Coadministration has not been studied. Vincristine is metabolized by CYP3A5/4. Lopinavir/ritonavir could potentially increase vincristine exposure. Close monitoring of vincristine induced toxicity is recommended. Consider temporarily withholding the ritonavir containing regimen in patients who develop significant haematologic or gastrointestinal side effects. If the antiretroviral regimen must be withheld for a prolonged period, consider initiating a revised regimen that does not include a CYP3A or P-gp inhibitor. An empirical dose reduction of vincristine may also be considered, especially in patients who have previously experienced or are at high risk for toxicity.