Most mouth ulcers are caused by herpes simplex virus. It is very common and not to be confused with genital herpes. The good news is that there is a cure even though medical doctors say it cant be cured. An American Indian showed me how, take one tablespoon of whole sage (like you cook with) in a half cup of water, heat it until it starts to boil, turn the heat off and cover it for 10 minutes. Then strain it through a clean white cloth or I used a coffee filter, put it in a small clean jar with a lid so you can use it as needed. You should make a fresh batch each day. Take a cotton ball or q-tip if it is a small ulcer, dip it in the mixture and place it on the ulcer. Hold it there for at least 5 minutes and throw it away. Do this 3 to 4 times a day for three days. It will almost immediately eliminate the pain, and the ulcer will be gone twice as fast as without the mixture. Most people only have to go through this two to three times before they stop coming back. Thank God for Indian Medicine Men. Oh it smells strong but there is almost no taste. Let me know if this helps you, it did me. I haven't had one in over 10 years!