Answer:
Canker sores are caused by a variety of sources.
The most common source is a tiny injury to the lining of the mouth (from braces, misaligned teeth, crunchy foods, dentistry, etc.)
Another likely cause is toothpaste containing a harsh chemical detergent called sodium lauryl sulfate (or SLS).
Food allergies including allergies to milk and nuts are believed to trigger canker sores in some people.
Diets lacking B-vitamins, folic acid, and iron can be the culprit.
Many canker sores are worsened by the appearance of helicobacter pylori; this is the same organism that causes most stomach ulcers.
Switching to a natural toothpaste and non-alcoholic mild mouthwash help most people who suffer from these annoying ulcerations in the mouth.