Compositions and methods for increasing the metabolic lifetime of dextromethorphan and related pharmacodynamic effects

A method of administering dextromethorphan twice a day without loss of efficacy, comprising orally co-administering threohydroxybupropion or bupropion with dextromethorphan to a human being twice a day for at least eight consecutive days, wherein the human being is an extensive metabolizer of dextromethorphan in need of treatment with dextromethorphan, wherein threohydroxybupropion or bupropion is administered in an amount that results in an AUC0-12 of threohydroxybupropion in the human being, on the eight day that is at least about 2,000 ng·hr/mL, 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 threohydroxybupropion or bupropion for eight consecutive days.

A method of increasing the metabolic lifetime of dextromethorphan, comprising co-administering threohydroxybupropion or bupropion with dextromethorphan to a human being once a day for at least eight consecutive days, wherein threohydroxybupropion or bupropion is administered in an amount that results in an AUC0-12 of threohydroxybupropion in the human being, on the eighth day, that is at least about 1,000 ng·hr/mL, wherein the human being is an extensive metabolizer of dextromethorphan in need of treatment with dextromethorphan, wherein the co-administration results in an AUC0-12 of dextromethorphan that, on the eighth day, is at least about 30 times the AUC0-12 that would be achieved by administering the same amount of dextromethorphan without threohydroxybupropion or bupropion for eight consecutive days, and wherein dextromethorphan is present in the body of the human being at the same time as threohydroxybupropion or bupropion.