The interaction between nevirapine and the antithrombotic agent warfarin is complex, with the potential for both increases and decreases in coagulation time when used concomitantly. Close monitoring of anticoagulation levels is warranted.
Viramune Summary of Product Characteristics, Boehringer Ingelheim Ltd, November 2019.

The in vitro interaction between nevirapine and the antithrombotic agent warfarin in complex. As a result, when giving these drugs concomitantly, plasma warfarin levels may change with the potential for increases in coagulation time. When warfarin is co-administered with nevirapine, anticoagulation levels (INR) should be monitored frequently.
Viramune Prescribing Information, Boehringer Ingelheim Pharmaceuticals Inc, October 2019.

In case studies of three Italian patients all taking antiretroviral therapy including nevirapine plus warfarin, all experienced increased Quick times. In the first case 5 mg of warfarin per day was able to stabilise anticoagulant activity. In the second case 12 mg of warfarin was required and in the third case only after stopping nevirapine was 7.5 mg of warfarin enough to achieve the therapeutic range needed.
Need for increased dose of warfarin in HIV patients taking nevirapine. Dionisio D, Mininni S, Bartolozzi D et al. AIDS, 2001,15:277–78.