This disclosure relates to methods of improving the efficacy of dextromethorphan, or providing beneficial pharmacokinetic effects to dextromethorphan, comprising co-administering erythrohydroxybupropion, or a prodrug thereof, and dextromethorphan to a human being. Dosage forms, drug delivery systems, and methods related to dextromethorphan and erythrohydroxybupropion or a prodrug of erythrohydroxybupropion are also disclosed.