Coadministration of warfarin (5 mg single dose) and ritonavir (400 mg twice daily) increased both the AUC and Cmin of S-warfarin by 9%; R-warfarin AUC was decreased by 33%, but there was no change in Cmin. Induction of CYP1A2 and CYP2C9 lead to decreased levels of R-warfarin while little pharmacokinetic effect is noted on S- warfarin when co-administered with ritonavir. Decreased R-warfarin levels may lead to reduced anticoagulation, therefore it is recommended that anticoagulation parameters are monitored when warfarin is co-administered with ritonavir dosed as an antiretroviral agent or as a pharmacokinetic enhancer.
Norvir Summary of Product Characteristics, AbbVie Ltd, September 2016.

The coadministration of ritonavir (400 mg every 12 hours for 12 days) and a single 5 mg dose of warfarin was investigated in 12 subjects. S-warfarin AUC was increased by 9% and the Cmax decreased by 9%. The AUC for the R-enantiomer was decreased by 33% and the Cmax unaltered with coadministration of ritonavir. Initial frequent monitoring of the INR during ritonavir and warfarin coadministration is indicated.
Norvir Prescribing Information, AbbVie Inc, December 2016.

There is a case report where the warfarin dose had to be almost doubled to maintain INR whilst on ritonavir. When ritonavir was discontinued, the INR rose rapidly and the warfarin requirement decreased significantly. A potentiation of warfarin effect was expected when coadministered with ritonavir, however the opposite occurred. Frequent, careful monitoring of warfarin is recommended when coadministered with ritonavir. Potential for either an increase or decrease in INR.
Potential interaction involving warfarin and ritonavir. Knoell KR, Young TL, Cousins ES. Ann Pharmacother, 1998, 32, 1299–1301.