Bupropion as a modulator of drug activity

A method of treating treatment resistant depression, comprising orally co-administering:

• a daily dose of bupropion that is about 150 mg to about 250 mg and a daily dose of dextromethorphan that is about 60 mg to about 100 mg, to a human being in need thereof for at least 8 consecutive days, wherein the bupropion is bupropion hydrochloride, or the free base form or another salt form of bupropion and the dextromethorphan is dextromethorphan hydrobromide, or the free base form or another salt form of dextromethorphan;
• wherein the bupropion and the dextromethorphan are the only therapeutically active compounds to be co-administered;
• wherein the dextromethorphan is formulated for immediate release and the bupropion is formulated for sustained release;
• wherein orally co-administering the bupropion and the dextromethorphan results in the human being experiencing a therapeutic effect;
• and wherein orally co-administering the bupropion and the dextromethorphan is more effective in treating treatment resistant depression than orally administering the same amount of the dextromethorphan alone for the same number of consecutive days.