Co-administration of zidovudine results in a 13% increase in zidovudine exposure (AUC) and a 28% increase in peak plasma levels. This is not considered to be of significance to patient safety and therefore no dosage adjustments are necessary. Zidovudine has no effect on the pharmacokinetics of lamivudine. The possibility of interactions with other medicinal products administered concurrently should be considered, particularly when the main route of elimination is active renal secretion via the organic cationic transport system. Zidovudine is not eliminated by this mechanism and are unlikely to interact with lamivudine.
Epivir Summary of Product Characteristics, ViiV Healthcare UK Ltd, February 2019.

No clinically significant alterations in lamivudine or zidovudine pharmacokinetics were observed in 12 asymptomatic HIV-infected adult patients given a single dose of zidovudine (200 mg) in combination with multiple doses of lamivudine (300 mg twice daily).
Epivir Prescribing Information, ViiV Healthcare, May 2019.

A modest increase in Cmax (28%) was observed for zidovudine when administered with lamivudine, however overall exposure (AUC) was not significantly altered. Zidovudine has no effect on the pharmacokinetics of lamivudine.
Retrovir Summary of Product Characteristics, ViiV Healthcare UK, Ltd, December 2018.

Coadministration of a single dose of zidovudine (200 mg) and lamivudine (300 mg every 12 hours) increased zidovudine AUC by 13% and had no effect on the pharmacokinetics of lamivudine. Routine dose modification of zidovudine is not warranted.
Retrovir Prescribing Information, ViiV Healthcare, September 2018.