Canker sores form when your immune system overreacts to minor irritation inside your mouth, breaking down a small patch of the soft tissue lining your cheeks, gums, or tongue. Unlike cold sores, which are caused by a virus and appear on or around the lips, canker sores are not infections and are not contagious. They affect anywhere from 5% to 66% of the population depending on the group studied, and most people who get them will notice they happen less often with age.
Physical Injury Is the Most Common Trigger
The single most frequent cause of a canker sore is mechanical damage to the soft tissue inside your mouth. This can be as minor as accidentally biting your cheek while chewing, scraping your gums with a sharp chip, or burning the roof of your mouth on hot food. Dental work is another reliable trigger: having a cavity filled, getting a crown fitted, or even a routine cleaning that scrapes tender spots can set one off within a day or two.
Orthodontic hardware is a particularly common culprit. Braces, retainers, and aligners create constant low-grade friction against the inner lips and cheeks. The repeated rubbing wears through the top layer of tissue, and your immune system responds by sending inflammatory cells to the area, which can turn a small irritation into a full ulcer. Even a toothbrush with hard bristles or an aggressive brushing technique can do enough damage to start the process.
Your Toothpaste May Be Contributing
Sodium lauryl sulfate, the foaming agent in most toothpastes, has been linked to canker sores in multiple studies. In a 2012 clinical trial, 90 participants alternated between toothpaste with and without this ingredient for eight weeks at a time. When using the version without it, they reported that their canker sores healed faster and caused less pain. If you get sores frequently, switching to a toothpaste labeled “SLS-free” is one of the simplest changes you can make. Several major brands sell versions without it.
Nutritional Deficiencies That Raise Your Risk
Your body needs certain nutrients to maintain the mucosal lining inside your mouth, and running low on them makes that tissue more vulnerable. The three most studied deficiencies are vitamin B12, folate, and iron. In one study comparing people with recurrent canker sores to a healthy control group, about 50% of those with frequent sores were deficient in B12, compared to none in the control group. Roughly 46% had low folate levels as well.
These deficiencies are common in people who eat limited diets, have absorption issues, or follow strict vegetarian or vegan diets without supplementation. A simple blood test can check all three levels. Correcting a deficiency often reduces how frequently sores come back, though it won’t necessarily stop them entirely.
Stress and Hormonal Shifts
If you notice canker sores appearing during exam weeks, work deadlines, or emotionally difficult periods, the connection is real. Stress elevates cortisol, your body’s primary stress hormone, which suppresses normal immune function and reduces the resistance of your oral tissue to damage. At the same time, stress increases the production of reactive oxygen species, molecules that can directly damage cells lining the mouth. The combination makes it easier for a minor irritation that would normally heal quietly to spiral into a visible ulcer instead.
Some women notice canker sores appearing at predictable points in their menstrual cycle, particularly in the days before a period when hormone levels shift. This pattern suggests that fluctuations in estrogen and progesterone play a role in mucosal vulnerability, though the exact mechanism is less well understood than the stress connection.
Food Sensitivities and Acidic Foods
Certain foods trigger canker sores in susceptible people without causing any kind of allergic reaction in the traditional sense. The most commonly reported offenders are citrus fruits, tomatoes, strawberries, chocolate, coffee, and spicy foods. These tend to be highly acidic or contain compounds that irritate the mucosa directly. Keeping a simple food diary when sores appear can help you identify your personal triggers, since they vary widely from person to person.
Underlying Health Conditions
For most people, canker sores are a nuisance with an identifiable trigger. But frequent or severe sores that keep returning can occasionally signal an underlying condition. Celiac disease, the autoimmune reaction to gluten, commonly presents with recurrent mouth ulcers, sometimes before any digestive symptoms appear. Crohn’s disease, an inflammatory bowel condition, can also cause oral ulcers as one of its earliest signs.
Behçet syndrome, a rarer inflammatory disorder, is characterized by recurring ulcers in the mouth and other areas of the body. HIV infection and other conditions that weaken the immune system can produce frequent, slow-healing sores. In children, a condition called PFAPA syndrome causes periodic fevers alongside canker sores, sore throat, and swollen neck glands. These conditions are uncommon causes, but worth considering if you’re getting large or unusually frequent sores that don’t match typical patterns.
The Three Types and How They Differ
Not all canker sores are the same. Minor aphthous ulcers account for 75% to 85% of cases. They’re less than 1 centimeter across, last 7 to 14 days, and heal without leaving a scar. These are the small, round, white or yellowish sores with a red border that most people picture when they think of a canker sore.
Major aphthous ulcers make up 5% to 10% of cases and are noticeably larger, over 1 centimeter. They’re significantly more painful, can take up to six weeks to heal, and often leave scarring on the tissue afterward. The third type, called herpetiform ulcers (despite having nothing to do with the herpes virus), also accounts for 5% to 10% of cases. These appear as clusters of tiny sores, sometimes as many as 100 at once, each only 1 to 3 millimeters across. They can merge into larger irregular ulcers but typically heal within two weeks.
Canker Sores vs. Cold Sores
The easiest way to tell these apart is location. Canker sores occur inside the mouth, on soft tissue like the inner cheeks, gums, tongue, or soft palate. Cold sores (fever blisters) occur outside the mouth, typically on or around the border of the lips. They also look different: cold sores start as clusters of small fluid-filled blisters that eventually crust over, while canker sores are single round or oval ulcers with a white or yellow center. Cold sores are caused by the herpes simplex virus and are contagious. Canker sores are neither viral nor contagious.
What Healing Looks Like
Most canker sores follow a predictable timeline. You may feel a tingling or burning sensation for a day or two before the sore becomes visible. The ulcer itself typically peaks in pain during the first three to four days, then gradually shrinks. Minor sores resolve completely within 10 to 14 days without any treatment. During healing, the pain usually drops significantly after the first few days, and you’ll notice the white or yellow center slowly being replaced by normal pink tissue.
Major sores follow the same general pattern but on a much longer timeline, sometimes taking six weeks to fully close. If a sore lasts longer than three weeks, is unusually large, comes with a fever, or makes it difficult to drink fluids, that’s worth a medical evaluation to rule out something beyond a typical canker sore.