herpes outbreaks and promoted recovery, as compared with placebo treatment. Another study found that ribavirin potentiated the antiviral effect of acyclovir. Some interest has been seen in its possible use as a treatment for cancers, especially acute myeloid leukemia. Adverse effects The medication has two FDA "black box" warnings: One raises concerns that use before or during pregnancy by either sex may result in birth defects in the baby, and the other is regarding the risk of red blood cell breakdown. Ribavirin should not be given with zidovudine because of the increased risk of anemia; concurrent use with didanosine should likewise be avoided because of an increased risk of mitochondrial toxicity. Mechanisms of action It is a guanosine (ribonucleic) analog used to stop viral RNA synthesis and viral mRNA capping, thus, it is a nucleoside inhibitor. Ribavirin is a prodrug, which when metabolized resembles purine RNA nucleotides. In this form, it interferes with RNA metabolism required for viral replication. Over five direct and indirect mechanisms have been proposed for its mechanism of action. The enzyme inosine triphosphate pyrophosphatase (ITPase) dephosphorylates ribavirin triphosphate in vitro to ribavirin monophosphate, and ITPase reduced enzymatic activity present in 30% of humans potentiates mutagenesis in hepatitis C virus. RNA viruses Ribavirin's amide group can make the native nucleoside drug resemble adenosine or guanosine, depending on its rotation. For this reason, when ribavirin is incorporated into RNA, as a base analog of either adenine or guanine, it pairs equally well with either uracil or cytosine, inducing mutations in RNA-dependent replication in RNA viruses. Such hypermutation can be lethal to RNA viruses. DNA viruses Neither of these mechanisms explains ribavirin's effect on many DNA viruses, which is more of a mystery, especially given the complete inactivity of ribavirin's 2' deoxyribose analogue, which suggests that the drug functions only as an RNA nucleoside mimic, and never a DNA nucleoside mimic. Ribavirin 5'-monophosphate inhibits cellular inosine monophosphate dehydrogenase, thereby depleting intracellular pools of GTP. History Ribavirin was first made in 1972 under the national cancer institute's Virus-Cancer program. This was done by researchers from International Chemical and Nuclear Corporation including Roberts A. Smith, Joseph T. Witkovski and Roland K. Robins. It was reported that ribavirin was active against a variety of RNA and DNA viruses in culture and in animals, without undue toxicity in the context of cancer chemotherapies. By the late 1970s, the Virus-Cancer program was widely considered