A drug used in the treatment of infections caused by viruses. Or A synthetic nucleoside analogue of deoxyadenosine in which the 3' hydroxyl on the ribose moiety is replaced by a hydrogen atom. In vivo, didanosine is converted to the active triphosphate form, which is incorporated into DNA. The absence of the 3' hydroxyl inhibits DNA elongation because phosphodiester bonds cannot be made with the substituted hydrogen.