Cognitive reversal of expected nitrous oxide analgesia for acute pain.

In a laboratory experiment, the expected analgesic action of 33% nitrous oxide was reversed by creating the expectancy of heightened awareness of bodily sensations. Pain threshold and tolerance of electrical tooth-pulp stimulation were significantly reduced. Results from a control study gave us a basis for comparison of changes in the verbal expression of pain when nitrous oxide was administered without introducing expectancies beyond those already held by the subjects. Contrasting results from the experimental and control studies confirm the powerful role of mental processes in mediating pain experience.