Dose ratio between morphine and hydromorphone in patients with cancer pain: A retrospective study

Morphine (M) and hydromorphone (HM) are commonly used opioid analgesics for cancer pain. Opioid rotation is often necessary in the event of toxicity and/or inadequate analgesia. Equianalgesic reference tables based on single dose comparisons are possibly inadequate for patients on chronic treatment and developing tolerance. This retrospective study of opioid rotation involving M and HM sought to determine the equianalgesic dose ratio for 91 rotations in 74 consecutively evaluable cancer pain patients. Only rotations involving subcutaneous (sc-sc) and oral (po-po) routes were evaluated. There were 44 rotations from M-HM (34: sc-sc, 10: po-po) and 47 rotations from HM- M (35: sc-sc, 12: po-po). Expressing all ratios as M/HM, the median dose ratios (lower-upper quartiles) for sc and po rotations were 4.92 (4.1-5.9) vs. 5.76 (4.9-5.8) for M-HM (P = 0.28, NS) and 4.0 (3.1-4.8) vs. 3.45 (2.8- 4.2) for HM-M (P = 0.4, NS), respectively. Pain intensity, as measured on a visual analogue scale (VASP), showed no significant difference between mean values pre- and post-rotation. A unified overall median dose ratio of 4.29 (3.3-5.3, lower-upper quartiles) was calculated by expressing all of the HM- M dose ratios as M/HM and combining them with the dose ratios for all of the M-HM rotations. This suggests a potency ratio of approximately 4.3:1 between M and HM. When expressed as M/HM for dose ratio comparison, the median dose ratio for all HM-M rotations was 3.7 (2.9-4.5, lower-upper quartiles) vs. 5 (4.2-5.9) for M-HM rotations (P = 0.0001), suggesting that the opioid to which rotation is taking place is more potent than our proposed unified overall median dose ratio of 4.29:1 would predict. Our data suggests that HM is 5 times more potent than M when given second (M-HM), but is only 3.7 times more potent when given first (HM-M). We therefore recommend a ratio of 5 for M/HM in rotating from M to HM and ratio of 3.7 for M/HM when rotating from HM to M in patients exposed to chronic dosing of these opioids. There was no correlation observed between M-HM and HM-M dose ratios and the level of previous opioid dose, in contrast to HM to methadone rotation where the dose ratio was higher in patients receiving higher doses of HM.