Canker sores (aphthous ulcers) do not have one single proven cause, but several common triggers make them more likely to appear. They are not contagious and are different from cold sores, which come from the herpes virus.

Main triggers

• Minor injury to the mouth, such as biting the cheek or tongue, rubbing from braces or sharp teeth, or irritation from hard or abrasive foods.

• Certain foods, especially acidic or spicy items and some trigger foods like chocolate, coffee, nuts, cheese, strawberries, and citrus, which can irritate the mouth lining in susceptible people.

• Ingredients like sodium lauryl sulfate in some toothpastes and mouthwashes, which can irritate the oral tissues in some individuals.

Internal factors

• Deficiencies of nutrients such as iron, folate (folic acid), vitamin B12, zinc, or overall poor nutrition, which are linked with more frequent sores.

• Hormonal changes (for example around menstruation) and emotional or physical stress, which can alter immune responses and trigger outbreaks.

• Immune system factors, where the immune system may mistakenly attack cells in the mouth lining; this is more common in people with autoimmune conditions or weakened immunity (such as HIV/AIDS).

Medical conditions and genetics

• Associated conditions like celiac disease, inflammatory bowel disease (Crohn’s disease, ulcerative colitis), Behçet’s disease, and other systemic illnesses can make recurrent canker sores more likely.

• Family history and genetics also play a role; people who get frequent canker sores often have relatives with the same problem, suggesting inherited susceptibility or shared environmental triggers (like diet or allergens).

If sores are very large, extremely painful, or keep coming back, it is important to see a healthcare professional to check for nutritional deficiencies or underlying conditions.