When ritonavir is coadministered with quinine, plasma concentrations of quinine may be increased and a decrease in dose may be needed.
Norvir Prescribing Information, AbbVie Inc, December 2016.

The pharmacokinetic interaction between ritonavir (200 mg twice daily for 9 days) and single doses of quinine (600 mg) was studied in healthy volunteers during a three-period pharmacokinetic non-randomized sequential design study. Pharmacokinetics of ritonavir, quinine and its major metabolite, 3-hydroxyquinine, were determined. Ritonavir increased quinine Cmax and AUC by 2.8-fold and 3.4-fold, respectively, with a 77% decrease in clearance. This, together with a decrease in the Cmax (50%) and AUC (60%) of the metabolite, resulted in a pronounced reduction in the ratio of metabolite:unchanged drug AUC. Quinine caused modest but significant increases in the Cmax (15%), AUC (21%) and elimination half life (32%) of ritonavir. Downward dosage adjustment of quinine appears necessary when concurrently administered with ritonavir.
Pharmacokinetic interactions between ritonavir and quinine in healthy volunteers following concurrent administration. Soyinka JO, Onyeji CO, Omoruyi SI, et al. Br J Clin Pharmacol, 2010, 69(3): 262-270.