AIDS drug capsules. Hand with capsules of the drug AZT (azidodideoxythymidine, also known as zidovudine or Retrovir). AZT was the first drug effective against the human immunodeficiency virus (HIV), which causes AIDS. AZT blocks the function of reverse transcriptase, an enzyme that the virus uses to copy its genetic information. AZT's side effects are severe in some people. It is now usually taken in combination with newer drugs to prevent the virus developing resistance. AIDS (acquired immune deficiency syndrome) results when HIV attacks the immune system, leaving affected people extremely vulnerable to other infections.