Ribavirin (Copegus; Rebetol; Ribasphere; Vilona,Virazole, also generics from Sandoz, Teva, Warrick) is an anti-viral drug which is active against a number of DNA and RNA viruses. It is a member of the nucleoside antimetabolite drugs that interfere with duplication of viral genetic material. Though not effective against all viruses, ribavirin is remarkable as a small molecule for its wide range of activity, including important activities against influenzas, flaviviruses and agents of many viral hemorrhagic fevers.

Ribavirin is a pro-drug, meaning that it is a chemical precursor for the actual phamacologically active molecule. When the pro-drug is administered, the body converts it into the desired chemical. Ribavirin is activated by cellular kinases which change it into the 5' triphosphate nucleotide. In this form it interferes with aspects of RNA metabolism related to viral reproduction. A number of mechanisms have been proposed for this (see Mechanisms of Action, below) but none of these is proven. More than one mechanism may be active.

In the U.K. & the U.S. the oral (capsule or tablet) form of ribavirin is used in the treatment of hepatitis C, in combination with interferon drugs. The aerosol form is used to treat respiratory syncytial virus-related diseases in children. In Mexico, ribavirin (\"ribavirina\") has been sold for use against influenza.

The primary observed serious adverse side-effect of ribavirin is hemolytic anemia, which may worsen preexisting cardiac disease. The mechanism for this effect is unknown. It is dose-dependent and may sometimes be compensated by decreasing dose. Ribavirin is also a teratogen in some animals species and thus poses a theoretical reproductive risk in humans, remaining a hazard as long as the drug is present, which is as 6 months after a course of the drug has ended.