Serum concentrations of venetoclax may be increased due to CYP3A inhibition by ritonavir, resulting in increased risk of tumour lysis syndrome at the dose initiation and during the ramp-up phase. For patients who have completed the ramp-up phase and are on a steady daily dose of venetoclax, it is advised that dose be reduced by at least 75% when used with strong CYP3A inhibitors (refer to venetoclax SPC for dosing instructions).