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;

• wherein about 50 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 about 40 mg to about 60 mg of dextromethorphan in a salt form, or a molar equivalent amount of the free base form of dextromethorphan, once a day or twice a day;
• and wherein co-administering the bupropion and the dextromethorphan results in a Cmin of dextromethorphan on the eighth day that is about 20 times to about 150 times the Cmin of dextromethorphan that results after the first dose of the bupropion and the dextromethorphan are co-administered.