Aphthous ulcers, commonly known as canker sores, are painful white or yellow sores that develop inside the mouth, affecting roughly one in five people at some point in their lives. While these sores typically heal on their own within a week or two, they can cause significant discomfort and may return again and again, disrupting daily activities like eating and speaking.

How Common Are Aphthous Ulcers?

Aphthous ulcers represent one of the most widespread oral health issues worldwide. Research indicates that approximately 20 to 25 percent of the general population experiences these painful mouth sores at least occasionally^[1][4]. Some studies from different countries have reported prevalence rates ranging from as low as 1.4 percent to as high as 21.4 percent, depending on the region and population studied^[4].

These ulcers don’t affect everyone equally. They most commonly first appear during childhood or adolescence, typically between the ages of 10 and 19^[5]. Young adults are particularly susceptible, with the condition being most frequent in teenagers and people in their twenties^[3]. Interestingly, females tend to develop canker sores more often than males, a pattern that researchers believe may be linked to hormonal differences^[1][3].

For many people who develop aphthous ulcers, the condition is not just a one-time occurrence. The ulcers tend to recur repeatedly over time, with rates of recurrence within three months as high as 50 percent^[4]. Despite this tendency toward repeated episodes, there is good news: many individuals find that the frequency and severity of outbreaks naturally decrease as they reach their thirties, and the condition often resolves on its own over several years^[6].

What Causes Aphthous Ulcers?

The exact reason why aphthous ulcers develop remains something of a medical mystery. Despite extensive research, scientists have not yet identified a single clear cause for these painful sores. Current understanding suggests that the body’s immune system, which normally protects against illness and infection, becomes disturbed by external factors and begins to react abnormally against proteins in the delicate tissue lining the mouth^[1].

What researchers do know is that these ulcers are not contagious. Unlike cold sores caused by the herpes virus, you cannot catch aphthous ulcers from someone else, nor can you spread them through kissing, sharing utensils, or any other form of contact^[3][5]. No virus or bacteria has been consistently linked to causing these sores, even though infectious agents like Helicobacter pylori (a stomach bacteria) and herpes simplex virus have been investigated^[8].

The mechanism by which these ulcers form appears to involve immune-related processes. Studies have found that people with recurrent aphthous ulcers may have changes in how their immune system functions locally in the mouth. Both T-cells (a type of white blood cell that helps fight infection) and B-cells (white blood cells that produce antibodies) show altered responses in patients who experience recurring ulcers^[8]. When examined under a microscope, the sores contain specific types of immune cells and are covered with a protein called fibrin^[8].

Genetics appears to play a significant role in who develops these ulcers. Approximately 40 percent of people who get aphthous ulcers have a family history of the condition^[1][5]. Research has also identified certain genetic markers that may increase susceptibility, suggesting that some people are simply born with a greater tendency to develop mouth ulcers.

Risk Factors and Triggers

While the underlying cause remains unclear, many factors are known to trigger outbreaks of aphthous ulcers in susceptible individuals. Understanding these triggers can help people identify what might be causing their particular episodes.

Stress and sleep are among the most important triggers. Emotional stress and lack of adequate sleep can prompt the development of new ulcers^[1]. Stressful life events have been found to nearly triple the odds of experiencing an outbreak^[4]. The stress doesn’t directly cause the ulcers, but it appears to increase the likelihood that they will develop and can slow down the healing process^[5].

Physical injury to the inside of the mouth is another common trigger. This can include accidentally biting the inside of your cheek while eating, burns from hot food or drinks, or irritation from dental work, rough tooth brushing, or ill-fitting dentures or orthodontic devices like braces^[5][3]. Even minor trauma to the soft tissues of the mouth can set off an ulcer in someone who is prone to them.

Nutritional deficiencies can contribute to the development of aphthous ulcers in some people. Deficiencies in vitamin B12, folic acid (also called folate), iron, or zinc have all been associated with these sores^[1][3]. When the body lacks these essential nutrients, the tissues lining the mouth may become more vulnerable to ulceration.

Certain foods and beverages can trigger outbreaks in susceptible individuals. Common culprits include chocolate, coffee, strawberries, eggs, nuts, cheese, and foods that are acidic, spicy, or salty^[1][5]. Different people react to different foods, so keeping track of what you eat before an outbreak can help identify your personal triggers.

Dental care products containing sodium lauryl sulfate, a foaming agent found in many toothpastes and mouthwashes, may trigger or prolong mouth ulcers in some people^[1][5]. While this ingredient helps toothpaste foam up, it may irritate the delicate tissues inside the mouth.

