Canker sores don’t have a single cause. They result from a combination of factors, including immune system overreaction, nutritional gaps, physical irritation, and genetic susceptibility. Unlike cold sores, they are not caused by a virus and are not contagious. Up to 20% of the general population gets them repeatedly, and for most people, multiple triggers overlap to produce an outbreak.
Your Immune System Attacks Your Own Tissue
The core mechanism behind a canker sore is your immune system turning against the thin tissue lining the inside of your mouth. White blood cells designed to fight infections mistakenly target healthy cells in the oral lining, creating a painful open wound. This immune overreaction releases inflammatory molecules that break down the surface tissue and expose the nerve-rich layers underneath, which is why even a tiny canker sore can produce sharp, disproportionate pain when you eat or talk.
What triggers this immune misfiring varies from person to person. Stress, lack of sleep, hormonal shifts (especially around menstruation), and minor injuries to the mouth can all set off the process. A sharp chip, a slip of the toothbrush, biting the inside of your cheek, or irritation from braces or dentures is often enough to start an ulcer in someone who’s prone to them.
Nutritional Deficiencies That Fuel Outbreaks
Low levels of certain vitamins and minerals are closely linked to recurring canker sores. The three most common deficiencies are vitamin B12, iron, and folate. In clinical studies, researchers use thresholds of B12 below 220 pg/mL, folate below 280 ng/mL, and ferritin (iron stores) below 10 ng/mL in men or 20 ng/mL in women to define deficiency levels associated with recurrent ulcers.
These nutrients all play roles in maintaining healthy mucosal tissue and supporting normal immune function. When they’re depleted, the oral lining becomes more fragile and the immune system more prone to the kind of misfiring that produces canker sores. If you get frequent outbreaks with no obvious trigger, a simple blood test checking these three levels can be revealing. Correcting a deficiency through diet or supplements often reduces how often sores return.
Foods That Trigger Sores
Certain foods act as direct irritants to the mouth’s lining and can provoke canker sores in susceptible people. The most common culprits are citrus fruits, tomatoes, chocolate, coffee, and spicy foods. These don’t cause canker sores in the way a virus causes an infection. Instead, they create chemical irritation or minor tissue damage that, in someone already predisposed, is enough to kick off an immune response and ulcer formation.
Nuts and hard, crunchy foods can also contribute by physically scratching the inner cheek or gum tissue, giving the immune system a site to overreact. Keeping a food diary during outbreaks can help you identify your personal triggers, since the specific foods vary widely from person to person.
Your Toothpaste May Be Making It Worse
Sodium lauryl sulfate (SLS), a foaming agent found in most commercial toothpastes, is a well-documented canker sore trigger. In one clinical study, patients who switched from an SLS-containing toothpaste to an SLS-free one saw their number of ulcers drop by roughly 60 to 70%. All ten participants in the study experienced fewer sores after making the switch.
SLS strips away the protective mucous layer inside the mouth, leaving the tissue more vulnerable to irritation and immune attack. If you deal with frequent canker sores, switching to an SLS-free toothpaste is one of the simplest and most effective changes you can make. Several major brands now offer SLS-free options, usually labeled as such on the packaging.
Underlying Health Conditions
Recurrent canker sores can be an early or ongoing sign of certain systemic diseases, particularly celiac disease and inflammatory bowel diseases like Crohn’s disease and ulcerative colitis. In an Italian survey of patients with these conditions, about 65% reported oral symptoms during active disease flares, and canker sores were the most common complaint, affecting roughly 63% of those with oral symptoms.
The connection works in two directions. These conditions cause widespread inflammation that can affect the mouth, and the nutrient malabsorption they produce (especially of iron, B12, and folate) compounds the problem. In people with celiac disease specifically, canker sores sometimes appear before any digestive symptoms do. Persistent, recurring canker sores that don’t respond to the usual fixes can be worth investigating further, particularly if you also experience unexplained fatigue, digestive issues, or unintentional weight loss.
Genetics and Family History
If your parents got canker sores frequently, you’re significantly more likely to get them too. The genetic component is strong enough that researchers have identified specific immune system markers associated with higher risk. Studies of twins and families consistently show that susceptibility to recurrent canker sores runs in families, influencing how your immune system responds to minor oral tissue damage.
This genetic predisposition helps explain why some people can bite their cheek and heal without incident, while others develop a painful ulcer from the same minor injury. It also explains why certain individuals get canker sores regardless of how carefully they manage dietary triggers and oral care products.
Canker Sores vs. Cold Sores
Because both involve painful sores around the mouth, canker sores and cold sores are frequently confused. They are completely different conditions. According to the National Institute of Dental and Craniofacial Research, the key distinction is location: cold sores appear outside the mouth, typically along the lip border, while canker sores occur inside the mouth on soft tissue like the inner cheek, gums, or tongue.
They also look different. Cold sores are clusters of small fluid-filled blisters, while canker sores are single, round ulcers that are white or yellow with a red border. Cold sores are caused by herpes simplex virus type 1 and are contagious. Canker sores have no known infectious cause and cannot be spread from person to person. This distinction matters because the treatments and prevention strategies are entirely different for each condition.
Stress and Hormonal Shifts
Emotional stress is one of the most commonly reported triggers for canker sore outbreaks, and the connection has a physiological basis. Stress hormones suppress certain immune functions while amplifying others, creating the kind of dysregulated immune response that leads to tissue destruction in the mouth. Many people notice outbreaks clustering around exams, work deadlines, or emotionally difficult periods.
Hormonal fluctuations play a similar role. Some women experience canker sores predictably during specific phases of their menstrual cycle, particularly just before or during menstruation. Pregnancy and menopause can also shift the pattern of outbreaks. These hormonal links suggest that the immune dysregulation behind canker sores is sensitive to the body’s broader chemical environment, not just what’s happening locally in the mouth.