Bupropion as a modulator of drug activity

A method of treating major depressive disorder, comprising administering a dosage form once a day or twice a day, for at least 8 days, to a patient in need thereof, wherein the dosage form comprises a combination of dextromethorphan, or a pharmaceutically acceptable salt thereof, and an inhibitor of dextromethorphan metabolism in the human liver, wherein the dose of the inhibitor of dextromethorphan metabolism in the human liver is adjusted to achieve an AUC0-12 of dextromethorphan on day 8 that is about 300 ng*hr/mL to about 1,686 ng*hr/mL.