Coadministration of sulfamethoxazole/trimethoprim (800/160 mg single dose) and ritonavir (500 mg twice daily) decreased sulfamethoxazole AUC by 20% and increased trimethoprim AUC by 20%; there was no change in Cmin of either sulfamethoxazole or trimethoprim. Dose alteration of sulfamethoxazole/trimethoprim during concomitant ritonavir therapy should not be necessary.
Norvir Summary of Product Characteristics, AbbVie Ltd, September 2016.

When ritonavir (500 mg every 12 hours for 12 days) was coadministered with a single dose of trimethoprim/sulfamethoxazole (160/800 mg), in 15 subjects, trimethoprim AUC increased by 20% where as sulfamethoxazole AUC decreased by 20%. The Cmax of neither drug was altered by coadministration with ritonavir.
Norvir Prescribing Information, AbbVie Inc, December 2016.

Coadministration of ritonavir (500 mg 12 hourly) and a single dose of a combined formulation containing trimethoprim (160 mg) and sulfamethoxazole (800 mg) was studied in 15 healthy volunteers. There was a 20% decrease in AUC of sulfamethoxazole, a 10% decrease in AUC of N-acetyl sulfamethoxazole and the AUC of trimethoprim increased by 20%. There was no significant change in the Cmax for sulfamethoxazole or trimethoprim. The half-life of sulfamethoxazole decreased by 1.6 h and that of trimethoprim increased by 2 h. The minor PK changes for trimethoprim and sulfamethoxazole are likely not to be clinically relevant given the wide safety margin.
Effect of ritonavir on the pharmacokinetics of trimethoprim/sulphamethoxazole. Bertz RJ, Cao G, Cavanaugh JH, et al. 11th International Conference on AIDS, 1996, abstract Mo.B.1197.