Coadministration of alprazolam (1 mg single dose) and ritonavir (200 mg twice daily for 2 days) increased alprazolam AUC by 2.5-fold but had no effect on Cmin. Coadministration with ritonavir (500 mg twice daily for 10 days) decreased alprazolam AUC by 12% and Cmin by 16%. Alprazolam metabolism was inhibited following the introduction of ritonavir. After ritonavir use for 10 days, no inhibitory effect of ritonavir was observed. Caution is warranted during the first several days when alprazolam is co-administered with ritonavir dosed as an antiretroviral agent or as a pharmacokinetic enhancer, before induction of alprazolam metabolism develops.
Norvir Summary of Product Characteristics, AbbVie Ltd, September 2016.

Coadministration of ritonavir (500 mg every 12 hours for 10 days) with a single dose of alprazolam, in 12 subjects caused a 12% and 16% decrease in alprazolam AUC and Cmax, respectively.
Norvir Prescribing Information, AbbVie Inc, December 2016.

Following coadministration of ritonavir (500 mg 12 hourly) and a single dose of alprazolam (1 mg) to 12 healthy volunteers, it was found that alprazolam AUC was reduced by 12% and alprazolam Cmax was reduced by 16% in the presence of ritonavir. Ritonavir AUC was in the expected range. Modification of the alprazolam dose in patients receiving ritonavir is unnecessary. Contraindication of alprazolam and ritonavir is unwarranted.
Effect of ritonavir on the pharmacokinetics and pharmacodynamics of alprazolam Frye R, Bertz R, Granneman GR, et al. 37th International Conference on Antimicrobial Agents & Chemotherapy, 1997, abstract A-59.

Coadministration of ritonavir (200 mg 12 hourly, 4 doses) and alprazolam (1 mg single dose) was investigated in 10 healthy volunteers. Coadministration of ritonavir produced a highly significant prolongation of alprazolam elimination half-life, an increase in AUC and a 41% decrease in clearance. The findings emphasise the complex time dependent consequences of ritonavir co-treatment on the pharmacokinetics of alprazolam. Therapeutic guidance contained in the product labelling should reflect these complexities rather than simply allowing or prohibiting specific drug combinations.
Alprazolam-ritonavir interaction: implications for product labelling. Greenblatt DJ, von Molke LL, Harmatz JS, et al. Clin Pharmacol Ther, 2000, 67:335–41.