The purpose of this study is to determine whether different types of antidepressant medicines relieve back pain that has lasted at least six months on a daily basis. Study participants will be assigned to treatment with either a antidepressant acting on the serotonin system in the brain (fluoxetine), one acting on the noradrenaline system (desipramine, or to a control medication not expected to relieve pain (benztropine). Each participant will be seen at least nine times during their 12 weeks on medication. This is a phase 2/3, outpatient study.