Zidovudine in the treatment of HIV:

Zidovudine inhibits the action of reverse transcriptase. It functions as an analog for thymidine (one of the nucleotide building blocks of DNA).

Zidovudine has the same shape as thymidine and can be incorporated into the growing strand of DNA. It is first phosphorylated to the triphosphate form by host cell enzymes. After thymidine has been incorporated into the DNA chain, the 3' -OH group (i.e. the -OH group attached to the 3' carbon of the sugar) can bind to the phosphate group of the next nucleotide. However, zidovudine does not have such an -OH group and instead has an azido (-N₃) group. The azido group cannot form a bond with a phosphate group, and so once zidovudine has been included in the DNA chain, no more nucleotides can be added. This means reverse transcription stops, and the chain is terminated after zidovudine is incorporated.