Study of D-Methadone in Patients With Chronic Pain

Inclusion Criteria: Phase I and Phase II portions of the study: * 18 years of age or older * Chronic pain with average 24 hour intensity rated at least 3 on a verbal numerical scale from 0-10 during the 24 hours prior to study entry. * Give informed consent to participate in this study. * Karnofsky Performance Score (KPS) \>= to 80 * Negative urine pregnancy test, verified by the study nurse, at study entry (for women of child-bearing potential). Patients must also use a medically approved contraceptive method during the study period. Phase I only: * Responsible companion living with patient during study. Phase II only: * Group 1 -- Patients must be taking chronic opioid therapy (long acting morphine, long acting oxycodone, transdermal fentanyl) at a stable dose for a minimum of four days. The dose of as needed short acting opioid does not need to be stable. * Group 2 -- Patients must not be receiving opioids and must have cancer related neuropathic pain secondary to post-chemotherapy peripheral neuropathy, post-radiation and/or post surgical plexopathy, radiculopathy or neuropathy, or post- herpetic neuralgia. Exclusion Criteria: Phase I and Phase II: * Known hypersensitivity to methadone * Patient taking methadone or with a history of methadone treatment within one month of study enrollment. * Patient that requires changes in the dose of one of the following medications within 2 weeks of study enrollment: * Abacavir, * Benzodiazepines, * Carbamazepine, * Efavirenz, * Fluconazole, * Fluvoxamine, * FOS amprenavir, * Fosphenytoin, * Naltrexone, * Nelfinavir, * Nevirapine, * Phenytoin, * Rifampin, * Rifapentine, * Risperidone, * Ritonavir, * St. John's Wort, * Zidovudine * Hepatic function tests (SGOT, alkaline phosphatase, bilirubin) greater than 2 times the upper limit of normal or creatinine greater than 1.4 within 30 days of study entry. * Neurologic or psychiatric disease sufficient, in the investigator's opinion, to compromise data collection. * Women who are pregnant or nursing. * Women of childbearing potential who do not agree to use a medically recognized method of contraception during the study period.