You know, I havent had a canker sore in dogs years, since I got my braces taken off [well except for whenever i got hit in the face and the inside of my mouth got cut.] what I get is this annoying exterior cold sore dead center of my bottom lip=( but I discovered a tea tree oil lip balm-stick called lip rescue in the healthfood store, and using that instead of generic lip balm keeps it away like a charm.] Maybe they make a tea tree mouth wash, or you could get the tiny bottle of tea tree oil and put a dab of that on the inside of your lip when it flares up?