Cymbalta (duloxetine) is a selective serotonin and norepinephrine reuptake inhibitor (SNRI) used for treating depression, anxiety disorder and pain associated with diabetic peripheral neuropathy or fibromyalgia. The most common side effects noted with Cymbalta are nausea, dry mouth, constipation, diarrhea, fatigue, difficulty sleeping, and dizziness. Some patients may experience withdrawal reactions such anxiety, nausea, nervousness, and insomnia.