Canker sores: what they are, why they appear and when to see a dentist

A canker sore (aphthous ulcer) is a small, round, painful ulcer inside the mouth that, in most cases, heals on its own within one to two weeks. It is not contagious, it is not an infection, and it rarely leaves a mark. It is the most common ulcerative condition of the oral mucosa (Akintoye and Greenberg, 2014), so you have probably had one. What helps is knowing why they appear, how to ease them while they last, and which signs mean a sore is not an ordinary canker sore.

What a canker sore is and how to recognize it

A typical canker sore is round or oval, with a yellowish or grey base and a well-defined red border (Scully and Porter, 2008). Many people feel burning or tingling in the area a day or two before it shows. After that it hurts, especially when eating, brushing or talking.

Canker sores form on the soft lining of the mouth: the inside of the lips and cheeks, the sides and underside of the tongue, the floor of the mouth and the soft palate. They are uncommon on the firm gum around the teeth and on the hard palate, which helps tell them apart from other lesions.

When canker sores keep coming back, the medical name is recurrent aphthous stomatitis. It usually starts in childhood or adolescence and often runs in families, because there is a genetic predisposition (Scully and Porter, 2008).

Types of canker sores and how long they last

• Minor canker sores. The vast majority. Smaller than one centimeter, single or in small groups, they heal in 7 to 14 days without scarring.
• Major canker sores. Larger than one centimeter and deeper, they can last several weeks. They sometimes scar, and a dentist should see them.
• Herpetiform canker sores. Dozens of tiny ulcers that may merge. Despite the name, they are not caused by the herpes virus.

Why canker sores appear

The exact cause is still unknown. What is known is that, in a predisposed person, certain triggers set off a local immune response that ends in an ulcer (Akintoye and Greenberg, 2014). These are the best studied.

Small injuries and friction

An accidental bite, harsh brushing, hard food, or the rubbing of a bracket, a denture or a tooth with a rough edge can be the starting point.

Stress and fatigue

Many people notice they get more of them during exams, heavy workloads or short nights. In a case-control study, people with recurrent canker sores had higher psychological stress, and the authors describe stress as a trigger rather than the underlying cause (Gallo et al., 2009). Stress shows in the mouth in other ways too, such as bruxism.

Your toothpaste

Many toothpastes contain sodium lauryl sulfate (SLS), the detergent that makes foam. A systematic review of clinical trials found that, in people with recurrent canker sores, switching to an SLS-free toothpaste reduced the number of ulcers, their duration and pain (Alli et al., 2019). The trials are few and small, but it is an easy change to try: check the label for sodium lauryl sulfate. SLS-free does not mean fluoride-free, and fluoride remains the basis of cavity prevention.

Low iron, vitamin B12 or folate

A meta-analysis of nine studies found vitamin B12, folic acid, ferritin and hemoglobin deficiencies more often in people with recurrent canker sores (Chen et al., 2015). A more recent meta-analysis confirmed the association with low vitamin B12, low ferritin and low hemoglobin (Mousavi et al., 2024). Association is not causation, and not every canker sore comes from a deficiency. But if they keep coming back, it is worth asking your physician for a blood test and looking at your diet.

Conditions linked to canker sores

Sometimes canker sores point to something systemic. A review of controlled studies found canker sores more often in people with celiac disease, although most of the data come from children and there are few studies in adults (Nieri et al., 2017). They are also associated with inflammatory bowel disease such as Crohn's disease (Akintoye and Greenberg, 2014), and with Behçet's disease, in which mouth ulcers come with genital ulcers and eye problems (Scully and Porter, 2008). It is a concrete example of the mouth-body connection.

Canker sores on the tongue, gums, lip or throat

• On the tongue. Among the most annoying, because the tongue never stops moving. They usually appear on the sides or underside. A lesion on the side of the tongue that does not heal in two weeks should be checked by a professional.
• On the gums. They can form on the soft lining near the fold between the gum and the lip or cheek. On the firm gum attached to the teeth they are rare: a sore there is usually a brushing injury or herpes.
• On the lip. Very common on the inside of the lip. On the outside, on the skin or the red border of the lip, it is almost never a canker sore: a cold sore is far more likely.
• In the throat. They can appear on the soft palate or near the tonsils. If they come with fever, severe sore throat or feeling unwell, an infection is more likely, and you should see a physician.

Canker sore, cold sore or angular cheilitis: telling them apart

Three different lesions that are often confused:

• Canker sore: a round ulcer inside the mouth, on the soft lining. Not contagious.
• Cold sore: caused by the herpes simplex virus. It starts with tingling and forms small clustered blisters, almost always on the border of the lip, which break and crust. It is contagious, especially while blisters are present. When it appears inside the mouth, it usually does so on the firm gum or hard palate, exactly where canker sores rarely appear.
• Angular cheilitis: red, painful cracks at the corners of the mouth. Fungi and bacteria are usually involved, and it can be a sign of another problem, such as a denture that no longer fits or a nutritional deficiency (Chiriac et al., 2024).

If you are not sure which one it is, do not self-treat with cold sore products. A dentist can usually tell them apart at a glance.

Are canker sores contagious?

No. Canker sores do not spread through kissing, sharing glasses or any other contact. Cold sores do, which is why it matters not to confuse them.

How to heal a canker sore and ease the pain

No treatment makes a canker sore vanish overnight, and none prevents new ones. The goal is to reduce pain, help it heal sooner and space out recurrences (Akintoye and Greenberg, 2014). While it lasts:

• Avoid acidic, very salty, spicy and crunchy foods, which irritate the ulcer.
• Brush with a soft toothbrush and go gently around the sore.
• If you use a mouthwash, choose an alcohol-free one. Here is which mouthwash to use, and when.
• Pharmacies carry gels and films that cover the sore and protect it from friction, and topical anesthetics that ease pain for a short while. Use them following the package instructions.
• If canker sores keep returning, try an SLS-free toothpaste.

Avoid applying irritants such as alcohol or pure baking soda directly on the sore: they increase pain and do not close it any faster.

When canker sores are large, very frequent or stop you from eating, a dentist can prescribe a treatment such as a topical corticosteroid. It reduces the severity of an episode, although they do not prevent new sores (Scully and Porter, 2008).

When a canker sore needs a dentist

Most canker sores do not need a visit. Book an appointment if:

• The ulcer has not healed in two weeks. If it lasts more than three weeks without a clear explanation, the UK NICE guideline recommends considering referral to rule out oral cancer (NICE NG12).
• It is larger than one centimeter or very deep.
• Many appear at once, or you are almost never free of them.
• They first started in adulthood.
• The lesion is painless but hard, or comes with a white or red patch that does not go away.
• You have fever or swollen glands in the neck, or you struggle to swallow or drink.
• You also have genital ulcers, eye discomfort, persistent diarrhea or unexplained weight loss. In that case, see your physician as well.

How we approach it at AS Odontología Digital

Most of the sores patients show us are ordinary canker sores, and confirming that is enough. We also check whether something in the mouth is causing them, such as a tooth with a rough edge, a worn restoration or an appliance that rubs, because that is a cause we can correct. When canker sores are frequent, we talk it through with you and suggest looking into possible systemic causes with your physician.

If you have an ulcer that won't heal, or you simply want to be sure, you can book an evaluation with us in Vitacura.

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