Co-administration of nevirapine and ketoconazole (400 mg once daily) decreased ketoconazole AUC and Cmax by 72% and 44%, respectively. Nevirapine plasma concentrations increased by 15-28% compared to historical controls. It is not recommended to co-administer ketoconazole and nevirapine.
Viramune Summary of Product Characteristics, Boehringer Ingelheim Ltd, November 2019.

Coadministration of nevirapine (200 mg once daily for 2 weeks then 200 mg twice daily for 2 weeks) with ketoconazole (400 mg once daily), in 21 HIV+ subjects, caused a 72% and 44% reduction in ketoconazole AUC and Cmax. Ketoconazole Cmin was below the limit of detection for the assay. The effect on nevirapine pharmacokinetics was not significant when compared to historical controls. Nevirapine and ketoconazole should not be administered concomitantly because decreases in the ketoconazole plasma concentrations may reduce the efficacy of the drug.
Viramune Prescribing Information, Boehringer Ingelheim Pharmaceuticals Inc, October 2019.

Coadministration of nevirapine (200 mg twice daily) with ketoconazole (400 mg once daily) to 22 HIV+ individuals resulted in a 63% reduction in ketoconazole AUC and a 40% reduction in Cmax. There was a 15–28% increase in plasma concentration of nevirapine.
The pharmacokinetic interactions of nevirapine and ketoconazole. Lamson M, Robinson P, Lamson M et al. 12th World AIDS Conference, 1998, abstract 12218.