Abemaciclib and Phenobarbital: 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
Phenobarbital
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.
Phenobarbital can make abemaciclib (Verzenio) work less well. Phenobarbital speeds up a liver enzyme called CYP3A that breaks down abemaciclib, a medicine used to treat certain breast cancers. When that enzyme works faster, your body clears the cancer medicine more quickly, so there is less of it in your blood to do its job.
This matters because a lower level could make your cancer treatment less effective. The good news is that your care team can watch for this. Please don't change or stop either medicine on your own. Talk with your oncologist or pharmacist, who can adjust your treatment or monitor you more closely to keep things on track.
Effect: Phenobarbital, a strong CYP3A4 inducer, increases CYP3A-mediated metabolism of abemaciclib (a CYP3A4 substrate), lowering abemaciclib AUC/Cmax and its active metabolites, with potential loss of antineoplastic efficacy.
- Direction: reduced abemaciclib exposure and effect (induction)
- Onset: unspecified; enzyme induction is typically delayed (days to weeks)
- Evidence: theoretical
- Magnitude: not quantified
Management: Avoid concomitant use where possible. If unavoidable, the abemaciclib dose may need to be adjusted and individualized by the care team, with closer monitoring of treatment response. Consider alternative non-inducing agents.
What happens
Reduced CYP3A substrate exposure and reduced efficacy of CYP3A substrate
Interaction Deep Dive
When PHENobarbital, which induces CYP3A, is administered together with a CYP3A substrate, the resulting reduction in substrate exposure can diminish the therapeutic effect of that CYP3A substrate. In situations where combined use is unavoidable, neonates should be observed carefully, and raising the dose of the CYP3A substrate as needed should be considered1.
Why it happens (mechanism)
Induction of CYP3A-mediated metabolism of CYP3A substrates
How to manage this interaction
Keep taking both medicines as prescribed unless your care team tells you otherwise. This is something your oncology team can manage.
- Your team may prefer to avoid this combination if another option is available.
- If both must be used, your abemaciclib dose may need to be adjusted and individualized by your care team.
- Expect closer monitoring of how well your cancer treatment is working.
What to do: Tell your oncologist and pharmacist that you take both drugs, and check before starting, stopping, or changing either one.
Management is individual — confirm any change with your pharmacist or prescriber.
Common questions
Can I take Abemaciclib and Phenobarbital together?
Phenobarbital can lower abemaciclib levels and may make it work less well, so tell your oncology team; they can adjust the dose or monitor you more closely, or pick a different medicine. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Abemaciclib and Phenobarbital 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 Phenobarbital interaction managed?
Keep taking both medicines as prescribed unless your care team tells you otherwise. This is something your oncology team can manage. Your team may prefer to avoid this combination if another option is available. If both must be used, your abemaciclib dose may need to be adjusted and individualized by your care team. Expect closer monitoring of how well your cancer treatment is working. What to do:… 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 Phenobarbital 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: SEZABY(TM) intravenous injection, phenobarbital sodium intravenous injection. Sun Pharmaceutical Industries Inc (per manufacturer), Cranbury, NJ, 2022. DailyMed
Keep reading about Phenobarbital
Keep reading about Abemaciclib
These medications also interact with supplements
Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:
major · moderate · minor — check everything you take with our drug–supplement interaction checker.
Check another combination
Our instant two-drug interaction checker is almost here.
Still have questions about this combination?
Every question gets a real answer from a licensed pharmacist — free, and usually within a day.
Ask the pharmacist