Hormonal changes can also play a role, particularly in women. Some women notice that they develop ulcers in relation to their menstrual cycle or when taking certain oral contraceptives^[3][5]. Pregnancy can also be associated with hormonal shifts that trigger these sores.

Certain medications have been linked to the development of aphthous ulcers. These include beta-blockers (used for heart conditions and high blood pressure), immunosuppressants (drugs that suppress the immune system), and NSAIDs (nonsteroidal anti-inflammatory drugs like ibuprofen)^[3]. Additionally, a medication called nicorandil, which is given for angina (chest pain from heart disease), has been associated with mouth ulcers^[1].

Paradoxically, smoking appears to offer some protection against aphthous ulcers, even though smoking worsens many other oral and skin conditions^[1][8]. However, quitting smoking can sometimes trigger an outbreak as the mouth adjusts to the change^[7]. This does not mean smoking is beneficial overall—the harmful effects of tobacco use far outweigh any protective effect against canker sores.

Recognizing the Symptoms

Aphthous ulcers have distinctive features that make them relatively easy to identify, though they can vary in size and severity. The typical sore appears as a round or oval-shaped shallow crater inside the mouth, with a white, yellow, or grayish center and a red border or halo around the edge^[1][2]. The surrounding tissue usually looks healthy and unaffected.

Before a visible ulcer appears, many people experience warning signs. You might feel a tingling, burning, or prickling sensation in a spot inside your mouth a day or two before the actual sore develops^[2][3]. This prodromal (early warning) sensation can last anywhere from two to 48 hours before you can see the ulcer^[9].

These sores develop only on areas of the mouth where the tissue is not tightly attached to bone underneath. That means they typically appear on the inside of the lips and cheeks, on or under the tongue, at the base of the gums, on the soft palate (the soft back portion of the roof of your mouth), or in the area around the tonsils^[1][9]. They do not occur on the external surface of your lips or outside the mouth—that feature helps distinguish them from cold sores.

The main symptom of an aphthous ulcer is pain, which can range from mild to quite severe. The discomfort is often worse when the sore is irritated by movement of the mouth during talking, or by contact with certain foods and drinks, particularly those that are citrus, acidic, salty, or spicy^[1]. Even routine activities like brushing your teeth can become uncomfortable when you have an active ulcer.

You might develop just a single ulcer, or you could have multiple sores at once. When multiple ulcers occur, they tend to be spread throughout different areas of the mouth rather than clustered in one spot^[1]. The ulcers can also affect the genital area in both males and females, though this is less common^[1].

There are three main types of aphthous ulcers, classified by their size and characteristics. Minor aphthous ulcers are the most common type, accounting for about 80 percent of cases^[1][4]. These are small sores, typically less than 5 millimeters in diameter (about the size of a pencil eraser or smaller), and they heal completely within one to two weeks without leaving any scars^[2].

Major aphthous ulcers are larger and more serious, though they occur in only about 10 to 15 percent of people with canker sores^[4]. These ulcers are often more than 10 millimeters in diameter (larger than a pea), penetrate deeper into the tissue, and are extremely painful^[1][2]. They can take weeks or even months to heal and may leave behind scars. Major ulcers can appear anywhere in the mouth and may affect the area at the back of the throat^[4].

Herpetiform ulcers are the least common type, affecting somewhere between 1 and 10 percent of people with aphthous ulcers^[4]. Despite the name, these are not caused by the herpes virus. They appear as multiple tiny pinpoint sores, sometimes as many as 100 at a time, each measuring just 2 to 3 millimeters across^[4]. These small ulcers often merge together to form larger, irregularly shaped sores. Most herpetiform ulcers heal within about two weeks to a month^[1][3].

In severe cases—though these are extremely rare—people might experience additional symptoms such as swelling of the lymph nodes, fever, physical fatigue, or a general feeling of being unwell^[5]. If you develop these more serious symptoms along with mouth ulcers, it’s important to seek medical attention.

Prevention Strategies

While there’s no guaranteed way to prevent aphthous ulcers completely, there are several practical steps you can take to reduce the frequency of outbreaks and minimize your risk of developing new sores.

Practice gentle oral hygiene. Taking care of your mouth is important, but being too aggressive can cause problems. Use a soft-bristled toothbrush and brush gently to avoid injuring the delicate tissues inside your mouth^[3]. Use waxed dental floss rather than unwaxed, as it’s less likely to cause small cuts or scrapes. If you wear dentures, orthodontic devices, or mouth splints, make sure they fit properly. Ill-fitting dental appliances can rub against the inside of your mouth and trigger ulcers^[3]. Orthodontic wax can be used to cover sharp edges on braces or other devices^[5].

