1. A method of administering a drug for which the major clearance mechanism in humans is CYP2D6 (CYP-cytochrome P-450) mediated oxidative biotransformation or a pharmaceutically acceptable salt thereof in combination with a CYP2D6 inhibitor or a pharmaceutically acceptable salt thereof to a human in need of the intended pharmaceutical activity of such drug wherein said drug for which the major clearance mechanism in humans is CYP2D6 mediated oxidative biotransformation is selected from the group...