The analgesic effect of codeine as compared to imipramine in different human experimental pain models

The hypoalgesic effect of single oral doses of 100 mg imipramine and 125 mg codeine was evaluated in a randomised, placebo-controlled. double-blind, 3 -way cross-over experiment including 18 healthy volunteers. Pain tests were performed before and 90, 180, 270, 360 and 450 min after medication. The t ests included determination of pain tolerance thresholds to pressure. pain detection/tolerance thresholds to single electrical sural nerve stimulation and pain summation at tolerance threshold to repetitive electrical sural n erve stimulation (temporal summation) and pain experienced during the cold presser test, rated as peak pain intensity, pain average intensity and disc omfort. Compared to placebo, imipramine significantly increased pressure pa in tolerance threshold (P = 0.03) and increased pain tolerance threshold (P = 0.05) and pain summation threshold (P = 0.03), but not pain detection th reshold to electrical stimulation, Imipramine did not cause significant cha nges in pain perception during the cold presser test. Codeine significantly increased pressure pain tolerance threshold (P = 0.02), pain detection (P = 0.04) and pain tolerance threshold (P = 0.01) and pain summation threshol d (P = 0.02) to electrical stimulation. In addition. codeine reduced the pa in experienced during the cold presser test (P = 0.04-0.003). It is conclud ed that both imipramine and codeine inhibit temporal pain summation, wherea s only codeine reduces cold presser pain. Pain summation may be a key mecha nism in neuropathic pain. Imipramine has a documented effect on such pain c onditions on temporal summation. The present study showed that codeine also inhibits temporal summation, which is in line with the clinical observatio ns indicating that opioids relieve neuropathic pain. Copyright (C) 2001 Els evier Science B.V.. Published by Elsevier Science B.V. All rights reserved.