Effects of dextromethorphan in clinical doses on capsaicin-induced ongoing pain and mechanical hypersensitivity

The analgesic properties of the noncompetitive N-methyl-D-aspartate (NMDA)-receptor antagonist dextromethorphan, available for clinical use as an antitussive, have been studied in the human capsaicin pain model to determine a possible clinical effect on pain due to central sensitization. Ninety milligrams dextromethorphan or vehicle was given orally to ten volunteers, each at two different occasions in a double-blind fashion, prior to an intradermal injection of 300 micrograms capsaicin. Ongoing pain, pain evoked by von Frey filament stimulation, and pressure pain thresholds were assessed before and after the capsaicin injection. The area in which von Frey filament stimulation evoked pain was mapped after the capsaicin injection. There were no significant group effects on ongoing pain, or on von Frey or pressure hypersensitivity. There was also no significant effect on the area of mechanical hypersensitivity. These results show that clinical doses of dextromethorphan do not effect ongoing or mechanically evoked pain after capsaicin injection.