Adverse effects and drug interactions of clinical importance with antiviral drugs.

Most antiviral drugs are nucleoside analogues with potential teratogenic, embryotoxic, carcinogenic and antiproliferative activities. They must be administered with caution during pregnancy, because some are known teratogens (e.g. amantadine) and a similar propensity cannot be entirely excluded for others (e.g. aciclovir). Their adverse effects mostly involve bone marrow depression (e.g. granulocytopenia with ganciclovir, anaemia with zidovudine) or neurotoxicity (e.g. seizures with interferon-alpha, peripheral neuropathy with zalcitabine), although gastrointestinal effects are also seen. Idiosyncratic reactions include didanosine-induced acute pancreatitis. Only inosine pranobex is largely free from toxicity. Idoxuridine must be administered topically, given the severity of its systemic adverse effects. Drug interactions involving antiviral agents mostly reflect shared toxicity with other agents (e.g. neutropenia with ganciclovir and zidovudine, pancreatitis with didanosine and alcohol), although renal excretion or hepatic metabolism may be implicated. Given the possibility of severe adverse reactions and drug interactions, antiviral chemotherapy should only be used for potentially serious virus infections. Topical administration avoids systemic adverse effects but not mutagenic risks, and may result in exposure of individuals other than the patient (e.g. aerosolised ribavirin).