• Bradley S Galer, David Lee, Tina Ma, Barbara Nagle, and Thomas G Schlagheck.
• Endo Pharmaceuticals, Inc., Clinical Development, 100 Endo Blvd., Chadds Ford, PA 19317, USA. galer.bradley@endo.com
• Pain. 2005 Jun 1;115(3):284-95.

AbstractWhile many pre-clinical and clinical studies have suggested that the addition of N-methyl-D-aspartate (NMDA) receptor antagonists, such as dextromethorphan (DM), to opioid analgesics, such as morphine (MS), may enhance the analgesic effects and prevent the tolerance that may result from chronic opioid administration, others have not. The potential for reduced doses, enhanced opioid analgesia, and decreased analgesic tolerance associated with the MS/DM combination were evaluated in a series of three large, randomized, double-blind, parallel group, phase 3, multicenter trials each of 3 months duration in patients with chronic, non-malignant, non-neuropathic pain. To evaluate these unique endpoints, novel study designs were employed. In Study A, patients received fixed doses of MS or MS/DM, based on the stable dose of MS/DM attained during a Run-in period; changes from baseline in average daily pain intensity were compared. In Studies B and C, patients self-titrated doses of MS or MS/DM, based on stable doses of MS or other opioids attained during Run-in periods, to maintain pain relief; percentage changes from baseline in MS (or MS-equivalent) doses were compared. No statistically significant differences between treatment groups in any primary or secondary efficacy variables were demonstrated in any trial. These results suggest that adding the NMDA antagonist, dextromethorphan, to opioids does not add any clinical benefit.

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