Canker sores are caused by an overactive immune response that attacks the lining of your mouth, but the specific trigger varies from person to person. Unlike cold sores, they aren’t caused by a virus and aren’t contagious. Most people get them from a combination of factors: physical irritation, certain foods, stress, nutritional gaps, or genetics.
Your Immune System Turns on Your Own Tissue
At the cellular level, canker sores form when your immune system mistakenly targets the thin tissue lining your mouth. Certain immune cells, including T cells that normally fight infections, begin destroying the oral surface instead. This process is fueled by inflammatory signaling molecules that recruit even more immune cells to the area, creating a self-reinforcing cycle of tissue damage. The result is that familiar shallow, painful crater.
People who get canker sores repeatedly show higher blood levels of several inflammatory markers compared to people who don’t. Their immune pathways involved in detecting threats appear to be overactive, essentially lowering the threshold for triggering an attack on their own tissue. This is why so many different things can set off a canker sore: the underlying immune response is primed, and it just needs a nudge.
Physical Irritation and Mouth Trauma
One of the most common nudges is simple mechanical damage. Biting the inside of your cheek, jabbing yourself with a chip or cracker, or burning your mouth on hot food can all create just enough tissue injury for the immune response to overreact. Orthodontic braces are a frequent culprit, especially in the first weeks after they’re placed, because the brackets and wires rub against tissue that hasn’t toughened up yet. Dental work, aggressive tooth brushing, and even a sharp edge on a broken tooth can do the same thing.
Foods That Trigger Outbreaks
Certain foods reliably provoke canker sores, though the specific triggers differ between people. The most common categories include acidic foods like citrus fruits and tomatoes, salty foods like pretzels and processed meats, and spicy dishes. Coffee and alcohol are also well-known triggers. Both tend to be acidic, can dry out the mouth, and irritate tissue that’s already vulnerable. Some research links higher coffee intake to more frequent outbreaks.
Food sensitivities play a role too. In some people, specific proteins or additives provoke a localized immune reaction in the mouth that leads directly to ulceration. Common offenders include dairy proteins, gluten, eggs, shellfish, and certain artificial colors or preservatives. Chocolate can also be a trigger, partly because of its acidity and partly because of allergenic ingredients it’s often paired with, like milk and additives. Nuts are a double threat: their rough texture can physically scrape the tissue, and they’re a common allergen.
If you notice a pattern between certain foods and your outbreaks, an elimination approach (cutting suspect foods for a few weeks, then reintroducing them one at a time) can help you identify your personal triggers.
Nutritional Deficiencies
Low levels of specific nutrients are strongly associated with recurrent canker sores. In one study of people with recurring mouth ulcers, about half were deficient in vitamin B12, nearly 46% had low folate levels, and roughly 11% had low iron stores. These nutrients all play roles in maintaining healthy mucosal tissue and supporting normal immune function, so when they’re depleted, the mouth lining becomes more vulnerable to breakdown.
If you get canker sores frequently, it’s worth having your B12, folate, and iron levels checked through a simple blood test. Correcting a deficiency sometimes reduces or eliminates outbreaks entirely.
Stress and Hormonal Changes
Stress is one of the most commonly reported triggers, and the connection is biological, not just anecdotal. Stress hormones alter immune function in ways that can tip the balance toward the kind of inflammatory overreaction that produces canker sores. Many people notice outbreaks clustering around exams, deadlines, or periods of emotional strain.
Hormonal shifts during the menstrual cycle also appear to play a role. The rise in progesterone in the days before a period can trigger oral changes including swollen gums and canker sore development. Some women notice their outbreaks follow a predictable monthly pattern.
Your Toothpaste May Be Contributing
Sodium lauryl sulfate (SLS), a foaming agent found in most commercial toothpastes, is a well-documented canker sore trigger. A systematic review of clinical trials found that switching to an SLS-free toothpaste significantly reduced the number of ulcers, the duration of each ulcer, the number of episodes, and pain levels. On average, people using SLS-free toothpaste developed about one fewer ulcer per observation period and had ulcers that healed roughly two days faster.
This is one of the simplest changes you can make if you deal with recurring canker sores. SLS-free toothpastes are widely available at most pharmacies and grocery stores.
Genetics and Family History
If your parents get canker sores, you’re more likely to get them too. In studies of people with recurrent outbreaks, roughly 46% reported a family history of the condition. Researchers have investigated specific immune-related genetic markers, but no single gene has been definitively linked to canker sores in the general population. The hereditary component likely involves multiple genes that collectively influence how reactive your immune system is to minor triggers in the mouth.
The Three Types of Canker Sores
Not all canker sores are the same, and the type you get affects how long you’ll be dealing with it.
- Minor canker sores are the most common type. They’re smaller than a pea (under one centimeter), heal within a few weeks, and don’t leave scars.
- Major canker sores are larger than one centimeter, significantly more painful, and can take months to heal.
- Herpetiform canker sores are rare. They appear as clusters of tiny pinpoint sores that typically heal within about two weeks without scarring. Despite the name, they have nothing to do with the herpes virus.
Canker Sores vs. Cold Sores
These two conditions are frequently confused, but they’re entirely different. Canker sores appear inside the mouth, on the inner cheeks, lips, or tongue. They look like single round white or yellow sores with a red border. They are not contagious. Cold sores (fever blisters) appear outside the mouth, typically around the lip border. They look like clusters of small fluid-filled blisters, are caused by herpes simplex virus, and are highly contagious.
When a Mouth Sore Needs Attention
A typical canker sore heals on its own within two to three weeks. Any mouth sore that persists beyond two to four weeks without healing warrants a professional evaluation. A nonhealing ulcer is one of the early signs of oral cancer, along with persistent red or white patches in the mouth. This doesn’t mean every lingering sore is cancer, but any oral lesion that doesn’t resolve on its own or respond to typical care should be examined and, if necessary, biopsied to rule out something more serious.