Cymbalta (duloxetine) is a selective serotonin and norepinephrine reuptake inhibitor antidepressant (SSNRI). Duloxetine affects chemicals in the brain that may become unbalanced and cause depression.Cymbalta is used to treat major depressive disorder in adults. Cymbalta is also used in adults to treat fibromyalgia (a chronic pain disorder), or chronic muscle or joint pain (such as low back pain and osteoarthritis pain).