Zalcitabine (trade name Hivid) is a nucleoside analog HIV reverse transcriptase inhibitor (NARTI). It is an antiviral pyrimidine nucleoside analogue effective against HIV replication. 2′,3′-Dideoxycytidine, when activated to its triphosphate, is incorporated into DNA by HIV-1 RT (HIV-1 reverse transcriptase), causing DNA chain termination and viral replication. Zalcitabine was the third antiretroviral to be approved by the Food and Drug Administration (FDA) for the treatment of HIV/AIDS. It is used as part of a combination regimen. Zalcitabine appears less potent than some other nucleoside RTIs, has an inconvenient three-times daily frequency and is associated with serious adverse events. For these reasons it is now rarely used to treat human immunodeficiency virus (HIV), and it has even been removed from pharmacies entirely in some countries.