Choose your toothpaste carefully. Consider switching to a toothpaste that doesn’t contain sodium lauryl sulfate, the foaming agent that may trigger or prolong ulcers in some people^[3][5]. You can ask your dentist or dental care provider to recommend suitable brands.

Avoid mouthwash with alcohol. Alcohol-based mouthwashes can cause irritation to the tissues inside your mouth and may make you more susceptible to developing ulcers^[20].

Be mindful of what you eat and drink. If you’ve noticed that certain foods or drinks seem to trigger your ulcers, try to avoid them^[3]. Common triggers include acidic foods and beverages, spicy dishes, salty snacks, coffee, chocolate, eggs, cheese, peanuts, and almonds. Eating carefully can also help you avoid accidentally biting the inside of your cheek or lips, which can lead to ulcer formation.

Address nutritional deficiencies. If you experience frequent ulcers, talk to your healthcare provider about testing for nutritional deficiencies^[3]. Ensuring adequate intake of vitamin B12, vitamin D, folic acid, iron, and zinc may help reduce outbreaks. If a deficiency is found, your provider can recommend appropriate supplements or dietary changes.

Manage stress effectively. Since stress is a significant trigger for many people, finding ways to reduce stress in your life can be helpful^[5]. This might include practicing relaxation techniques such as meditation, deep breathing exercises, yoga, or other stress-reduction activities. Getting adequate sleep is also important, as lack of sleep can increase the likelihood of developing ulcers^[1].

Keep a diary. If you get ulcers frequently, consider keeping a food and symptom diary to help identify your personal triggers^[5]. Write down what you eat and drink, note any stressful events or changes in your routine, and record when ulcers appear. Over time, patterns may emerge that can help you understand and avoid your specific triggers.

How Aphthous Ulcers Affect the Body

Understanding what happens in your body when an aphthous ulcer forms can help explain why these sores are so painful and why they take time to heal. The process involves a breakdown in the normal protective barrier of the mouth and an inappropriate immune response that damages healthy tissue.

The inside of your mouth is lined with a special type of tissue called mucous membrane or mucosa. This delicate, moist tissue serves as a protective barrier and contains many nerve endings, which is why the mouth is so sensitive. In most parts of the mouth, this mucosa is not tightly bound to the bone underneath, which allows for the flexibility needed for speaking, eating, and other mouth movements. However, this same quality makes these areas more vulnerable to ulceration.

When an aphthous ulcer develops, there is a loss or erosion of part of this protective mucosal layer. The process appears to involve the immune system malfunctioning in a localized area. Instead of protecting the tissue, certain immune cells begin attacking it. White blood cells that normally fight infection start to target proteins in the mouth’s lining^[1].

This immune reaction causes inflammation, which brings increased blood flow to the area—that’s what creates the characteristic red border or halo around the ulcer. The center of the sore becomes covered with a loose membrane made up of dead cells, fibrin (a protein involved in blood clotting), and other debris, which gives the ulcer its white, yellow, or grayish appearance^[1].

The exposed tissue in the crater of the ulcer is raw and highly sensitive because the protective mucous membrane that normally covers it is gone. This is why ulcers are painful, especially when they come into contact with food, drinks, or are touched by the tongue or teeth. The many nerve endings in the area send pain signals to the brain, alerting you to the damage.

For people with recurrent aphthous stomatitis (the medical term for repeatedly occurring ulcers), there appear to be alterations in how the immune system functions locally in the mouth. Studies have shown changes in cell-mediated immunity, which is the part of the immune system that involves T-cells directly attacking perceived threats^[8]. There may also be systemic changes in how both T-cells and B-cells respond throughout the body in people who experience frequent ulcers.

The exact trigger that sets off this immune reaction is still not fully understood. It may be that in susceptible individuals, various factors—such as trauma, certain foods, stress, or other triggers—cause the immune system to temporarily misidentify the mouth’s own tissue as foreign or dangerous. The immune system then launches an attack, creating the ulcer.

Over time, the body’s natural healing processes work to repair the damage. New cells gradually grow from the edges of the ulcer, filling in the crater and rebuilding the protective mucosal layer. For minor ulcers, this healing typically takes one to two weeks. The tissue regenerates without forming scar tissue, so the area returns to normal once healing is complete. However, major ulcers that penetrate more deeply may take much longer to heal and can leave permanent scars because the damage extends beyond the mucosa into deeper layers of tissue.