SWITCHING FROM MORPHINE TO ORAL METHADONE IN TREATING CANCER PAIN - WHAT IS THE EQUIANALGESIC DOSE RATIO

Purpose: To define the dose ratio between morphine and methadone in re lation to the previous morphine dose and the number of days needed to achieve the same level of analgesia in a group of patients with advanc ed cancer with pain who switched from morphine to oral methadone. Pati ents and Methods: A cross-sectional prospective study of 38 consecutiv e cancer patients who switched from morphine to oral methadone was per formed. The intensity of pain before, during, and after the switching period was assessed through a four-point verbal Likert scale. The rela tionship between previous morphine dose and the final equianalgesic me thadone dose, dose ratio between morphine and methadone, and the numbe r of days required to achieve equianalgesia have been examined by mean s of pearson's correlation coefficient, scatter plots, and Cuzick's te st for trend respectively. Results: Before the switch, the median oral equivalent daily dose of morphine was 145 mg/d; after the switch, the median equianalgesic oral methadone dose wets 21 mg/d. A median time of 3 days (range, 1 to 7 days) was necessary to achieve the equianalge sia with oral methadone; the lower the preswitching morphine dose, the fewer days necessary to achieve equianalgesia with oral methadone (P <.001). Dose ratios ranged from 2.5:1 to 14.3:1 (median, 7.75:1), whic h indicated that, in most cases, the dose ratio was much higher than t hat suggested by the published equianalgesic tables. A strong linear p ositive relationship between morphine and methadone equianalgesic dose s was obtained (Pearson's correlation coefficient, 0.91), The dose rat io increased with the increase of the previous morphine dose with a mu ch higher increase at low morphine doses. Conclusion: The results of o ur study confirm that methadone is a potent opioid, more potent than b elieved. Caution is recommended when switching from any opioid to meth adone, especially in patients who are tolerant to high doses of opioid s. (C) 1998 by American Society of Clinical Oncology.