Coadministration with nirmatrelvir/ritonavir has not been studied. Vinblastine is metabolized by CYP3A4. Concentrations are expected to increase due to strong inhibition of CYP3A4 by nirmatrelvir/ritonavir. Limited case reports with ritonavir-boosted HIV protease inhibitors suggest an ~2-fold increase in vinblastine AUC when coadministered with ritonavir (100 mg once daily). Increased risk of autonomic and peripheral neurotoxicity and neutropenia have been reported with coadministration of ritonavir and vinblastine. Decisions to pause or dose-adjust should be made in conjunction with the patient's oncologist. Vinblastine may be paused in the context of acute infection. Restart vinblastine 3 days after completing nirmatrelvir/ritonavir treatment given that CYP3A4 inhibition takes several days to resolve. Alternatively, vinblastine may be coadministered with close monitoring for haematologic and neurotoxicity. An empirical dose reduction of vinblastine may be considered, especially in patients who have previously experienced or are at high risk for toxicity.