According to WebMD, weight gain is an infrequent but possible side effect of Cymbalta. More common side effects include dry mouth, drowsiness, difficulty sleeping, reduced energy level, nausea, appetite loss and diarrhea.
Cymbalta is prescribed for depression and anxiety in order to improve mood, stimulate appetite and increase energy, WebMD explains. The drug achieves these results by balancing brain chemicals. It is also occasionally used for ongoing pain conditions, such as arthritis, fibromyalgia and chronic back pain.
WebMD lists several substances that have negative interactions with Cymbalta, including blood thinners, ibuprofen, certain antibiotics, some antidepressants and St. John's wort. In addition, the combination of aspirin and Cymbalta sometimes increases the risk of bleeding, but low doses prescribed for heart attack or stroke prevention should not be discontinued without a doctor's orders.