AZT

How does it stop the virus?

HIV carries its genetic information as RNA and has to convert that into DNA before it can insert itself into a human cell's chromosomes. The enzyme doing the conversion is reverse transcriptase, and it assembles the new DNA strand from individual building blocks. Zidovudine closely resembles one of those blocks, thymidine, but lacks the chemical attachment point that the next block would need. The enzyme picks it up, adds it, and the chain stops there. Without a complete DNA copy the virus cannot integrate, and that cell produces no new virus.

Why is it never given alone?

HIV copies itself in enormous numbers and makes frequent errors while doing so. Against a single drug, a variant carrying the right mutation appears quickly and takes over the population, so any benefit fades within months. Combining three drugs that act at different points makes simultaneous resistance to all of them extremely unlikely. That shift to combination treatment turned AIDS from a uniformly fatal illness into a manageable long-term condition, and no current regimen depends on one agent alone.

What is it used for now?

Newer agents have largely displaced it from first-line use because they are better tolerated and taken less often, but zidovudine remains in use, particularly where cost and availability shape the choice.

What are its side effects?

The bone marrow is affected most, so anemia and low white cell counts are the characteristic problems and blood counts are monitored during treatment. Headache, nausea and muscle aching are common early and often settle. Long use can damage muscle and the mitochondria, the compartments that generate energy inside cells, and it contributes to the redistribution of body fat seen with older drugs of this class. It entered use before those long-term effects were understood, which is part of why treatment for this immunodeficiency moved toward newer agents.