Abemaciclib and Ritonavir: 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
Ritonavir
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.
Here's what's going on. Abemaciclib (Verzenio) is a cancer medicine that your body breaks down using an enzyme called CYP3A4. Ritonavir (Norvir) slows that enzyme down. So when you take them together, abemaciclib can build up to higher levels than usual. That can mean more side effects, like diarrhea, low blood counts, tiredness, or nausea.
The good news is this is very manageable. Your cancer team knows about this and can simply lower your abemaciclib dose while you're on ritonavir, then raise it back later. Please don't change anything on your own, but do let your pharmacist or doctor know you're taking both so they can tailor your dose and watch you a little more closely.
Mechanism: Ritonavir is a strong CYP3A4 inhibitor; it reduces CYP3A4-mediated metabolism of abemaciclib, increasing plasma levels of abemaciclib and its active metabolites (neither is a prodrug, so effect and toxicity increase).
- Direction: Increased abemaciclib exposure, higher toxicity risk (diarrhea, neutropenia, hepatotoxicity, VTE).
- Evidence: Theoretical/labeling-based; onset unspecified.
- Management: If starting dose was 200 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. After discontinuing ritonavir, resume the pre-inhibitor dose following 3 to 5 inhibitor half-lives. Monitor for abemaciclib-related toxicity.
What happens
Increased abemaciclib plasma concentration
Interaction Deep Dive
Taking abemaciclib together with a potent CYP3A4 inhibitor can raise plasma concentrations of abemaciclib as well as its active metabolite, potentially producing greater toxicity. When these agents are given together and abemaciclib was originally started at 200 mg or 150 mg twice daily, lower the abemaciclib dose to 100 mg twice daily. For patients whose dose was already cut to 100 mg twice daily because of adverse reactions, reduce abemaciclib further to 50 mg twice daily. Once the strong CYP3A inhibitor has been stopped, allow 3-5 half-lives of that inhibitor to pass, then raise the abemaciclib dose back to the level used prior to starting the strong inhibitor1.
Why it happens (mechanism)
Inhibition of CYP3A4-mediated abemaciclib metabolism
How to manage this interaction
This interaction is manageable with dose adjustment and monitoring. Keep taking both medicines exactly as prescribed unless your care team tells you otherwise.
- Your team may lower your abemaciclib dose while you are on ritonavir (for example, from 150 or 200 mg twice daily down to 100 mg twice daily, and further if needed).
- When ritonavir is stopped, they can raise abemaciclib back to your earlier dose after the ritonavir has cleared.
- Report any worsening diarrhea, fatigue, fever, bruising, or signs of infection promptly.
Ask your pharmacist or oncologist to confirm your correct dose whenever ritonavir is started or stopped.
Management is individual — confirm any change with your pharmacist or prescriber.
Literature reports
2 reports — tap to read
a) In drug interaction studies, taking abemaciclib together with itraconazole (a strong CYP3A inhibitor) is projected 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, giving a single 50 mg dose of abemaciclib (0.3 times the approved 150 mg dosage) alongside clarithromycin 500 mg twice daily (a strong CYP3A inhibitor) raised the AUC(0 to infinity) of abemaciclib plus its active metabolites (M2, M18, and M20) by 1.7-fold relative to abemaciclib given alone 1.
Common questions
Can I take Abemaciclib and Ritonavir together?
Ritonavir raises abemaciclib levels and can increase its side effects, so your care team will likely lower the abemaciclib dose while you take both and adjust it again when ritonavir stops. Keep taking both as prescribed and confirm your dose with your oncologist or pharmacist. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Abemaciclib and Ritonavir 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 Ritonavir interaction managed?
This interaction is manageable with dose adjustment and monitoring. Keep taking both medicines exactly as prescribed unless your care team tells you otherwise. Your team may lower your abemaciclib dose while you are on ritonavir (for example, from 150 or 200 mg twice daily down to 100 mg twice daily, and further if needed). When ritonavir is stopped, they can raise abemaciclib back to your earlier… 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 Ritonavir 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, 2017. DailyMed
Keep reading about Ritonavir
Keep reading about Abemaciclib
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