Coadministration of didanosine (200 mg twice daily) given 2 hours prior to ritonavir (600 mg twice daily) decreased didanosine AUC by 13% and had no effect on Cmin. As ritonavir is recommended to be taken with food and didanosine should be taken on an empty stomach, dosing should be separated by 2.5 h. Dose alterations should not be necessary.
Norvir Summary of Product Characteristics, AbbVie Ltd, September 2016.

Coadministration of ritonavir (600 mg every 12 hours for 4 days) with didanosine (200 mg every 12 hours for 4 days), in 12 subjects, caused no alteration in ritonavir AUC, Cmax or Cmin but decreased didanosine AUC and Cmax by 13% and 16%, respectively with no change in didanosine Cmin.
Norvir Prescribing Information, AbbVie Inc, December 2016.

When the buffered formulation of ddI (200 mg twice daily for 4 days) was coadministered with ritonavir (600 mg twice daily for 4 days) to 12 HIV-negative subjects, ddI AUC and Cmax decreased by 13% and 16% respectively. There was no change in the AUC or Cmax of ritonavir.
Videx Prescribing Information, Bristol-Myers Squibb Company, December 2018.

Coadministration of ritonavir (600 mg every 12 h) and didanosine (200 mg every 12 h) to 12 HIV+ individuals found that the AUC and Cmax of didanosine were both reduced by ~15%. The relative small magnitude of change in ddI pharmacokinetics is of doubtful clinical significance and probably does not warrant an alteration in ddI dosage. However, dosing of ritonavir and ddI should be separated by 2.5 h to avoid formulation incompatibilities. Didanosine has little, if any, effect on ritonavir pharmacokinetics.
Pharmacokinetic interaction between ritonavir and didanosine when administered concurrently to HIV-positive patients. Cato A, Qian J, Hsu A, et al. J Acq Immune Def Syndr Hum Retrovirol, 1998, 18:466–72.