Compositions and methods comprising erythrohydroxybupropion and related compounds for improving the efficacy of dextromethorphan

A method of increasing dextromethorphan plasma levels in a human being that is an extensive metabolizer of dextromethorphan in need of treatment with dextromethorphan, comprising co-administering a) erythrohydroxybupropion or bupropion with b) dextromethorphan to the human being once or twice a day for at least eight consecutive days, wherein the daily dose of dextromethorphan is about 40 mg to about 600 mg, and co-administering erythrohydroxybupropion or bupropion with dextromethorphan 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 erythrohydroxybupropion or bupropion for eight consecutive days.