Canker Sores / Aphthous ulcers.  The basics -

What are they? | What causes them to form? | Outbreak triggers and risk factors. | What to avoid.

What are Canker Sores?

Canker sores are a type of mouth ulceration. (They're actually one of the most common kinds of mouth sores.)

The lesions themselves are easy to identify (based on their appearance and location where they've formed). And they're easy to distinguish from other types of mouth sores (like intraoral herpes).

(Use the links above for more detailed information, additional pictures and comparisons.)

The characteristic appearance of a canker sore.

Diagram showing the physical characteristics of an aphthous ulcer.

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

Other terms.
Canker sores are formally referred to as aphthous ulcers.
And in scientific literature, the routine form of canker lesion that we discuss on our pages is customarily referred to as: "recurrent minor aphthous ulcers" or "recurrent minor aphthous stomatitis."
While there's no true cure or preventive for canker sores, they're routinely managed via home remedies, the use of over-the-counter products or prescription medications.

What causes canker sores to form?

At this point in time, the biological mechanism that results in aphthous ulceration formation isn't entirely clear. (We discuss the predominant theory below on this page.)

There are, however, a number of known and suspected risk factors that are apparently associated with triggering outbreaks. Although, while many of these issues and events are significant for some people, for others they're not factors at all.

That's an interesting point.

The wide variety of outbreak triggers suggests that a common disease process exists. Yet the way it's initiated in different subgroups of people varies.

Another point that's interesting to note is that most, but not all (see below), people who experience canker sores are otherwise healthy individuals.

Section references - Neville

Identified/Suspected triggers and predisposing factors for canker sores. -

A) Mechanical trauma.

For many people, canker sore formation is frequently preceded by some type of tissue damage to the affected area.

  • A study by Casiglia reported that 38% of its participants felt that their sores were initiated by some type of tissue trauma.

In some instances, the type of tissue trauma that's occurred may not be so obvious.

  • A review of this subject by Chavan reported that variations in the composition of a person's saliva, or events associated with smoking, or even smoking cessation, may lead to tissue changes that trigger canker sore formation.

Section references - Casiglia, Chaven

B) Psychological stress.

Many people find that their outbreaks coincide with periods of emotional stress. This can even include those kinds of fluctuations caused by the relatively routine events and changes that we tend to experience on a day-to-day basis.

As evidence of the correlation, it's reported 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.

Section references - Casiglia

C) Dietary issues.


  • Nutrient deficiencies: zinc, folic acid, iron, selenium, calcium

Section references - Ship, Neville

D) Food allergies.

Sensitivities to some types of foods and compounds have been reported to be triggers for outbreaks. These items include:

  • Dairy: cow's 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), dyes

Section references - Woo, Neville

Keeping a food diary.

As a way of understanding what correlations exists for them, a person might keep a diary where they record the types of suspected foods and compounds they have had any exposure to that day. Then, later on if an outbreak occurs, they'll have a record they can refer back to that can help them to identify the offending item(s).

E) Sodium lauryl sulfate (SLS).

Some studies have suggested that the use of 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 breakouts.

A proposed mechanism.

As an explanation, it's suggested that SLS has a denaturing (molecular altering) effect on the surface of oral tissues.

As the level of protection provided by the surface layer of skin is degraded, the tissues lying underneath become ever-increasingly exposed to the effects of oral irritants. It's tissue trauma related to this loss of protection that then triggers the formation of an aphthous lesion.

Research findings.

  • Several early studies reported that test subjects who used SLS-free toothpaste experienced fewer canker sores. One study found this reduction to be as high as 81%. (Chahine)

    This same study also stated that some subjects reported that their aphthous lesions that did form were less painful than the ones experienced during periods when they continued to use products that contained SLS.

  • It should be noted that other studies have not been able to replicate all of these same outcomes.

    For example, a more recent study by Shim found that while the use of an SLS-free toothpaste did reduce ulcer duration and pain levels, its use did not help to reduce the overall number of outbreaks that the test subjects experienced.

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

Section references - Chahine, Shim

F) Hormonal changes.

For women, there may be a relationship between ulcer formation and certain phases of their menstrual period. It's 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 data suggests that people may tend to have a genetic predisposition for experiencing canker sores.

  • It's been reported that 35% of people who get lesions have at least one parent who suffers from them too.

    If both parents experience them, there's a 90% chance that their child will too.

  • In the case of identical twins, if one suffers there's a 91% chance that the sibling will too. The correlation with fraternal twins is only 57%.
  • Those people who have a positive family history typically get their first ulcers at an earlier age and experience more intense symptoms.

Section references - Casiglia, Akintoye

H) Infectious agents -


The current thought is that bacteria are not a causative agent for canker sores. However, chemical compounds typically associated with infections have been isolated from lesions.

  • 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)
  • Some studies have suggested a relationship between Streptococcus sanguis (a type of bacteria that may cause tissue damage) and the formation of canker sores. However, this theory is still unproven. (Chavan)



Section references - Casiglia, Chavan

I) Medical conditions.

Several medical conditions have been reported to have a correlation with canker sore formation (and other forms of aphthous ulcers as well). This finding suggests 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.

Section references - Neville, Chavan, Ship

J) Medications.


Section references - Neville

K) Are they contagious?

No, canker sores are neither contagious nor infectious.

What causes canker sores to form at the biological level?

The mechanism of aphthous ulcer formation is not well understood but the predominating theory suggests that it involves an inappropriate/inaccurate response of the person's immune system.

  1. As previously mentioned, the specific triggering event that sets the process off varies with each person. (Hence the list of outbreak factors discussed above.)
  2. Once set off, the person's immune system (via T-cells, a type of white blood cell) begins to produce "tumor necrosis factor-alpha" (TNF-α).
  3. This compound acts as a marker that assists in targeting the surface of areas of skin for destruction by the immune system. (The actual process is an inflammatory response that results in tissue destruction and thus ulcer formation.)
  4. As the process progresses, the extent of the lesion grows.

Section references - Neville, Akintoye

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 from 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.


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

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


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


Our hypothesis.
