Coadministration has not been studied. Vincristine is metabolized by CYP3A4/5. Concentrations are expected to increase due to strong inhibition of CYP3A4 by nirmatrelvir/ritonavir. Decisions to pause or dose-adjust should be made in conjunction with the patient's oncologist. Vincristine may be paused in the context of acute infection. Restart vincristine 3 days after completing nirmatrelvir/ritonavir treatment given that CYP3A4 inhibition takes several days to resolve. Alternatively, vincristine may be coadministered with close monitoring for haematologic and neurotoxicity. An empirical dose reduction of vincristine may be considered, especially in patients who have previously experienced or are at high risk for toxicity. Increased rates of hematologic toxicity and neuropathy (including autonomic neuropathy) have been reported with coadministration of ritonavir and vincristine.