liz
i'm not sure if canker sores are the same thing as cold sores or fever blisters but if they are herpes related you may want to try the amino acid L-Lysine, it is a natural product that our liver makes and some people who suffer from severe fever blisters are deficient in this amino acid. My husband went on it for about a year and now he rarely gets them. Give it a try-it is available where ever vitamins are sold.