Ribavirin is a synthetic triazole nucleoside (an analogue of guanosine) with a broad spectrum of activity against many RNA and DNA viruses, both in vitro and in vivo. Susceptible viruses include adenoviruses, herpesviruses, orthomyxoviruses, paramyxoviruses, poxviruses, picornaviruses, rhabdoviruses, rotaviruses, and retroviruses. Viral resistance to ribavirin is rare. The action of ribavirin involves specific inhibition of viral-associated enzymes, inhibition of the capping of viral mRNA, and inhibition of viral polypeptide synthesis. It is well absorbed, widely distributed in the body, eliminated by renal and biliary routes as both parent drug and metabolites, and has a plasma half-life of 24 hr in people. It does not have a wide margin of safety in domestic animals. Toxicity is manifest by anorexia, weight loss, bone marrow depression and anemia, and GI disturbances. It has been successfully administered by topical, parenteral, oral, and aerosol routes. Efficacy depends on the site of infection, method of treatment, age of the animal, and infecting dose of virus. Results of human influenza studies with ribavirin have been equivocal.