Abemaciclib and Nirmatrelvir: 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
Abemaciclib
Nirmatrelvir
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.
You're taking abemaciclib (Verzenio), a cancer medicine, along with nirmatrelvir, which is part of the COVID-19 treatment Paxlovid. Nirmatrelvir (with ritonavir) strongly slows down the liver enzyme that clears abemaciclib from your body. That means abemaciclib can build up to higher levels than expected, which can raise the risk of side effects like diarrhea, low blood counts, tiredness, and nausea.
The good news is that this is very manageable. Your cancer team may lower your abemaciclib dose while you're on the COVID medicine and then raise it back afterward. Please don't change anything on your own, just check in with your oncologist or pharmacist so they can guide the timing and dose.
Effect: Nirmatrelvir/ritonavir is a strong CYP3A4 inhibitor; abemaciclib is a CYP3A4 substrate. Inhibition reduces metabolism, increasing abemaciclib and active metabolite exposure (AUC/Cmax) and toxicity risk (GI, hematologic, hepatic).
Severity: Major. Onset: unspecified. Evidence: theoretical.
Management (per labeling):
- If starting dose was 200 mg or 150 mg BID, reduce abemaciclib to 100 mg BID.
- If already reduced to 100 mg BID for adverse reactions, reduce further to 50 mg BID.
- On inhibitor discontinuation, resume prior dose after 3 to 5 inhibitor half-lives.
Monitor for diarrhea, neutropenia, and hepatotoxicity.
What happens
Increased abemaciclib plasma concentration
Interaction Deep Dive
Taking abemaciclib together with a potent CYP3A4 inhibitor (for example, nirmatrelvir/ritonavir) can raise the plasma concentrations of abemaciclib2 along with its active metabolite, potentially heightening toxicity. When these agents are given together and the initial abemaciclib dose was 200 mg or 150 mg twice daily, the abemaciclib dose should be lowered to 100 mg twice daily. For patients whose dose has already been decreased to 100 mg twice daily because of adverse reactions, the abemaciclib dose should be reduced further to 50 mg twice daily. Once the strong CYP3A inhibitor has been stopped, the abemaciclib dose can be raised (following 3-5 half-lives of the inhibitor) back to the dose used prior to starting the strong inhibitor 1.
Why it happens (mechanism)
Inhibition of CYP3A4-mediated abemaciclib metabolism
How to manage this interaction
Keep taking both medicines as prescribed unless your care team tells you otherwise. This combination is well understood and can be managed.
- Your oncology team will likely lower your abemaciclib dose while you are on nirmatrelvir, then return it to your previous dose a few days after the COVID medicine is finished.
- Watch for and report diarrhea, unusual tiredness, fever or signs of infection, or nausea.
- Before starting a short COVID course, tell the prescriber you take Verzenio so the dosing can be coordinated.
Ask your oncologist or pharmacist to confirm the right dose and timing for you.
Management is individual — confirm any change with your pharmacist or prescriber.
Literature reports
2 reports — tap to read
a) In drug interaction studies, giving abemaciclib together with itraconazole (a potent CYP3A inhibitor) is expected to raise the AUC of abemaciclib along with its active metabolites (M2, M18 and M20) by 2.2-fold 1.
b) In drug interaction studies conducted in cancer patients, administering a single 50 mg dose of abemaciclib (0.3 times the approved 150 mg dosage) alongside clarithromycin 500 mg twice daily (a potent CYP3A inhibitor) raised the AUC(0 to infinity) of abemaciclib together with its active metabolites (M2, M18, and M20) by 1.7-fold relative to abemaciclib given by itself 1.
Common questions
Can I take Abemaciclib and Nirmatrelvir together?
Nirmatrelvir (Paxlovid) can raise abemaciclib levels and side-effect risk, so your oncology team will usually lower the abemaciclib dose during treatment and restore it afterward. Keep both as prescribed and coordinate with your oncologist or pharmacist. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Abemaciclib and Nirmatrelvir 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 is the Abemaciclib and Nirmatrelvir interaction managed?
Keep taking both medicines as prescribed unless your care team tells you otherwise. This combination is well understood and can be managed. Your oncology team will likely lower your abemaciclib dose while you are on nirmatrelvir, then return it to your previous dose a few days after the COVID medicine is finished. Watch for and report diarrhea, unusual tiredness, fever or signs of infection, or na… 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 Nirmatrelvir 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: VERZENIO(TM) oral tablets, abemaciclib oral tablets. Lilly USA LLC (per FDA), Indianapolis, IN, 2017. DailyMed
- Pfizer Inc: Fact sheet for healthcare providers: Emergency Use Authorization for PAXLOVID(TM). US Food & Drug Administration (FDA). Silver Spring, MD. 2021.
Keep reading about Abemaciclib
Keep reading about Nirmatrelvir
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