antiviral drug
any substance that interferes with the normal functioning of viruses. Antiviral drugs may act by blocking host-cell enzyme systems required for viral reproduction, by blocking signals carried in messenger RNA, or by uncoating the nucleic acid molecule of the virus. These drugs are difficult to manage in clinical practice because chemicals that block viral processes may also interfere with the patient’s normal cell functions. However, antivirals occasionally interact with substances in human tissues to yield unexpected benefits, as with amantadine, which can be used as an antiparkinsonian agent.