Coadministration has not been studied. Abemaciclib is primarily metabolised by CYP3A4 and concentrations are expected to increase significantly due to strong inhibition of CYP3A4 by nirmatrelvir/ritonavir. Coadministration with strong CYP3A4 inhibitors is not recommended. The decision to pause or dose-adjust abemaciclib should be made in conjunction with the patient’s oncologist. Cyclin-dependent kinase inhibitors may be paused in the context of acute infection. Restart abemaciclib 3 days after completing nirmatrelvir/ritonavir treatment given that CYP3A4 inhibition takes several days to resolve. Alternatively, if coadministration is necessary, consider a dose reduction to 50 mg once daily with close monitoring for toxicity.