Based on parallel group comparison, coadministration of theophylline (3 mg/kg three times daily) and ritonavir (500 mg twice daily) decreased theophylline AUC by 43% and Cmin by 32%. An increased dose of theophylline may be required when co-administered with ritonavir, due to induction of CYP1A2.
Norvir Summary of Product Characteristics, AbbVie Ltd, September 2016.

Coadministration of ritonavir (500 mg every 12 hours for 10 days) with theophylline (3 mg/kg every 8 hours for 15 days) was investigated using a study with parallel group design. Theophylline AUC, Cmax and Cmin were reduced by 43%, 32% and 57%, respectively. Increased dosage of theophylline may be required; therapeutic drug monitoring should be considered.
Norvir Prescribing Information, AbbVie Inc, December 2016.

Coadministration of ritonavir (500 mg 12 hourly) and theophylline (3 mg/kg 8 hourly) was investigated in 27 healthy volunteers. Ritonavir decreased theophylline AUC by 43%, Cmax by 32% and Cmin by 57%. Since theophylline has a relatively narrow therapeutic range, dosage adjustment is probably necessary when ritonavir is coadministered.
Assessment of multiple doses of ritonavir on the pharmacokinetics of theophylline. Hsu A, Granneman GR, Witt G, et al. 11th International Conference on AIDS, 1996, abstract Mo.B.1200.