Chronic low back pain (CLBP) is one of the most frequent forms of chronic pain and can result in significant functional impairment. This is often associated with major depression too. Previous research reported significant beneficial effects of antidepressant medication in alleviating depression and pain intensity. The aim of this study is to evaluate the efficacy of Escitalopram, a new kind of Selective Serotonin Reuptake Inhibitor (SSRI) in patients with CLBP in a prospective, randomized and double-blind clinical trial. The main hypothesis is: