Compositions and methods for increasing the metabolic lifetime of dextromethorphan and related pharmacodynamic effects

A method of enhancing dextromethorphan plasma levels in a human being, comprising co-administering a bupropion with a dextromethorphan, to the human being, for at least nine consecutive days, wherein the human being is an extensive metabolizer of dextromethorphan in need of treatment with dextromethorphan, and the human being is not suffering from neuropathic pain, wherein the bupropion and the dextromethorphan, are the sole active agents administered to the human being, wherein the bupropion is administered in an amount that results in a Cmax of dextromethorphan that is at least about 40 ng/mL, and wherein, on the ninth day that the bupropion and the dextromethorphan, are co-administered, the Cmax of dextromethorphan is at least about 15 times the Cmax of dextromethorphan that would result from administering the same amount of the dextromethorphan without the bupropion for nine consecutive days.