Canker sores develop when the soft tissue inside your mouth breaks down, usually from a combination of physical irritation, immune response, and individual susceptibility. Unlike cold sores, which are caused by a virus, canker sores aren’t contagious and aren’t triggered by a single clear cause. Instead, a mix of factors converge to create the conditions for one to form.
Physical Trauma to the Mouth
The most straightforward trigger is mechanical damage to the tissue inside your mouth. Biting your cheek, jabbing yourself with a toothbrush, or eating something with a sharp edge (like a tortilla chip) can break the surface of the soft tissue and start the process. Dental appliances, braces, and ill-fitting dentures cause repeated friction against the inner cheeks and gums, which is another common trigger. Canker sores tend to appear on the softer, non-keratinized surfaces inside the mouth, the areas that lack the tougher protective layer found on your gums and the roof of your mouth.
Once that tissue is damaged, your immune system mounts a response. The damage is driven primarily by a specific type of white blood cell (T-cells), and inflammatory signaling molecules amplify the process, turning a small injury into a visible, painful ulcer.
Your Toothpaste May Be a Factor
Sodium lauryl sulfate (SLS), the foaming agent in most toothpastes, strips away the protective mucus layer coating the inside of your mouth. With that barrier compromised, the underlying tissue is left exposed and more vulnerable to irritation. In one study, participants who switched from an SLS-containing toothpaste to an SLS-free version saw a 64% reduction in canker sore occurrence, dropping from an average of 14.3 ulcers over three months to just 5.1. A 2019 systematic review of four clinical trials involving 124 participants confirmed the pattern: SLS-free toothpaste consistently reduced the number of ulcers, how long each one lasted, and the pain involved.
If you get canker sores regularly, switching to an SLS-free toothpaste is one of the simplest changes you can make.
Nutrient Deficiencies That Set the Stage
Several specific nutrient gaps are linked to recurrent canker sores:
- Iron: Iron deficiency, even before it progresses to full anemia, has been associated with canker sores. You might also notice fatigue and weakness.
- Vitamin B12: A B12 deficiency can show up as canker sores along with numbness in fingers and toes, fatigue, and heart palpitations.
- Folate: People with frequent canker sores often have lower blood levels of folate. This vitamin is essential for cell growth and tissue repair, so low levels may slow the mouth’s ability to heal minor damage before it becomes an ulcer.
- Zinc: Zinc plays a central role in wound healing, and a deficiency slows that process, giving small injuries more time to develop into sores.
If you’re getting canker sores repeatedly and can’t identify an obvious trigger, a blood test checking these levels can be worthwhile.
Stress and Hormonal Shifts
Stress is one of the most well-documented triggers. The classic example: college students developing canker sores during final exams. Psychological stress appears to shift the immune response in ways that make the mouth’s soft tissue more vulnerable to breakdown.
Hormonal fluctuations play a role too. Some women who are prone to canker sores find they recur in a predictable pattern tied to their menstrual cycle. The connection isn’t fully mapped, but the correlation between hormonal shifts and flare-ups is consistent enough that many women can anticipate outbreaks.
Foods That Trigger Flare-Ups
Certain foods are known to worsen or trigger canker sores in susceptible people. Chocolate, peanuts, and eggs are among the most commonly reported culprits. These aren’t true allergic reactions in most cases. Rather, something about these foods seems to provoke the inflammatory process in people who are already prone to sores. Acidic foods like citrus fruits and tomatoes can also irritate existing tissue damage and accelerate ulcer formation.
Keeping a simple food diary when sores appear can help you identify your personal triggers, since the specific foods vary from person to person.
Underlying Health Conditions
Occasional canker sores are extremely common and usually harmless. But frequent or severe outbreaks can sometimes signal an underlying condition. Celiac disease, inflammatory bowel disease (including Crohn’s disease), and Behçet’s syndrome all feature recurrent oral ulcers as a symptom. In these cases, the canker sores are part of a broader pattern of inflammation throughout the body, and treating the underlying condition typically reduces the frequency of mouth sores as well.
The Three Types and How Long They Last
Not all canker sores are the same. Minor canker sores, the most common type, are smaller than a pea (under one centimeter), and they heal within a few weeks without leaving a scar. Major canker sores are larger than one centimeter, significantly more painful, and can take months to heal. They often leave scars. Herpetiform canker sores are rare and look different from the other types: instead of a single ulcer, tiny pinpoint sores cluster together. Despite the name, they have nothing to do with the herpes virus. They typically heal within about two weeks.
Canker Sores vs. Cold Sores
People often confuse these, but they’re different in almost every way. Canker sores appear inside the mouth and look like a single round white or yellow sore with a red border. Cold sores (fever blisters) appear outside the mouth, typically around the border of the lips, and look like a cluster of small fluid-filled blisters. Cold sores are caused by herpes simplex virus and are contagious. Canker sores are neither viral nor contagious.
If your sore is inside your mouth, round, and not in a cluster of blisters, it’s almost certainly a canker sore.