Canker sores -

What causes them? / Risk factors

Canker sores (also referred to as "recurrent minor aphthous ulcers") are the most common type of recurring mouth ulceration.

This page explains what causes them and risk factors known to trigger their formation.

Other pages of our topic discuss their identification and how to distinguish them from other types of lesions. We also explain their management, including the use of home remedies, over-the-counter products and prescription medications.

What causes them to form?

There are a number of risk factors that can play a role in triggering canker sore outbreaks. (Either alone or in combination with others.)

They include:

Diagram showing the characteristics of an aphthous ulcer.

Characteristics of canker sores: A) White membrane-coated center. B) Red border. C) Normal-looking surrounding skin.
(More details/pictures)

A) Mechanical trauma.

A sore's formation is often preceded by some type of tissue damage.

This might be an accidental self-inflicted bite, irritation caused by the sharp edge of a tooth that needs repair, trauma from food (like a crisp chip) or even just overenthusiastic tooth brushing.

One study investigating this issue found that 38% of its participants felt that their ulcerations were triggered this way. (Casiglia 2002) [topic references]

B) Psychological stress.

It's common that people will find that their outbreaks coincide with periods of emotional stress, like dealing with the usual type of life events that tend to wear us down from time to time.

As evidence of this general correlation, it's been found that patients who have been diagnosed with psychological disorders tend to have a higher incidence rate. And those who have genetic conditions linked to increased anxiety traits do too.

C) Dietary issues.

Research suggests that canker sore outbreaks may be associated with nutritional deficiencies.

Some of the vitamins and nutrients thought to be important in this relationship are:

  • Vitamins: B1 (thiamine deficiency), B2 (riboflavin deficiency), B6 (pyridoxine deficiency), B12 (pernicious anemia), C
  • Other nutrients: zinc, folic acid, iron, selenium, calcium

(Ship 2000)

D) Food allergies.

Sensitivities primarily to foods but also to some other substances have been suggested as being triggers for outbreaks. Some of the suspected items include:

  • Cereal grains: buckwheat, wheat, oats, rye, barley, the gluten protein found in grains
  • Fruits and vegetables: lemons, oranges, pineapples, apples, figs, tomatoes, strawberries, eggplant
  • Dairy: milk, cheeses
  • Other foods: nuts (walnuts), chocolate, shellfish, soy, vinegar, French mustard, tea, cola
  • Additives: cinnamonaldehyde / cinnamon oil (a flavoring agent), benzoic acid (a preservative)
  • Other substances: toothpastes, mints, gums, dental materials, metals, medications

(Woo 1996)

As a way of sorting this issue out, a person can keep a diary where they record the types of things they have had an exposure to that day. Then, later on if an outbreak occurs, they'll have a record they can refer back to which can help them identify the offending item.

E) Sodium lauryl sulfate (SLS).

Studies suggest that products that contain sodium lauryl sulfate ("SLS"), a foaming agent found in most toothpaste and mouthwash formulations, may place a person at increased risk for break outs.

This may be due to the drying effect it has on the outer protective layer of oral tissues. Once this layer has been degraded, the tissue underneath is more vulnerable to the effects of irritants.

Several studies have reported that participants who use SLS-free toothpaste experienced fewer sores. This reduction was found to be as high as 81% in one study. (Chahine 1997)

This same study also reported that some participants felt that those lesions that did form were less painful than the ones they experience during periods when they were using products that did contain SLS.

Not all studies, however, have been able to demonstrate all of these same outcomes. For example, a recent study (Shim 2012) found that while the use of a SLS-free toothpaste did reduce ulcer duration and pain levels, its use did not help to reduce the overall number of outbreaks the test subjects experienced.

F) Hormonal changes.

For women, there may be a relationship between ulcer formation and certain phases of their menstrual period. It has also been reported that some women notice a remission of their sores during pregnancy. Neither of these observations has been adequately documented nor explained by research.

G) Genetics.

Some people may have a genetic predisposition for canker sores.

  • One study found that 35% of people who get them have at least one parent who also suffers from them too.
  • Another study found that 91% of identical twins both experienced them whereas only 57% of fraternal twins did.
  • People who have a positive family history for them typically first develop theirs at an earlier age and experience more intense symptoms.

(Casiglia 2002)

H) Infectious agents - Bacterial.

Current thought is that bacteria are not a causative agent but chemical compounds typically associated with infections have been isolated from canker sores.

In one research study, 95 different bacterial species were associated with the test group's ulcerations. But only 3 of these species were common to both the study group and the control group. (Casiglia 2002)

I) Infectious agents - Viral.

Viruses may play a role in ulcer formation by way of causing an immune response.

Some of those suspected are: cytomegalovirus (CMV), human papilloma virus (HPV), human herpes virus-8 (HHV-8), Epstein-Barr virus (EBV), human immunodeficiency virus (HIV) and herpes simplex virus (HSV-1).

J) Medical conditions.

Several medical conditions are associated with canker sore formation (and other forms of aphthous ulcers as well). That means, for those people who experience persistent difficulties with them, that the possibility of an underlying undiagnosed systemic condition should be considered.

A few of the medical conditions know to have this association are: Behcet's disease, neutrophil dysfunction diseases, inflammatory bowel diseases (celiac and Crohn's), HIV-AIDS, MAGIC disease, Reiter's syndrome, systemic lupus erythematosus and Sweet's syndrome. (Ship 2000)

K) Medications.

The use of nonsteroidal anti-inflammatory drugs (NSAIDs), beta blockers, chemotherapeutic agents, or nicorandil has been suggested as a possible risk factor for outbreaks.

Biologically speaking, what causes canker sores?

The fundamental cause of aphthous ulcers is not well understood.

One theory suggests that it's related to an inappropriate or inaccurate response of the person's immune system.

1) The immune system encounters molecules that it identifies as "foreign."

2) This triggers an assault on the offending molecules (somewhat similar as to what happens when a transplanted organ is attacked).

3) The result of this process is the formation of an ulcer.

When do most outbreaks occur?

There may be a seasonal tendency for canker sore outbreaks. If so, it seems that people tend to suffer with them more during the summer months (in the USA).

Our chart below is based on 5 years of data and it seems to suggest that web search volume for the term "canker sores" rises into and subsequently tops out during the summer months (August into September). Thereafter, search activity tends to wane, reaching a low point in December into January.

Monthly web searches for keywords 'canker sores.'

Similar analysis involving the term "mouth ulcers" seems to show a similar seasonal bias. Since aphthous ulcers are the most common type of recurring mouth ulcer. And for those who initially don't know what type of lesion they have, the generic term "mouth ulcers" seems a logical choice of terms.

Monthly web searches for keywords 'mouth ulcers.'

We aren't suggesting that our charts and observations are hard facts. But what they show is interesting.

In regard to an explanation, one of the factors that has been suggested as a trigger for outbreaks is food allergies. This includes reactions to many fresh fruits and vegetables. The increased availability of these items during the summer months might explain the seasonal effect illustrated above.


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