I only seem to get them when I am generally run down in other ways. I take vitamin C every day and I found that after I started doing that the incidenced of mouth ulcers did reduce. However, its sods law. I walway get one on the day of a concert, like right now with four hours till I have to start playing. And of course, its right under my mouthpiece. My normal procedure is to lay off playing if it happens athe day before a performance and use some bonjella to try and aid the healing process. Years ago i was getting lots of them and my dentist found I had a sharp edge on a tooth that was cutting into me. He smoothed it off and that stopped the problem. The current ones are not related to any sort of abrasion that I can find. Obviously the lips are very sensitive and having anything painful under the mouthpiece is not good news. How do you deal with this?