Concomitant use of ritonavir dosed as an antiretroviral agent and voriconazole is contraindicated due to reduction in voriconazole concentrations and possible loss of effect. Coadministration of voriconazole (200 mg twice daily) and ritonavir (400 mg twice daily) decreased voriconazole AUC and Cmin by 82% and 66%, respectively. Co-administration of voriconazole and ritonavir dosed as a pharmacokinetic enhancer should be avoided, unless an assessment of the benefit/risk to the patient justifies the use of voriconazole. Coadministration of voriconazole (200 mg twice daily) and ritonavir (100 mg twice daily) decreased voriconazole AUC and Cmin by 39% and 24%, respectively.
Norvir Summary of Product Characteristics, AbbVie Ltd, September 2016.

Voriconazole is contraindicated with ritonavir doses of 400 mg every 12 h or greater due to the potential for loss of antifungal response. Coadministration of voriconazole with ritonavir 400 mg every 12 hours significantly decreases voriconazole plasma concentrations and may lead to loss of antifungal response. Coadministration of voriconazole (400 mg twice daily for 1 day, then 200 mg twice daily for 8 days) and ritonavir (400 mg twice daily for 9 days) resulted in no change in ritonavir AUC or Cmax; however voriconazole AUC and Cmax decreased by 82% and 66%, respectively. Coadministration of voriconazole and ritonavir 100 mg should be avoided unless an assessment of the benefit/risk to the patient justifies the use of voriconazole.
Norvir Prescribing Information, AbbVie Inc, December 2016.

The pharmacokinetics of voriconazole and ritonavir were determined in 2 groups of HIV- subjects (n=17 per group) receiving voriconazole (200 mg twice daily) and low or high dose ritonavir (100 or 400 mg twice daily). High dose ritonavir substantially decreased voriconazole AUC (82%) and Cmax (66%); the effect of low dose ritonavir was less pronounced with AUC decreasing by 39% and Cmax by 24%. Two subjects (one per group) experienced increases in voriconazole exposure (2.5-3-fold) which the authors suggest were due to lack of CYP2C19. Voriconazole had no apparent effect on the pharmacokinetics of high dose ritonavir, but significantly decreased the AUC (14%) and Cmax (24%) of low dose ritonavir. Coadministration with high dose ritonavir is contraindicated due to the significant decrease in voriconazole exposure. Coadministration with low dose ritonavir should be avoided unless the benefit to the patient justifies the use.
Steady state pharmacokinetic and safety profiles of voriconazole and ritonavir in healthy male subjects. Liu P, Foster G, Gandelman K, et al. Antimicrob Agents Chemother, 2007, 51(10); 3617-3626.