Compositions and methods for reducing dextrorphan plasma levels and related pharmacodynamic effects

A method of increasing the metabolic lifetime of dextromethorphan, comprising co-administering hydroxybupropion or bupropion with dextromethorphan to a human being once or twice a day for at least eight consecutive days, wherein hydroxybupropion or bupropion is administered in an amount that results in an AUC0-12 of hydroxybupropion in the human being, on the eighth day, that is at least about 3,000 ng·hr/mL, wherein the human being is an extensive metabolizer of dextromethorphan in need of treatment with dextromethorphan, and wherein the co-administration results in an AUC0-12 of dextromethorphan that, on the eighth day, is at least about 20 times the AUC0-12 that would be achieved by administering the same amount of dextromethorphan without hydroxybupropion or a bupropion for eight consecutive days and wherein dextromethorphan is present in the body of the human being at the same time as hydroxybupropion or bupropion.