What Are Canker Sores? Why Mouth Ulcers Appear and How They Heal

What Are Canker Sores and Why Is It Important to Recognise Them?

Canker sores are shallow, painful lesions that appear inside the mouth — on the inner cheeks, inner lips, tongue, floor of the mouth, soft palate or gums. They are usually round or oval, with a whitish-yellow centre surrounded by a red halo. In medical literature, their recurring form is called recurrent aphthous stomatitis, and they are among the most common types of mouth ulcers; many people experience at least one canker sore at some point in their lives. They appear most often in young adulthood, and their frequency tends to decrease with advancing age.

Despite their small size, canker sores can noticeably affect daily life, causing distinct pain while eating, drinking, speaking and even brushing the teeth. Fortunately, most canker sores are benign and tend to heal on their own within one to two weeks, even without any treatment. Still, knowing how to distinguish a canker sore from other oral lesions, recognising its triggers and understanding when to see a clinician both prevents unnecessary worry and ensures that conditions requiring attention are not overlooked.

What Causes Canker Sores? Known Triggers

No single, definitive cause of canker sores has been established to date; the current understanding is that genetic predisposition and environmental triggers act together. An exaggerated immune response against the tissues lining the mouth is thought to initiate ulcer formation in susceptible individuals. The triggers most consistently reported in clinical observation and research include:

• Mechanical trauma: Biting the cheek or lip, aggressive brushing with a hard-bristled brush, or a sharp-edged filling or broken tooth rubbing against the tissue
• Stress and fatigue: Intense periods at work or school, irregular sleep and exam-related stress
• Nutritional deficiencies: Deficiencies of vitamin B12, iron, folic acid and zinc have been linked to canker sore formation
• Hormonal fluctuations: Some people notice that sores appear more frequently during certain periods
• Foods: Acidic, spicy, very hot or hard foods can irritate sensitive oral tissue
• Toothpaste ingredients: Toothpastes containing sodium lauryl sulfate (SLS) can act as a trigger in some individuals
• Conditions affecting immunity: Certain illnesses and medications can lower the resistance of the oral lining
• Family predisposition: Canker sores may occur more often in people whose family members are frequently affected

What Are the Types of Canker Sores? Minor, Major and Herpetiform Ulcers

Minor aphthous ulcers are the most common type; they are usually smaller than 1 centimetre and shallow. They generally tend to heal on their own within 7 to 14 days without scarring, and they make up the vast majority of canker sores encountered in everyday life.

Major aphthous ulcers are larger than 1 centimetre, deeper and more painful. Healing can take several weeks, and scarring may remain after they resolve. Because of their size and duration, major ulcers should be evaluated and followed up by a clinician.

Herpetiform ulcers are characterised by clusters of numerous pinhead-sized lesions; these clusters can merge into larger sores with irregular borders. Despite the similarity in name, this type has no connection to the herpes virus that causes cold sores; the name refers only to the resemblance in appearance.

Whatever the type, the size, number and healing time of canker sores can vary from person to person; even in the same individual, episodes of differing severity can occur at different times. Noting when the sores appear, how long they last and what seems to trigger them therefore provides valuable information for an accurate assessment during a clinical examination.

What Is the Difference Between Canker Sores and Cold Sores? Are Canker Sores Contagious?

Canker sores and cold sores are the two most frequently confused conditions of the mouth area, yet their causes, locations and contagiousness are entirely different. A cold sore is a contagious infection caused by the herpes simplex virus; it usually appears on the outer part of the lip and around the mouth, starting as small fluid-filled blisters that later burst and crust over. A canker sore, by contrast, appears inside the mouth, is not caused by any virus or bacteria, and does not spread from person to person.

For a practical distinction, three points can be checked: location (canker sores occur inside the mouth, cold sores mostly on and around the outer lip), appearance (a canker sore is a single ulcer with a whitish-yellow base, while a cold sore begins as a cluster of blisters) and contagiousness (a cold sore is contagious during its active phase, whereas a canker sore is never contagious). Sharing a glass or kissing therefore cannot pass a canker sore to someone else. Even so, for any mouth lesion whose nature is unclear, the definitive distinction should be made through a clinical examination.

How Do Canker Sores Heal? Approaches That Support Recovery at Home

Most minor canker sores heal on their own within one to two weeks even without treatment. During this period, the aim is not so much to make the ulcer disappear instantly as to ease the pain, protect the sore from further irritation and maintain an oral environment that favours healing. The following approaches can support this process:

• Gently rinsing the mouth several times a day with warm salt water or a baking soda solution
• Avoiding acidic, spicy, very hot and hard foods until the sore has healed
• Continuing to brush gently with a soft-bristled brush; oral hygiene should not be neglected because of the pain
• Drinking plenty of water to prevent dry mouth
• Trying a toothpaste free of sodium lauryl sulfate for those who develop sores frequently
• Using protective oral gels that cover the sore and antiseptic mouth rinses, on the advice of a pharmacist or clinician

What Should Be Avoided When You Have a Canker Sore?

