. Onset unspecified.
- Monitoring: Watch for diarrhea, neutropenia, hepatotoxicity, fatigue, VTE.
- Management: If coadministered, monitor for adverse reactions and consider reducing abemaciclib in 50-mg increments per labeling (e.g., 150 mg BID down to 100 then 50 mg BID; 200 mg BID stepwise to 50 mg BID; discontinue if 50 mg BID not tolerated).
What happens
Increased abemaciclib plasma concentration
Interaction Deep Dive
Taking abemaciclib (which is metabolized by CYP3A4) together with a moderate CYP3A4 inhibitor can raise plasma concentrations of both abemaciclib and its active metabolite, potentially increasing toxicity. When these agents are given together, watch for adverse reactions and consider lowering the abemaciclib dose in 50-mg steps. For a patient on 150 mg twice daily, reduce first to 100 mg twice daily, then to 50 mg twice daily, and stop the drug if 50 mg twice daily is not tolerated. For a patient on 200 mg twice daily, reduce first to 150 mg twice daily, then to 100 mg twice daily, then to 50 mg twice daily, and discontinue if 50 mg twice daily is not tolerated1.
Why it happens (mechanism)
Inhibition of CYP3A4-mediated abemaciclib metabolism
How to manage this interaction
You can usually take both medicines together safely when your care team is aware and watching for side effects.
- Keep taking both as prescribed unless your prescriber tells you otherwise.
- Your team may monitor you more closely for abemaciclib side effects such as diarrhea, low blood counts, fatigue, or liver changes.
- If needed, your abemaciclib dose may be adjusted and individualized by your care team, typically lowered in 50-mg steps.
- Report new or worsening diarrhea, unusual bruising or fever, or severe tiredness to your pharmacist or doctor promptly.
Management is individual — confirm any change with your pharmacist or prescriber.
Literature reports
1 report — tap to read
a) Drug interaction studies predict that giving abemaciclib together with verapamil and diltiazem (moderate CYP3A inhibitors) will raise the AUC of abemaciclib along with its active metabolites (M2, M18, and M20) by roughly 1.6-fold and 2.4-fold, respectively 1.
Common questions
Can I take Abemaciclib and Letermovir together?
Letermovir can raise abemaciclib levels and side effects; keep taking both as prescribed but watch for stronger abemaciclib side effects, and your team may monitor you more closely or lower the abemaciclib dose. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Abemaciclib and Letermovir interaction?
It is rated moderate. Can be significant — usually manageable with monitoring.
How quickly could this interaction happen?
The documented onset is "unspecified". The timing of this interaction is not well characterized.
How is the Abemaciclib and Letermovir interaction managed?
You can usually take both medicines together safely when your care team is aware and watching for side effects. Keep taking both as prescribed unless your prescriber tells you otherwise. Your team may monitor you more closely for abemaciclib side effects such as diarrhea, low blood counts, fatigue, or liver changes. If needed, your abemaciclib dose may be adjusted and individualized by your care t… Management is individual — always follow your own care team's guidance.
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 Abemaciclib or Letermovir 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 anything you'd monitor while I'm on both?
References (1)
- Product Information: VERZENIO(TM) oral tablets, abemaciclib oral tablets. Lilly USA LLC (per FDA), Indianapolis, IN, 2018. DailyMed
Keep reading about Letermovir
Keep reading about Abemaciclib
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