Two antiviral drugs—amantadine and rimantadine—are currently approved for the treatment of influenza A. Neither has activity against influenza B. For maximal effectiveness, both need to be started within 48 h of onset of symptoms and can reduce the duration of systemic symptoms by 1 to 2 days. The dose is 100 mg bid for 5 days for both drugs. Amantadine is cleared renally, and the dose needs to be adjusted in the elderly and in renal insufficiency. The recommended dose for persons older than age 65 is 100 mg qd. Amantadine causes an increase in seizure activity in patients with a preexisting seizure disorder. Rimantadine is cleared hepatically, and a 50 percent dose reduction is recommended in severe liver dysfunction. Rimantadine has a significantly lower incidence of central nervous system (CNS) side effects than does amantadine. Neither drug should be used during pregnancy. Both amantadine and rimantadine can be used in children older than the age of 1 year, although only rimantadine is officially approved for prophylaxis in children. Amantadine costs approximately $20 for a course of therapy versus about $140 for a course of rimantadine in adults. A new medication—zanamavir—appears promising in clinical trials and has activity against both influenza A and B. 8
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