Canker sores (aphthous ulcers) happen when the delicate lining inside your mouth gets inflamed, usually from a mix of triggers rather than one single cause. They are not contagious and are different from cold sores on the lips.

What canker sores are

Canker sores are small, shallow ulcers that form on soft tissues inside the mouth (inside cheeks, lips, tongue, or at the base of the gums). They are typically round or oval, with a white or yellow center and a red border, and can make eating or talking painful.

Think of them as tiny “open patches” where the surface lining of your mouth has broken down and the immune system is reacting.

Why do canker sores happen?

There is no single proven cause, but several common triggers make them more likely.

1. Minor injury or irritation

Small injuries in the mouth can start the process.

  • Accidentally biting your cheek or tongue
  • Braces, dentures, or sharp/broken teeth rubbing on the lining
  • Brushing too hard or using a very hard toothbrush
  • Eating very crunchy or abrasive foods (chips, crusty bread)

These tiny injuries can damage the surface layer, and then your immune system reacts, leading to an ulcer.

2. Immune system changes

Your immune system seems to play a big role.

  • Some people’s immune systems overreact to minor irritations or bacteria in the mouth.
  • Autoimmune-type responses or an “imbalanced” immune reaction can lead to recurring sores.
  • People with weakened immune systems (certain infections or conditions) can get more frequent or more severe canker sores.

3. Stress and lack of rest

Emotional or physical stress is a major trigger for many people.

  • High stress levels can change immune function and healing speed.
  • Poor sleep or burnout can make your body less able to repair small injuries in the mouth.
  • Many patients notice sores appear during exams, work crunch time, or after big life changes.

4. Nutritional deficiencies

Missing certain nutrients can make the lining of your mouth more fragile.

Common culprits:

  • Vitamin B12
  • Folate (folic acid)
  • Iron
  • Zinc

When these are low, the mouth tissue doesn’t repair as quickly, and your immune system may not function optimally.

5. Food triggers and sensitivities

Certain foods can either irritate the mouth directly or trigger a sensitivity reaction.

Typical triggers include:

  • Very acidic foods: citrus fruits, tomatoes, vinegar-based foods
  • Very spicy foods
  • Hard, rough foods that scrape the lining
  • In some people, sensitivity to ingredients in toothpaste or mouthwash (for example, sodium lauryl sulfate)

Not everyone has food triggers, but when they do, sores often show up a day or two after eating the problem food.

6. Hormones

Hormonal changes—especially around menstruation—are linked to flares in some women.

  • Shifts in estrogen and progesterone can affect blood flow, inflammation, and immune response in the mouth.
  • Some people notice sores that seem to “cycle” with their period.

7. Genetics

Canker sores tend to run in families.

  • If parents or siblings get frequent canker sores, you are more likely to get them.
  • This may be due to inherited immune system tendencies or shared environmental triggers (like similar diets or stress levels).

8. Certain medications and conditions

Some medicines and medical conditions can increase your risk.

Medications that may be associated:

  • Certain pain relievers or anti-inflammatory drugs (in some people)
  • Some medications that affect the immune system

Health conditions that can be linked:

  • Inflammatory bowel diseases (Crohn’s disease, ulcerative colitis)
  • Celiac disease
  • Conditions that weaken the immune system (for example, HIV/AIDS)

In these situations, mouth ulcers are sometimes one of several symptoms, not the only problem.

Quick “why they happen” recap

You can think of canker sores as the result of three main forces mixing together:

  1. A small injury to the mouth lining (bite, scrape, hot or spicy food).
  2. A sensitive or overreactive immune system that inflames that small injury more than usual.
  3. Background factors that make you more vulnerable: stress, low vitamins, hormones, genetics, or certain illnesses/medications.

When to worry or see a doctor

While most canker sores are harmless and heal in 1–2 weeks, you should get checked if:

  • Sores are extremely large or very painful.
  • They last more than two weeks without improving.
  • You keep getting them very frequently.
  • You also have fever, weight loss, diarrhea, joint pain, or other body symptoms.
  • You have trouble eating or drinking enough because of the pain.

A doctor or dentist can look for underlying issues (like deficiencies or other diseases) and suggest stronger treatments if needed. TL;DR:
Canker sores happen because the inner lining of your mouth gets injured or irritated, and your immune system overreacts, especially if you’re also dealing with stress, vitamin deficiencies, hormones, genetics, or certain medical conditions.

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