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 (s.c.-s.c.) and oral (p.o.-p.o.) routes were evaluated. There were 44 rotations from M-HM (34: s.c.-s.c., 10: p.o.-p.o.) and 47 rotations from HM-M (35: s.c.-s.c., 12: p.o.-p.o.). Expressing all ratios as M/HM, the median dose ratios (lower-upper quartiles) for s.c. and p.o. 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.
Lawlor et al. (Fri,) studied this question.