or zidovudinezīdōˈvyoodēnˌ, drug used to treat patients infected with the human immunodeficiency virus ( HIV), which causes AIDS; also called azidothymidine. Originally developed in 1964 as an anticancer drug, AZT was never approved for that purpose. In 1984, Burroughs-Wellcome Company, which owned the rights to the drug, reexamined it as part of a search for any antiviral drug that might be effective against the AIDS virus. It was approved by the FDA in 20 months, rather than the usual 8 to 10 years, in part for humanitarian reasons; thousands of people were dying of AIDS, no other treatment was forthcoming, and AIDS activists were lobbying heavily for approval.

AZT affects HIV's ability to reproduce by inhibiting the transcription of RNA to DNA. Although AZT can be helpful in the short term by promoting weight gain, decreasing the number of opportunistic infections, and improving T4 (CD4) lymphocyte counts (see immunity), some researcher believe studies of its effectiveness to be flawed and regard the drug as too toxic for long-term use. There is also a question of whether it is helpful in HIV-positive, asymptomatic people. AZT does not cure or prevent AIDS, nor does it keep one from transmitting the virus to others, although some studies show that it does lessen the possibility that an HIV-infected mother will transmit the virus to her fetus.

Adverse effects include bone marrow depression, headache, nausea, muscle pain, and a reduction in the number of certain white blood cells. The risk of side effects increases when certain other drugs, including acetaminophen, are taken at the same time.

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