Zidovudine is a nucleoside reverse transcriptase inhibitor (NRTI) with activity against Human Immunodeficiency Virus Type 1 (HIV-1). Zidovudine is phosphorylated to active metabolites that compete for incorporation into viral DNA. They inhibit the HIV reverse transcriptase enzyme competitively and act as a chain terminator of DNA synthesis. The lack of a 3'-OH group in the incorporated nucleoside analogue prevents the formation of the 5' to 3' phosphodiester linkage essential for DNA chain elongation, and therefore, the viral DNA growth is terminated. Zidovudine, a structural analog of thymidine, is a prodrug that must be phosphorylated to its active 5′-triphosphate metabolite, zidovudine triphosphate (ZDV-TP). It inhibits the activity of HIV-1 reverse transcriptase (RT) via DNA chain termination after incorporation of the nucleotide analogue. It competes with the natural substrate dGTP and incorporates itself into viral DNA. It is also a weak inhibitor of cellular DNA polymerase α and γ. Zidovudine is used in combination with other antiretroviral agents for the treatment of human immunovirus (HIV) infections. Zidovudine is marketed as Retrovir.

Fozivudine tidoxil is a thioether lipid–Zidovudine (ZDV) conjugate. After intake it is split intracellularly into the lipid moiety and ZDV-monophosphate, which is subsequently phosphorylated to the active metabolite ZDV-triphosphate. The rationale behind the development of fozivudine (FZD) was to take advantage of the high cleavage activity in mononuclear cells and other organs resulting in increased amounts of intracellular ZDV available for phosphorylation to the active metabolite, and a very low activity in red blood and stem cells, which should result in reduced haematologic toxicity. It is member of the family of nucleoside reverse transcriptase (RT) inhibitors. Fozivudine tidoxil has been in Phase II clinical trials for the treatment of HIV infection. There were three adverse events possibly related to fozivudine: urine abnormality, gastrointestinal pain and abnormal dreams.