Some remedies popularly claimed to cure canker sores quickly can harm the tissue rather than help. Placing an aspirin or similar tablet directly on the sore, applying acidic substances such as pure lemon juice or vinegar, or pressing salt onto the wound can cause chemical irritation of the oral lining and delay healing.

Likewise, using products of unknown content or dosage without consulting a clinician or pharmacist is not recommended. Since smoking and alcohol can further irritate the oral lining while a sore is present, staying away from them during the healing period can make the process more comfortable. In addition, since hard foods such as crisps, nuts and crusty bread can mechanically irritate the sore, choosing soft-textured, lukewarm foods during this period can make the process easier.

What Does the Dentist Do to Treat Canker Sores?

For long-lasting, large or frequently recurring sores, a dental examination is important for managing the condition correctly. The dentist first assesses whether the lesion really is a canker sore, then looks for factors in the mouth that could be causing trauma. Smoothing a sharp-edged filling, repairing a fractured tooth surface or reviewing how appliances fit against the tissue can remove the source of recurring traumatic ulcers. For example, during orthodontic treatment brackets and wires rubbing against the soft tissue, or the edges of a denture pressing unevenly on the tissue, can set the stage for canker-like sores; in such cases protective wax and fit adjustments usually bring relief.

Where considered necessary, the dentist may recommend topical corticosteroid preparations or antiseptic products; in severe and widespread cases, blood tests may be requested to investigate underlying causes, or a referral may be made to the relevant medical speciality. The examination also takes overall oral health into account; addressing accompanying problems such as the causes of bleeding gums in the same visit can help strengthen the general resilience of the oral tissues.

Which Conditions Can Constantly Recurring Canker Sores Signal?

Canker sores that appear a few times a year and heal on their own are usually benign. However, sores that recur very frequently, appear in large numbers at once or take unusually long to heal can be the oral reflection of certain systemic conditions. In such conditions, mouth ulcers are usually not the only finding; accompanying signs such as fatigue, weight loss, digestive complaints or eye and skin findings may also be part of the picture. These possibilities include:

• Behçet's disease: An inflammatory condition that can involve recurrent mouth ulcers together with genital lesions and eye findings
• Coeliac disease and inflammatory bowel diseases: Absorption problems originating in the digestive system can show themselves in the oral lining
• Vitamin and mineral deficiencies: Deficiencies of B12, iron and folic acid, and some forms of anaemia, have been associated with recurrent ulcers
• Conditions affecting the immune system: Immunosuppressive illnesses and certain medications can increase how often sores appear

When Should You See a Clinician About a Canker Sore?

The oral lining is a sensitive tissue that can reflect the body's general condition; considering the link between oral health and general health, keeping track of changes in the mouth is valuable not only for the teeth but for overall wellbeing. Although most canker sores heal on their own, seeing a clinician without delay is recommended in the following situations:

• Sores lasting longer than two weeks with no tendency to shrink
• Sores that keep growing or feel unusually firm to the touch
• Sores that seriously interfere with swallowing, speaking or eating
• Cases accompanied by fever, fatigue or swollen lymph nodes under the jaw
• Sores that recur very frequently, or a new one appearing before the previous one has healed
• Mouth lesions that are painless yet fail to heal for weeks

Frequently Asked Questions About Canker Sores and Mouth Ulcers

How many days does a canker sore take to heal? Minor sores usually heal on their own within 7 to 14 days without scarring. Major ulcers can take several weeks to heal and may leave a scar. Any mouth sore that persists beyond two weeks without shrinking should be evaluated through a clinical examination.

What helps a canker sore? Warm salt water or baking soda rinses, avoiding irritating foods, and protective gels that cover the sore can help ease the pain. For products claimed to speed up healing, it is best to consult a pharmacist or clinician before use.

Are canker sores contagious? No; since canker sores are not caused by a virus or bacteria, they cannot be passed on through kissing or sharing glasses and cutlery. Cold sores are the contagious ones and usually appear on the outer lip; if there is any doubt about which condition is present, a clinical examination provides clarity.

Can a canker sore that will not heal be a sign of cancer? A canker sore itself does not turn into cancer. However, mouth lesions that persist beyond three weeks, fail to heal and are notably painless can be an early sign of other problems, so such lesions should be examined by a clinician without delay.

Which conditions can constant canker sores indicate? Very frequently recurring sores can be associated with conditions such as Behçet's disease, coeliac disease, inflammatory bowel diseases or deficiencies of B12 and iron. These possibilities can only be assessed through a clinical examination and, where needed, blood tests; getting checked is preferable to self-diagnosis.

Which specialist should be seen for canker sores? A dentist is an appropriate starting point for the first assessment; an intraoral examination can identify the type of lesion and possible traumatic causes. If considered necessary, a referral can be made to dermatology, ear-nose-throat or internal medicine.