Because of possible induction of amantadine or rimantadine resistance, treatment of persons who have influenza-like illness should be discontinued as soon as clinically warranted, generally after three to five days of treatment or 24 to 48 hours after symptoms disappear. Selection of Drugs Amantadine and rimantadine differ in their pharmacokinetic properties. Although both drugs can cause adverse reactions of the central nervous system and of the gastrointestinal system when administered to young, healthy adults at dosages of 200 mg per day, the incidence of central nervous system side effects is higher in persons taking amantadine than in those taking rimantadine. The incidence of gastrointestinal side effects is approximately 3 percent in persons taking either drug. Side effects associated with both drugs are usually mild and resolve after discontinuation of the drug.
Side effects can diminish or disappear even after continued use of the drug.
These more severe side effects have been related to high plasma drug concentrations and have been observed most often in persons who have renal insufficiency, seizure disorders or certain psychiatric disorders and in elderly persons who have been taking amantadine as prophylaxis at a dosage of 200 mg per day. Lowering the dose reduces the side effects. Recommendations for reduced dosages for these groups of patients have been made. Physicians should review the package insert before using amantadine or rimantadine for any patient. The patient's age, weight, renal function, use of other medications, presence of other medical conditions, indications for use of amantadine or rimantadine, and the potential for interaction with other medications must be considered.