After I started Lexapro I had the worst jaw clenching ever...bit raw sores on my lips and inside my cheeks. I still have ridges from where I chewed. I went to the dentist and had a very expensive mouth guard made and it gave me canker sores so that is still in the drawer. Don't know how long you have been on Cymbalta but it should go away with time, if it's the drug causing this. I am going through it again....now that I have stopped Lexapro. I am also grinding my teeth.