However, many equi-analgesic tables provide different information, depending on the source and the manner in which equivalency was calculated. There are drawbacks to these equivalency tables, in part because many do not consider a recommended 15% dose reduction for opioid cross-tolerance.[1,2,7]
Some resources actually recommend that a dose reduction of up to 50% is appropriate when switching from one opioid to an alternative.
Another common problem with conversion tables is that many are based on single doses rather than steady-state concentrations, so certain data will not apply to chronic opioid users.
Most opioid conversion tables fail to elucidate the potential problems when converting a patient to methadone from another opioid, or from another opioid to methadone. Methadone conversion requires careful consideration because of its long half-life and unusual pharmacokinetic profile compared with most other opioids. In addition, converting methadone to morphine, for example, is not bidirectional.[8,9]
Consider that the half-life of methadone is 15-30 hours. When switching from an established dose of methadone to another opioid, we must consider that measurable methadone serum levels will be around for days. Therefore, when placing a patient on a new opioid, even with the discontinuation of methadone, both drugs are now readily available to the mu receptors, increasing the overall risk for opioid toxicity.
When newly converting a patient on methadone from another opioid, the equivalent dose conversion changes in a triphasic pattern
: For example, the ratio of morphine (or a morphine equivalent) < 90 mg/day to methadone is 4:1; the ratio for morphine 90 mg/day - 300 mg/day is 8:1; and for morphine > 300 mg/day, the ratio is 12:1.[8,10]
Dosing opioids requires the clinician to account for a patient's opioid history, physical tolerance, consideration of agents in mixed preparations, cross-tolerance, and conversion irregularities. It is always best to use caution when initiating and increasing opioid regimens.