Bupropion as a modulator of drug activity

A method of increasing dextromethorphan plasma levels, comprising co-administering bupropion and dextromethorphan once a day or twice a day for at least 8 consecutive days to a human being who is an extensive metabolizer of dextromethorphan in need of treatment with dextromethorphan;

• wherein about 90 mg to about 220 mg of bupropion hydrochloride, or a molar equivalent amount of the free base form or another salt form of bupropion is co-administered with dextromethorphan hydrobromide, or the free base form or another salt form of dextromethorphan, once or twice a day;
• and wherein the bupropion and the dextromethorphan are the only therapeutically active agents to be co-administered and co-administering the bupropion and the dextromethorphan results in an AUC0-24 of dextromethorphan on the eighth day that is at least about 20 times the AUC0-24 of dextromethorphan that would result from administering the same daily dose of the dextromethorphan without the bupropion for eight consecutive days.