The present invention provides a drug for preventing or treating chronic pain and a method for preventing or treating chronic pain. A pharmaceutical composition comprising a serotonin-3 receptor agonist such as a compound represented by the following formula (I): [wherein: m is an integer of 1 to 4; and R1 is each independently selected from the group consisting of a hydrogen atom, a halogen atom, a methyl group optionally substituted with 1 to 3 halogen atom(s), a methoxy group optionally substituted with 1 to 3 halogen atom(s), and a methylthio group optionally substituted with 1 to 3 halogen atom(s)] or a pharmaceutically acceptable salt thereof is useful for preventing or treating chronic pain. The above pharmaceutical composition is also useful for preventing or treating acute pain. Further, the present invention provides a method for screening a compound for preventing or treating chronic pain, the method comprising measuring a serotonin-3 receptor agonist activity.