I always have them in spots where stuff that's been applied to it will just be rubbed off or licked away immediately. I had to keep my finger with toothpaste against the sore for about five minutes, and it was uncomfortable and it hurt like hell.
I never take Ibuprofen or Advil, so I need to know some other causes. I'd get rid of masturbation if it meant never having canker sores again.
