Rationale: Experimental data demonstrates a profound analgesic synergy between morphine and methadone when co-administered systemically in mice. If this type of synergy can be demonstrated in clinical pain, it will be a relatively unique example of translating concepts of fundamental aspects of opioid actions seen in the laboratory to the clinical area, and provide a basis for offering new and scientifically- based analgesic regimens. This may provide better pain relief with less opioid related side effects in clinical practice.