DOSE RATIO BETWEEN MORPHINE AND METHADONE IN PATIENTS WITH CANCER PAIN - A RETROSPECTIVE STUDY

BACKGROUND. Current equianalgesic reference tables, based largely on s ingle dose studies, give dose ratios of 1:1 to 4:1 for oral morphine t o oral methadone, which possibly are inaccurate in patients with cance r pain who are exposed to multiple doses of these opioids. The purpose of this study was to determine the equianalgesic dose ratio between m orphine and methadone in patients with cancer pain and to establish wh ether the dose ratio changes as a function of previous opioid dose. ME THODS. A retrospective analysis of consecutive rotations involving mor phine and methadone using standard selection criteria identified a tot al of 20 evaluable rotations (14 from morphine to methadone and 6 from methadone to morphine). Opioid doses and pain intensity levels pre-an d postrotation were analyzed. RESULTS. Median dose ratios (lower-upper quartiles) for morphine to methadone and methadone to morphine rotati ons were 11.36 (range, 5.98-16.27) and 8.25 (range, 4.37-11.3), respec tively (P = 0.23). Combining all 20 rotations, a unified median dose r atio of 11.2 (range, 5.06-13.24) was calculated. There was no signific ant difference in pain intensity levels pre-and postrotation as record ed on a visual analogue scale. Univariate correlational analysis of do se ratio and the level of daily morphine dose prior to rotation reveal ed a Spearman correlation coefficient of 0.86 (P = 0.0001). In patient s receiving >1165 mg per day prior to methadone rotation, a median dos e ratio of 16.81 (range, 12.25-87.95) was observed, which was approxim ately 3 times higher compared with a median dose ratio of 5.42 (range, 2.95-9.09) (P = 0.007) for the 50% of patients receiving lower morphi ne doses. CONCLUSIONS. The results highlight the general underestimati on of methadone potency and the consequent risk of potential life-thre atening toxicity, The strongly positive correlation between dose ratio and previous morphine dose suggests the need for a highly individuali zed and cautious approach when rotating from morphine to methadone in patients with cancer pain. (C) 1998 American Cancer Society.