Ribavirin (brand names: Copegus, Rebetol, Ribasphere, Vilona and Virazole) is an anti-viral drug indicated for severe RSV infection (individually), hepatitis C infection (used in conjunction with peginterferon alfa-2b or peginterferon alfa-2a) and other viral infections. Ribavirin is a prodrug, which when metabolized resembles purine RNA nucleotides. In this form it interferes with RNA metabolism required for viral replication. How it exactly affects viral replication is unknown; many mechanisms have been proposed for this (see Mechanisms of Action, below) but none of these has been proven to date. Multiple mechanisms may be responsible for its actions.

The primary observed serious adverse side effect of ribavirin is hemolytic anemia, which may worsen preexisting cardiac disease. The mechanism for this effect is due to ribavarin's buildup inside erythrocytes. Oxidative damage to erythrocyte cell membrane is usually inhibited by glutathione; however, with reduced ATP levels caused by ribavirin, glutathione levels are impaired, permitting oxidative erythrocyte cell lysis. The gradual loss of erythrocytes leads to anemia. The anemia 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 can be as long as 6 months after a course of the drug has ended.