Canker sores -

What are they? / What causes them? / Outbreak risk factors.

Canker sore

Link to aphthous ulcer diagram.


Link to outbreak incidence graph.

Canker sores (also referred to as "aphthous ulcers") are one of the most common types of mouth ulcerations.

And even though their underlying cause is not well understood (see below), they are easily identified. They're also easy to distinguish from other types of mouth sores (like intraoral herpes).

This page outlines known risk factors that can trigger outbreaks. Other pages of this topic discuss various methods that can be used to manage these mouth ulcers, including home remedies, over-the-counter products and prescription medications.

What are Canker Sores?

They're painful round or oval ulcerations that form inside a person's mouth.

They have a white membrane-coated center (B) that's surrounded by a red border (A).

The characteristic appearance of canker sores.

Diagram showing the physical characteristics of an aphthous ulcer.

For the legend to this graphic see: How to identify canker sores.

In scientific literature they're usually referred to as: "Recurrent minor aphthous ulcers" or "Recurrent minor aphthous stomatitis."

What causes them to form?

There are a number of different risk factors that can play a role (either alone or in combination) in triggering canker sore outbreaks. They include:

A) Mechanical trauma.

A canker 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 sores were triggered this way. (Casiglia 2002) [page references]

Abnormalities in the composition of a person's saliva may also cause local tissue trauma (Chavan 2012).

B) Psychological stress.

Many people find that their outbreaks coincide with periods of emotional stress (this can even include normal day-to-day types of events that tend to wear us down from time to time).

As evidence of this correlation, researchers have found that patients who have psychological disorders tend to have a higher incidence rate of aphthous ulcers. So do people who have genetic conditions that cause increased anxiety traits.

C) Dietary issues.

Canker sore outbreaks may be associated with nutritional deficiencies. Some of the vitamins and nutrients thought to be involved are:

  • Vitamin deficiencies: B1 (thiamine deficiency), B2 (riboflavin deficiency), B6 (pyridoxine deficiency), B12 (pernicious anemia), C
  • Nutrient deficiencies: zinc, folic acid, iron, selenium, calcium

(Ship 2000)

D) Food allergies.

Sensitivities to some foods (as well as some other substances) seem to be triggers for outbreaks. These 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), peanuts, almonds, chocolate, shellfish, soy, vinegar, French mustard, tea, coffee, 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 helping to sort this issue out, a person might 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).

Some studies have suggested 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 canker sore break outs.

This may be due to a damaging effect that SLS has on the outer protective layer of oral tissues. As this layer becomes degraded, the tissues that lie underneath it become more exposed to the effects of irritants, with this trauma resulting in the formation of a lesion.

Research findings.

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.

(Related page: The best SLS-free toothpastes.)

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 yet been adequately documented or 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 suffers from them too.
  • Another study found that 91% of identical twins both experienced aphthous ulcers. The correlation with fraternal twins was only 57%.
  • People who have a positive family history typically get their first ulcers 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 for canker sores. However, chemical compounds typically associated with infections have been isolated from them.

  • One research study found 95 different bacterial species associated with their test group's ulcerations. Only 3 of these species were found in the mouths of the control group. (Casiglia 2002)
  • Some studies have suggested a relationship between Streptococcus sanguis (which may cause tissue damage) and the formation of canker sores. However this theory is still unproven. Chavan (2012)

I) Infectious agents - Viral.

Viruses may play a role in aphthous 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 Varicella zoster.

J) Medical conditions.

Several medical conditions have a correlation with canker sore formation (and other types of aphthous ulcers as well). That means that people who experience persistent difficulties with mouth ulcers should consider the possibility that an underlying undiagnosed systemic condition exists.

Some of the medical conditions involved are: Behcet's disease, neutrophil dysfunction diseases, inflammatory bowel diseases (celiac, Crohn's, ulcerative colitis), HIV-AIDS, MAGIC syndrome, Reiter's syndrome, systemic lupus erythematosus, cyclic neutropenia, Marshal's syndrome and Sweet's syndrome. (Ship 2000, Chavan 2012)

K) Medications.

The use of nonsteroidal anti-inflammatory medications (NSAIDs), beta blockers, chemotherapeutic agents, and nicorandil have each been suggested as being risk factors for more frequent break outs.

At a biological level, 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. Their immune system encounters molecules in oral tissues 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. As a result, a mouth ulcer forms.

When do most outbreaks occur?

There may be a seasonal tendency for canker sore outbreaks. If there is, it seems that people tend to suffer with them more during the summer months.

Our "research."

What we did was evaluate Google search volume for the keywords "canker sores" over a 5 year period.

The chart we created from that data shows that web searches tend to rise into and subsequently top out during the summer months (August into September). They then fall off, reaching a low point in December and into January.

Graph of monthly volume of web searches for the keywords 'canker sores.'

As you can see below, similar analysis involving the term "mouth ulcers" seems to show the same seasonal bias.

(Aphthous ulcers are the most common type of recurring mouth ulcer. And for those who initially don't know what type of lesion they do have, the generic term "mouth ulcers" seems a logical choice to use for an initial search.)

Graph of monthly volume of web searches for the keywords 'mouth ulcers.'

We aren't trying to pass our simple study off as fact. But what it shows is interesting.

Our hypothesis.

In regard to an explanation, one risk factor for outbreaks is allergies to food, including fruits and vegetables. The increased availability of these items during the summer months might explain the seasonal effect demonstrated above.



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