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 a non-poor metabolizer of dextromethorphan in need of treatment with dextromethorphan;

• wherein about 50 mg to about 220 mg of the bupropion is co-administered with the dextromethorphan once or twice a day;
• and wherein co-administering the bupropion and the dextromethorphan results in an Cmin of dextromethorphan on the eighth day that is about 20 times to about 1,000 times the Cmin of dextromethorphan that would result from administering the same amount of the dextromethorphan without the bupropion for eight consecutive days.