Bupropion as a modulator of drug activity

A method of treating treatment resistant depression, comprising administering:

• bupropion, at a daily dose that is about 150 mg to about 250 mg, and dextromethorphan, at a daily dose 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 and the dextromethorphan are administered in a dosage form containing both the bupropion and the dextromethorphan as the only therapeutically active compounds;
• wherein the bupropion is bupropion hydrochloride, or the free base or another salt form of bupropion and the dextromethorphan is dextromethorphan hydrobromide, or the free base or another salt form of dextromethorphan;
• wherein the daily doses of the bupropion and the dextromethorphan are orally administered once a day or are a total of twice daily doses;
• wherein the dosage form provides immediate release of dextromethorphan and sustained release of bupropion;
• and wherein orally administering the dosage form is more effective in treating treatment resistant depression than orally administering the same amount of the bupropion alone for the same number of consecutive days.