Canker sores are small, painful ulcers that form inside the mouth, and they’re usually triggered by a combination of factors rather than a single cause. The most common triggers are minor mouth injuries, emotional stress, certain foods, nutritional deficiencies, and an overactive immune response that attacks the lining of your mouth. About 20% of the population gets them regularly, and while they’re not contagious or dangerous, understanding what sets them off can help you get fewer of them.
How Canker Sores Actually Form
A canker sore isn’t an infection. It’s your own immune system turning against the tissue inside your mouth. When something triggers the process, immune cells called T lymphocytes activate and begin attacking the cells that make up your oral lining. This sets off a chain reaction: inflammatory signals flood the area, more immune cells migrate in, and the tissue breaks down into an open ulcer. The result is that familiar round, white or grayish crater with a red border that hurts when anything touches it.
Because the underlying mechanism is immune-driven, anything that ramps up inflammation or weakens the protective lining of your mouth can start the process. That’s why the list of triggers is so varied. Stress, a sharp chip scraping your cheek, and a vitamin deficiency don’t seem related on the surface, but they all push the same immune pathway toward overreaction.
Mouth Injuries and Physical Irritation
The single most common trigger is minor trauma to the inside of your mouth. Biting your cheek, getting poked by a toothbrush bristle, scraping your gums on a tortilla chip, or irritation from braces or dental work can all kick off a sore within a day or two. The injury doesn’t need to be dramatic. Even aggressive brushing or a small burn from hot food is enough to break the mucosal barrier and invite the immune response that creates the ulcer.
Sodium lauryl sulfate (SLS), the foaming agent in most toothpastes, is another physical irritant worth knowing about. SLS is a detergent that strips the protective mucous layer inside your mouth, leaving the tissue more vulnerable. If you get canker sores frequently, switching to an SLS-free toothpaste is one of the simplest changes you can make.
Stress and Emotional Triggers
Stress is one of the best-documented triggers. The classic example is a college student breaking out in canker sores during finals week. Psychological stress increases levels of inflammatory signaling molecules throughout the body, and the thin, delicate lining of the mouth is especially sensitive to that shift. If you notice sores appearing during high-pressure periods at work or after a stretch of poor sleep, the connection is likely real and not coincidental.
Foods That Set Off Outbreaks
Certain foods are well-known triggers, though the specific culprits vary from person to person. The most frequently reported ones include chocolate, peanuts, eggs, coffee, tomatoes, and acidic fruits like oranges and pineapple. These foods don’t cause canker sores in everyone, but in people who are prone to them, they can irritate the oral lining or provoke an immune flare.
Acidic foods do the most straightforward damage: they chemically erode the mucous membrane, lowering the threshold for an ulcer to form. Chocolate and nuts seem to work through a different, less well-understood mechanism, possibly related to specific proteins that trigger an immune response in susceptible people. Keeping a food diary when you get an outbreak can help you identify your personal triggers over time.
Vitamin and Mineral Deficiencies
Low levels of certain nutrients are strongly linked to recurrent canker sores, and this is one of the most actionable causes to investigate. Vitamin B12 stands out as the biggest factor. In one study comparing people with recurrent mouth ulcers to healthy controls, about 50% of the ulcer group was deficient in B12, while none of the controls were. That’s a striking difference.
Folate (vitamin B9) deficiency also shows up frequently in people with recurrent sores, with roughly 46% of ulcer patients showing low levels in the same study. Iron deficiency plays a smaller but measurable role. All three nutrients are essential for maintaining healthy mucosal tissue and supporting normal immune function, so when levels drop, the mouth lining becomes more fragile and the immune system more reactive.
If you’re getting canker sores regularly and can’t pin them on an obvious trigger like stress or injury, it’s worth having your B12, folate, and iron levels checked through a simple blood test. This is especially relevant if you follow a vegetarian or vegan diet, since B12 comes almost exclusively from animal products.
Hormonal Fluctuations
Some women notice canker sores appearing in the days before their period. Hormonal shifts during the menstrual cycle, particularly the drop in progesterone and estrogen right before menstruation, can affect the oral mucosa and trigger outbreaks. This isn’t universal. Most women don’t experience oral changes with their cycle. But if you see a pattern, hormones are a likely contributor. The same mechanism can cause swollen or bleeding gums at the same point in the cycle.
Underlying Health Conditions
Occasional canker sores are normal, but frequent or severe outbreaks can sometimes signal a systemic condition worth investigating.
Celiac disease is one of the more common hidden causes. People with undiagnosed celiac disease often develop recurrent mouth ulcers because the autoimmune damage to their small intestine impairs absorption of B12, iron, and folate. The sores may improve or disappear entirely after switching to a gluten-free diet.
Crohn’s disease and other inflammatory bowel conditions can also produce canker-like ulcers in the mouth. In Crohn’s, the sores typically resolve when the underlying intestinal inflammation is treated. Behçet syndrome, a rarer condition involving widespread blood vessel inflammation, frequently causes numerous, painful mouth ulcers as its first and most common symptom. These tend to cluster on the soft palate and back of the throat and are often more severe than typical canker sores.
If your sores are unusually large, appear in clusters, recur more than three or four times a year, or come alongside other symptoms like digestive problems, joint pain, or genital ulcers, it’s worth looking into whether a systemic condition is driving them.
The Three Types of Canker Sores
Not all canker sores are the same, and knowing which type you’re dealing with can help you set expectations for healing.
- Minor canker sores are the most common type. They’re less than 5 millimeters across (roughly the size of a pencil eraser), round with a white or gray center, and heal on their own within 10 to 14 days without scarring.
- Major canker sores exceed 1 centimeter in diameter and can appear on parts of the mouth that minor sores typically don’t affect. These take up to 6 weeks to heal and can leave scars. They’re significantly more painful and may interfere with eating and talking.
- Herpetiform canker sores are the least common but most unusual-looking. They start as many tiny ulcers, sometimes up to 100 at once, that merge together into large, irregularly shaped sores. Despite the name, they have nothing to do with the herpes virus.
Reducing Your Risk
Since canker sores result from an immune overreaction, the goal is to minimize the triggers that push your immune system in that direction. Switch to an SLS-free toothpaste. Use a soft-bristled toothbrush and avoid aggressive brushing. Track whether specific foods like chocolate, nuts, coffee, or acidic fruits seem to precede your outbreaks, and cut back on the ones that do.
Make sure your diet supplies adequate B12, folate, and iron. Good sources of B12 include meat, fish, eggs, and dairy (or a supplement if you eat plant-based). Folate is abundant in leafy greens, legumes, and fortified grains. Iron comes from red meat, beans, and spinach. Managing stress through whatever works for you, whether that’s exercise, sleep, or something else, can also reduce the frequency of outbreaks.
Most canker sores heal on their own within two weeks and don’t need treatment beyond basic pain management. Over-the-counter oral gels or rinses can take the edge off while you wait. The sores that warrant a closer look are the ones that are unusually large, last longer than three weeks, come back constantly, or make it difficult to eat or drink.