Effect of Combined Morphine and Duloxetine on Chronic Pain

Subjects will participate in a 10-week study consisting of three phases: Phase I is the dose titration period of 4 weeks, Phase II is the dose maintenance period of 4 weeks, and Phase III is the dose taper period of 2 weeks. Seven office visits (Initial visit/baseline, weeks 1, 3, 5, 7, 9, and end of week 10) and four follow up phone calls (weeks 2, 4, 6, and 10) will be used to collect data. Since this is a prospective study, we will be able to first determine baseline QST and VAS pain score, followed by assessing their longitudinal changes at each office visit. To assess OIH, QST will be performed at each office visit before subject takes the next dose of the study drugs. We will also measure plasma morphine concentration during the titration and maintenance phase to help validate morphine intake.