In most instances cankers are caused by an imbalance of good bacteria versus bad bacteria in the gut. Supplement with a good source of acidophilus. More severe instances may require action to get rid of the harmful bacteria and reintroduction of good bacteria. See: Candida
Raspberry leaf tea (Rubus idaeus). Steep two tablespoons of dried raspberry leaves in one cup boiling hot water until it cools. Strain the tea, and then swish a few mouthfuls for about one minute each three times a day. The tannins in this tea (which tastes a little like black tea) reduce the swelling and clean the sore so it can heal faster.
Swish Colloidal Silver around in the mouth and/or use a cotton ball soaked in colloidal silver to apply to the canker sores the help get rid of infection and heal the sores.
Black strap molasses is very healing in the mouth for sores. Take spoonfuls of it and swirl it in the mouth and slowly swallow. It will soothe and heal the canker sore(s). In addition, blackstrap molasses is full of good nutrition and is a good source of potassium.
One old home remedy uses the wooden pithy stuff that is inside pecan shells between the halves of the pecan and in the the folds of the nuts. If you have some pecans to shell, collect the material use thusly: Soak and lightly simmer, then when the solution has cooled and is no longer hot, wet a cotton ball with the solution and put it on the canker sore. Very soothiing. If you have any of the pecan stuff left over, bag it and save it.