Canker sores (aphthous stomatitis) are ulcerations of the mouth and lips. They can be caused by viral infection, niacin deficiency, and/or by insufficient lactic bacteria or probiotics in the diet. Hidden food sensitivities are often involved. Although almost any food could be a culprit (depending on biochemical individuality), the most common offenders tend to be milk, cheese, wheat, tomato, vinegar, lemon, pineapple and mustard.
See also the chapter, “One’s Food is Another’s Poison”, in NUTRITIONAL SOLUTIONS FOR 88 CONDITIONS (available at Amazon.com)