Actually the cause of canker sores remains unclear but there are several factors that contributes to mouth ulceration.

Possible triggers are:
 - A minor injury to your mouth from dental work, overzealous brushing, sports mishaps, spicy or acidic foods, or an accidental cheek bite
 - Toothpastes and mouth rinses containing sodium lauryl sulfate
 - Food sensitivities, particulazrly to chocolate, coffee, strawberries, eggs, nuts, cheese and highly acidic foods, such as pineapple
 - A diet lacking in vitamin B-12, zinc, folate (folic acid) or iron
 - An allergic response to certain bacteria in your mouth
 - Helicobacter pylori, the same bacteria that cause peptic ulcers
 - Hormonal shifts during menstruation
 - Emotional stress

Canker sores may also occur because of certain conditions and diseases, such as:
- Celiac disease, a serious intestinal disorder caused by a sensitivity to gluten, a protein found in most grains
- Inflammatory bowel diseases, such as Crohn's disease and ulcerative colitis
- Behcet's disease, a rare disorder that causes inflammation throughout the body, including the mouth
- A faulty immune system that attacks healthy cells in your mouth instead of pathogens, such as viruses and bacteria
- HIV/AIDS, which suppresses the immune system
- Unlike cold sores, canker sores are not associated with herpes virus infections.