Many cancer patients develop long-lasting nerve pain from chemotherapy, and the usual treatment often provides only limited relief. Methadone may work better for this type of pain, but it has not been well studied, so doctors are hesitant to use it. This study will compare methadone and duloxetine to see which one better reduces pain and improves daily life while remaining safe.

People in the study will be randomly assigned to take one of the two medications for five weeks. They will fill surveys to track changes in pain and how they are feeling over all. Researchers will then follow-up and adjust doses each week based on how each person is doing. The main goal of this study is to see whether methadone works better than duloxetine at reducing the average level of nerve pain caused by chemotherapy. The study will also look at whether methadone can help people function better in their daily lives, whether it improves overall quality of life, and whether methadone remains safe and effective over time without people needing increasing doses.