Mouth ulcers (canker sores) usually have several overlapping triggers rather than one single cause.

What mouth ulcers are

Mouth ulcers are small, painful sores on the inside of the lips, cheeks, tongue, or gums, often with a white or yellow centre and a red, inflamed edge. They are usually harmless and tend to heal on their own within 1–2 weeks.

Main things that cause mouth ulcers

Here are the most common factors linked to mouth ulcers:

  • Minor injury: Biting the cheek or tongue, harsh toothbrushing, sharp or broken teeth, braces, dentures, or burns from hot food can damage the lining and lead to an ulcer.
  • Stress and lack of sleep: Emotional stress and fatigue can weaken your immune response and make ulcers more likely.
  • Nutritional deficiencies: Low levels of vitamin B12, iron, folate (folic acid), and sometimes zinc are associated with recurrent mouth ulcers.
  • Hormonal changes: Many people notice more ulcers around menstruation, pregnancy, or other hormonal shifts.
  • Food triggers and irritation: Acidic, spicy, or rough foods (like citrus, tomatoes, crisps, toast), as well as chocolate, coffee, some nuts or strawberries, can irritate the lining or provoke a reaction in some people.
  • Toothpaste/mouthwash ingredients: Products containing sodium lauryl sulfate (SLS), a foaming agent in some toothpastes, may trigger ulcers in susceptible people.
  • Infections and illnesses: Viral and other infections (such as hand, foot and mouth disease, herpes infections, or other systemic illnesses) can cause or be associated with mouth ulcers.
  • Medical conditions: Coeliac (celiac) disease, Crohn’s disease, Behçet’s disease, some autoimmune conditions, and immune-suppressing illnesses like HIV can feature frequent or severe mouth ulcers.
  • Medications and treatments: Certain drugs such as nonsteroidal anti-inflammatory drugs (NSAIDs), beta‑blockers, some chemotherapy agents and other medicines can have mouth ulcers as a side effect.
  • Tobacco: Smoking or other tobacco use can irritate oral tissues; some people also notice ulcers when they stop smoking as the mouth environment changes.

Common causes and triggers of mouth ulcers

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Cause / trigger How it contributes
Minor trauma (bites, sharp teeth, braces, hot food) Physically damages the lining so a sore forms at the injury site.
Stress, poor sleep Alters immune function, making inflammation and ulcers more likely.
Nutrient deficiencies (B12, iron, folate, zinc) Impairs tissue repair and immune health, linked with recurrent ulcers.
Hormonal changes Fluctuations (e.g. around periods or pregnancy) are associated with flare‑ups.
Certain foods (acidic, spicy, rough, chocolate, nuts, coffee) Direct irritation or individual sensitivity can trigger sores.
Toothpaste with sodium lauryl sulfate (SLS) May irritate the lining and increase ulcer risk in some people.
Infections (e.g. viral) Some infections present with oral ulcers as part of the illness.
Medical conditions (coeliac, Crohn’s, Behçet’s, autoimmune) Chronic inflammation or immune changes can cause frequent or severe ulcers.
Medications (NSAIDs, beta‑blockers, chemotherapy, others) Ulcers can occur as a side effect of some drugs or treatments.
Tobacco use or quitting smoking Smoking irritates tissues; stopping can temporarily change ulcer patterns.

When mouth ulcers might be serious

Most mouth ulcers are not dangerous, but you should seek medical or dental advice if:

  1. An ulcer lasts longer than 2–3 weeks.
  2. Ulcers keep coming back very frequently.
  3. You have very large, deep, or unusually painful sores.
  4. You also have fever, weight loss, fatigue, rashes, or swollen glands.
  5. You take medicines that affect your immune system or have a known chronic disease.

Quick note on “latest news” and forum talk

In health forums over the last few years, many people discuss links between mouth ulcers and everyday stress, diet changes, and products like whitening toothpastes or energy drinks, but these are usually individual experiences rather than proven causes. Medical sources still emphasise the mix of trauma, stress, nutritional status, medications, and underlying conditions as the main drivers.

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