Differences in the ratios of morphine to methadone in patients with neuropathic pain versus non-neuropathic pain.

The use of methadone in the treatment of cancer pain is becoming more attractive mainly because of its known efficacy, lack of active metabolites, and low cost. Methadone also blocks the n-methyl-D-aspartate (NMDA) receptor, and this property may result in other clinical advantages. Because of this capacity of methadone to block the NMDA receptors, we have hypothesized that the equianalgesic dose ratio of hydromorphone or morphine to methadone will be different in patients with neuropathic pain than in patients with non-neuropathic pain. To explore this hypothesis, we reviewed computerized patient records and determined the ratio of morphine and hydromorphone (expressed as morphine subcutaneous equivalent dose) to methadone in patients who underwent rotation from morphine or hydromorphone to methadone. We found that the ratio of morphine subcutaneous equivalent dose to methadone is between 5 and 7, which is different from previously described dose ratios. However, our study failed to show a difference in the ratios of patients with neuropathic or non-neuropathic pain syndromes.