What Causes Canker Sores?

The exact cause of canker sores is not yet completely understood. However, there are several known triggers, including stress, injury, and nutritional.

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A man reveals a canker sore on the inside of his lip

Soumen Hazra / Getty images

Canker sores, also known as aphthous ulcers, are roundish, whitish sores that appear inside the mouth. The sores can develop on the gums, the roof of your mouth, and inside your cheeks. Canker sores are often painful but typically go away within a few days with or without treatment.

Experts do not know the exact cause of canker sores. However, the sores' development is believed to be connected to the activation of the immune system.

While their cause is not entirely understood, this is certain: Canker sores are not contagious—you cannot catch them or pass them on to others.

Canker sores also have known triggers and may be more likely to develop within families.

Canker Sore Triggers

Sometimes canker sores develop for no apparent reason. Other times, they may be triggered by a certain factor.


Stress can prompt a canker sore. This may be due to the changes in saliva, such as an increase in the stress hormone cortisol, that happen when you are experiencing stress.

Another theory as to why stress and canker sores are connected is because you may be more likely to perform actions that can injure the inside of your mouth while you are stressed, such as biting the inside of your cheek or lip. These injuries can lead to canker sores.

Nutritional Deficiencies

Some evidence suggests that deficiencies in certain key nutrients may lead to canker sore development. It’s estimated that 20% of canker sores are from low levels of the following nutrients needed in the formation of blood cells:

Low levels of the following nutrients may also trigger canker sores:

  • Thiamine
  • Zinc
  • Vitamin D

Injury to the Mouth

Mouth injuries may precede a canker sore. The following can cause an injury in your mouth:

  • Cutting your mouth with a sharp or broken tooth 
  • Wearing dentures that don’t fit well
  • Cutting your mouth with braces
  • Getting dental work
  • Brushing your teeth too hard
  • Biting the inside of your mouth
  • Burning your mouth with hot food or drink

Hormonal Changes

You may notice that you develop canker sores when your hormones fluctuate. This most commonly happens during menstruation. But the sores are also possible during other times when your hormone levels change.

Weakened Immune System

An immune system that is not at its healthiest may allow for the development of canker sores. For instance, sores may appear if you have a viral infection. Examples of viral infections include the cold and flu.

Exposure to Allergens or Toxins

Allergies or sensitivities to certain foods and ingredients may trigger the development of canker sores. These include:

  • The sodium lauryl sulfate in oral hygiene products
  • Cinnamon
  • Cheese
  • Citrus fruits

Toxin exposure, such as the nitrates in drinking water, can also trigger canker sores.

Are Canker Sores Hereditary?

Evidence suggests that canker sores run in families. So if someone in your family gets canker sores, you might be more likely to as well.

A child may have up to a 90% chance of repeatedly developing canker sores if both parents also experienced canker sores. If neither parent ever had canker sores, your risk drops to 20%.

About 24% to 46% of people who repeatedly get canker sores have a family history of canker sores.

Researchers have found that many of the genes potentially involved in the development of mouth ulcers include ones linked to the regulation of the immune system.

Risk Factors

Canker sores are one of the most common conditions that affect the mucous portion of the mouth. Although anyone can develop them, canker sores are more common among some groups.

Women have a higher chance of developing canker sores than men.

A person can start to develop the sores at any age, but they most commonly first appear in adolescents and young adults. Some studies indicate that the most likely years of their development fall between 10 and 19 years of age.

People living with certain underlying health conditions may also have a higher prevalence rate. This includes people with gastrointestinal diseases or human immunodeficiency virus (HIV) infection.

People with certain cancers may also develop canker sores. Those include cancers of the:

  • Mouth
  • Pancreas
  • Head and neck
  • Breast
  • Prostate

Race or ethnicity does not appear to affect a person’s likelihood of developing canker sores. 

Socioeconomic status appears to, with those in more affluent classes being more likely to develop canker sores.  

People who are less likely to get canker sores are those who practice good oral hygiene.

A Quick Review

Canker sores are non-contagious, painful sores that you may develop on your gums, on the inside of your cheeks, or on the roof of your mouth. The exact cause is not known, but experts believe that it may occur due to an immune system response. There are several things that can trigger a canker sore, including stress, nutritional deficiencies, and mouth injuries.  

Women, children, younger adults, and those whose parents have had canker sores are more likely to develop the sore. You may find that practicing good oral hygiene and avoiding any triggers, if possible, can prevent future canker sores.

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