Coadministration of zidovudine (200 mg three time daily) and ritonavir (300 mg four times daily) decreased zidovudine AUC by 25%. Ritonavir may induce the glucuronidation of zidovudine, resulting in slightly decreased levels of zidovudine. Dose alterations should not be necessary.
Norvir Summary of Product Characteristics, AbbVie Ltd, September 2016.

Coadministration of ritonavir (300 mg every 6 hours for 4 days) and zidovudine (200 mg every 8 hours for 4 days) to 10 subjects had no effect on ritonavir AUC, Cmax or Cmin, but decreased zidovudine AUC and Cmax by 25% and 27% respectively.
Norvir Prescribing Information, AbbVie Inc, December 2016.

Coadministration of zidovudine (200 mg three times daily for 4 days) and ritonavir (300 mg four times daily) to 9 subjects resulted in a 25% decrease in zidovudine AUC and no change in ritonavir AUC. Routine dose modification of zidovudine is not warranted with coadministration.
Retrovir Prescribing Information, ViiV Healthcare, September 2018.

Coadministration of ritonavir (300 mg 6 hourly) and zidovudine (200 mg 8 hourly) to 10 HIV+ individuals resulted in a 26% decrease in zidovudine AUC and a 27% decrease in Cmax. The clinical relevance of a 26% reduction in zidovudine exposure when administered concurrently with ritonavir is unknown. Zidovudine had no significant effect on ritonavir pharmacokinetics. 

Multidose pharmacokinetics of ritonavir and zidovudine in human immunodeficiency virus-infected patients. Cato A, Qian J, Hsu A, et al. Antimicrob Agents Chemother, 1998,42:1788–91.