Ritlecitinib and Abemaciclib: 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
Ritlecitinib
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.
You've been prescribed two medicines that can interact. Ritlecitinib (Litfulo) slows down a liver enzyme (called CYP3A) that your body uses to break down abemaciclib (Verzenio). When that enzyme is slowed, abemaciclib can build up to higher levels than usual.
Higher levels of abemaciclib can mean more side effects, such as diarrhea, low blood counts, tiredness, or belly upset. The good news is this is very manageable. Your care team can adjust your abemaciclib dose and watch you a little more closely. Please don't stop or change either medicine on your own, just check in with your doctor or pharmacist so they can tailor things for you.
Effect: Ritlecitinib is a moderate CYP3A inhibitor; abemaciclib is a sensitive CYP3A4 substrate. Concomitant use is expected to increase abemaciclib exposure (AUC/Cmax), raising the risk of dose-related toxicity (diarrhea, neutropenia, hepatotoxicity, fatigue).
- Direction: Increased abemaciclib exposure (inhibitor effect).
- Magnitude: Not directly studied; ritlecitinib raised midazolam AUC 2.69-fold, Cmax 1.81-fold.
- Onset: Unspecified; evidence probable. Severity major.
- Management: Consider abemaciclib dose reduction and enhanced monitoring (CBC, LFTs, GI toxicity) per oncology guidance.
What happens
Increased CYP3A substrate exposure and an increased risk of CYP3A substrate-related adverse reactions
Interaction Deep Dive
When sensitive CYP3A substrates are taken together with ritlecitinib, the exposure to those substrates rises, potentially heightening the likelihood of adverse reactions tied to them. In situations where minor changes in concentration could trigger serious adverse reactions during combined use with ritlecitinib, extra monitoring and dose adjustments should be considered. Administering ritlecitinib at 200 mg once daily over 11 days alongside midazolam (a sensitive CYP3A substrate) raised midazolam's AUC by 2.69-fold and its Cmax by 1.81-fold1.
Why it happens (mechanism)
Inhibition of CYP3A-substrate metabolism by ritlecitinib
How to manage this interaction
This combination is manageable with a little extra attention from your care team.
- Keep taking both exactly as prescribed unless your doctor tells you otherwise.
- Your abemaciclib dose may need to be adjusted and individualized by your care team since ritlecitinib can raise its levels.
- Your team may monitor you more closely, including blood counts and liver tests.
- Report side effects promptly, especially diarrhea, unusual fatigue, fever, or signs of infection.
Ask your pharmacist or oncologist before starting, stopping, or changing either medicine so they can tailor the plan for you.
Management is individual — confirm any change with your pharmacist or prescriber.
Literature reports
1 report — tap to read
a) Taking a 200 mg once daily dose of ritlecitinib for 11 days together with midazolam (a sensitive CYP3A substrate) raised the AUC(inf) and Cmax of midazolam by 2.69-fold and 1.81-fold, respectively 1.
Common questions
Can I take Ritlecitinib and Abemaciclib together?
Ritlecitinib can raise abemaciclib levels and increase side effects like diarrhea and low blood counts; your care team can manage this with dose adjustment and closer monitoring, so keep taking both as prescribed and check in with them. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Ritlecitinib and Abemaciclib 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 Ritlecitinib and Abemaciclib interaction managed?
This combination is manageable with a little extra attention from your care team. Keep taking both exactly as prescribed unless your doctor tells you otherwise. Your abemaciclib dose may need to be adjusted and individualized by your care team since ritlecitinib can raise its levels. Your team may monitor you more closely, including blood counts and liver tests. Report side effects promptly, espec… Management is individual — always follow your own care team's guidance.
How strong is the evidence for this interaction?
The evidence is graded "probable". Good supporting evidence, though not definitively proven.
Questions for your pharmacist
- Does my dose of Ritlecitinib or Abemaciclib 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: LITFULO(TM) oral capsules, ritlecitinib oral capsules. Pfizer Labs (per FDA), New York, NY, 2023. DailyMed
Keep reading about Ritlecitinib
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.
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