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.

Thymidine is a water-soluble pyrimidine deoxynucleoside and potent thymidine kinase substrate. Thymidine is known to be converted to dTTP and utilized for DNA synthesis. It is also known that thymidine is converted by catabolic enzymes to several end products. Excess thymidine results in high levels of dTTP, which exerts a regulatory feedback inhibition in the nucleotide biosynthesis pathways leading to DNA synthesis. This property of thymidine is used when it is given in high doses to patients with solid tumors. Thymidine is being tested in Phase II of clinical trials for the treatment of cancer and 5'-Nucleotidase Syndrome.