Abemaciclib and Cobicistat: 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
Cobicistat
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. Cobicistat (Tybost) is a strong "booster" that slows down a liver enzyme called CYP3A4. Your cancer medicine, abemaciclib (Verzenio), is broken down by that same enzyme. So when you take them together, abemaciclib can build up higher than usual in your blood.
Higher levels can mean more side effects, like diarrhea, low blood counts, tiredness, or nausea. The good news is this is very manageable. Your care team knows about this combination and can lower your abemaciclib dose to keep you safe, then adjust it back if the cobicistat is stopped. Please don't change anything on your own, just talk with your oncology team or pharmacist.
Mechanism: Cobicistat is a potent CYP3A4 inhibitor; abemaciclib is a CYP3A4 substrate (active drug, not a prodrug). Inhibition reduces metabolic clearance, raising abemaciclib and active-metabolite exposure (AUC/Cmax) and toxicity risk.
Evidence: Theoretical/label-based. Severity: moderate. Onset: unspecified.
Management:
- 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 discontinuing cobicistat, after 3-5 inhibitor half-lives, resume the pre-inhibitor dose.
- Monitor for diarrhea, hepatotoxicity, myelosuppression, and VTE.
What happens
Increased abemaciclib plasma concentration
Interaction Deep Dive
Taking abemaciclib together with a potent CYP3A4 inhibitor can raise plasma concentrations of both abemaciclib and 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 had already been reduced to 100 mg twice daily because of adverse reactions, the abemaciclib dose should be decreased further to 50 mg twice daily. Once the strong CYP3A inhibitor has been stopped, the abemaciclib dose should be raised back (following 3-5 half-lives of the inhibitor) to the dose that had been in use prior to starting the strong inhibitor1.
Why it happens (mechanism)
Inhibition of CYP3A4-mediated abemaciclib metabolism
How to manage this interaction
This combination can be managed. Because cobicistat slows the breakdown of abemaciclib, your team may lower your abemaciclib dose while you take both.
- Keep taking both medicines exactly as prescribed unless your care team tells you otherwise.
- Your team may reduce your abemaciclib dose (for example, stepping it down) and watch you more closely for side effects.
- If the cobicistat is later stopped, your dose can be increased back once it has cleared your system.
- Report side effects like diarrhea, unusual tiredness, fever, or signs of infection.
Ask your oncology pharmacist or prescriber before making any change.
Management is individual — confirm any change with your pharmacist or prescriber.
Literature reports
2 reports — tap to read
a) Drug interaction studies predict that giving abemaciclib together with itraconazole (a strong CYP3A inhibitor) would 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 strong 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 alone 1.
Common questions
Can I take Abemaciclib and Cobicistat together?
Cobicistat can raise abemaciclib levels and side-effect risk by blocking its breakdown, so your care team may lower the abemaciclib dose and monitor you more closely. Keep taking both as prescribed and check in with your oncology team. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Abemaciclib and Cobicistat 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 Cobicistat interaction managed?
This combination can be managed. Because cobicistat slows the breakdown of abemaciclib, your team may lower your abemaciclib dose while you take both. Keep taking both medicines exactly as prescribed unless your care team tells you otherwise. Your team may reduce your abemaciclib dose (for example, stepping it down) and watch you more closely for side effects. If the cobicistat is later stopped, y… 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 Cobicistat 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 Cobicistat
Keep reading about Abemaciclib
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