Canker sores are usually caused by a mix of mouth irritation, immune system reactions, and things like stress or vitamin deficiencies, rather than a single clear cause. They are not contagious and are different from cold sores, which are caused by herpes viruses and appear on the lips.

What a canker sore is

  • A canker sore (aphthous ulcer) is a small, painful open sore that appears inside the mouth, like on the inner lips, cheeks, tongue, or gums.
  • They often start as a small red bump, then develop a white or yellow center with a red border and usually heal on their own in 1–2 weeks.

Main triggers and causes

Experts do not know one single definitive cause , but several common triggers are strongly linked to canker sores.

  • Minor mouth injury: Biting your cheek or tongue, rubbing from braces, rough teeth, sharp foods, or aggressive brushing can damage the lining and trigger a sore.
  • Acidic or spicy foods: Citrus fruits, tomatoes, vinegar, soda, and spicy foods can irritate the mouth and either trigger or worsen canker sores.
  • Stress and lack of sleep: Psychological stress and fatigue are often associated with outbreaks, likely because they affect the immune system.
  • Nutritional deficiencies: Low levels of vitamin B12, folate (folic acid), iron, or zinc are linked with more frequent canker sores.
  • Immune system factors: Many sources suggest canker sores may be due to an abnormal immune reaction attacking the mouth lining.
  • Genetics: They tend to run in families, suggesting a hereditary component.
  • Toothpaste/chemical irritation: Products containing sodium lauryl sulfate (a foaming agent in some toothpaste) have been associated with more frequent sores in some people.
  • Hormonal changes: Some people notice flares around hormonal shifts, such as menstrual cycles.
  • Underlying medical conditions: More frequent or severe canker sores can occur in people with conditions like celiac disease, inflammatory bowel disease, Behçet disease, HIV/AIDS, or anemia.

What does NOT cause them

  • Canker sores are not caused by herpes viruses and are not contagious, unlike cold sores (fever blisters) on the lips.
  • They are not considered a form of oral cancer, though any sore lasting longer than about 2–3 weeks should be checked by a dentist or doctor to rule out other problems.

When to see a doctor or dentist

Seek professional advice if:

  1. A sore is very large, spreading, or extremely painful.
  1. Sores keep coming back very frequently or you always have several at once.
  1. A sore does not heal after about 2 weeks, or you have other symptoms like fever, weight loss, or severe fatigue.

A clinician can check for vitamin deficiencies, irritation sources, or underlying conditions and discuss treatments such as medicated mouth rinses or topical gels.

TL;DR: Canker sores happen when the lining inside your mouth gets irritated and your immune system reacts, with common triggers including mouth injuries, acidic foods, stress, and vitamin shortages; they are not contagious and not the same as cold sores.

Information gathered from public forums or data available on the internet and portrayed here.