Canker sores result from an overactive immune response in the lining of your mouth, but the triggers that set off that response vary widely from person to person. Unlike cold sores, canker sores are not caused by a virus and are not contagious. They form inside the mouth as round, white or yellow sores with a red border, and they can be triggered by everything from a minor bite on your cheek to a vitamin deficiency you didn’t know you had.
The Immune Response Behind the Sore
At a cellular level, canker sores involve a wave of immune cells rushing into the surface tissue of your mouth and breaking it down. White blood cells called T-cells drive most of the damage, producing an inflammatory signal called TNF-alpha that essentially attacks the mouth’s own lining. Your body also ramps up production of other inflammatory compounds, and the combination erodes a small patch of tissue until an open ulcer forms.
This is why canker sores often appear after some kind of local irritation. The injury triggers inflammatory signals that, in susceptible people, spiral into a full ulcer rather than healing quietly. It also helps explain why so many different triggers can lead to the same painful result: anything that kicks off or amplifies that inflammatory cascade in your mouth can cause one.
Physical Injury to the Mouth
The most straightforward trigger is mechanical damage. Accidentally biting your cheek, getting poked by braces or dentures, or even brushing too aggressively can all start the process. The injury doesn’t need to be dramatic. A sharp chip on a tooth, a slip of the toothbrush, or friction from orthodontic wire is often enough. In people prone to canker sores, what would be a minor scrape for someone else becomes a full ulcer within a day or two.
Toothpaste With SLS
Sodium lauryl sulfate (SLS) is a foaming agent found in most toothpastes, and it’s a well-documented irritant for people who get frequent canker sores. In one clinical study, patients who switched from an SLS-containing toothpaste to an SLS-free version saw their number of ulcers drop from about 14 over three months to roughly 5 in the same period. That’s a significant reduction from a simple product swap. If you get canker sores regularly, checking your toothpaste ingredient list is one of the easiest things you can try.
Vitamin and Mineral Deficiencies
Low levels of certain nutrients are closely linked to recurring canker sores, particularly vitamin B12, folate, and iron. Research comparing people with frequent canker sores to the general population found that affected individuals consumed about 7% less of the recommended daily intake of B12 and 20% less folate. Iron-deficiency anemia is also a recognized risk factor.
These deficiencies may contribute to canker sores because B12, folate, and iron all play roles in maintaining healthy mucosal tissue and supporting normal immune function. When levels drop, the mouth’s lining becomes more vulnerable to breakdown. If your canker sores keep coming back without an obvious trigger, a blood test checking these levels is a reasonable step.
Food Sensitivities and Irritants
Certain foods are common triggers, though the specific culprits vary by individual. The most frequently reported ones include coffee, chocolate, nuts, cheese, citrus fruits, and potatoes. Spicy, salty, and highly acidic foods can also provoke new sores or worsen existing ones. These foods likely act as direct chemical irritants to the mouth’s lining, lowering the threshold for that immune overreaction to kick in. Keeping a simple food diary when outbreaks occur can help you identify your personal triggers.
Stress and Hormonal Shifts
Stress is one of the most frequently cited triggers for canker sore flares. Studies of people with recurrent canker sores have found significantly elevated levels of cortisol (the body’s primary stress hormone) and higher anxiety scores, even during periods when they didn’t have an active sore. Stress appears to suppress parts of the immune system while amplifying the inflammatory pathways that drive ulcer formation.
Hormonal changes play a similar role, particularly for women. Some women develop canker sores on a predictable schedule tied to their menstrual cycle. In these cases, sores typically appear just before or during menstruation, when progesterone levels shift. The pattern is distinctive: a feeling of fullness or swelling in the lips one to two days before the period starts, followed by painful oral sores that resolve once menstruation ends. One review found the menstrual cycle was the precipitating factor in 73% of hormone-related cases.
Genetics and Family History
If both of your parents get canker sores, your chance of getting them is estimated at about 90%. If neither parent is affected, that risk drops to around 20%. This strong hereditary pattern suggests that the underlying immune tendency to overreact in the oral lining is largely inherited. It also means that some people are simply more biologically prone to canker sores regardless of lifestyle, while others can encounter every known trigger without ever developing one.
Underlying Medical Conditions
Frequent canker sores can sometimes signal a systemic health condition, especially when they first appear in someone over 40. Conditions most strongly associated with recurrent oral ulcers include celiac disease, Crohn’s disease, ulcerative colitis, iron-deficiency anemia, Behçet disease, and HIV/AIDS. In celiac disease, for example, canker sores may be one of the earliest or only noticeable symptoms, appearing because the autoimmune process damages the gut’s ability to absorb nutrients like iron and folate.
If you’re developing canker sores frequently for the first time later in life, or if they’re accompanied by other symptoms like digestive problems, joint pain, or fatigue, it’s worth investigating whether an underlying condition is involved.
Three Types of Canker Sores
Not all canker sores are the same, and the type you get affects how long it lasts and how much it hurts.
- Minor canker sores are the most common. They measure about 4 to 5 millimeters across (smaller than a pencil eraser), heal within 10 to 14 days, and don’t leave scars.
- Major canker sores are larger than 10 millimeters, dig deeper into the tissue, and can take weeks to months to heal. These often leave scars.
- Herpetiform canker sores appear as clusters of tiny 1 to 2 millimeter ulcers, sometimes 5 to 100 at once. Despite the name, they have nothing to do with herpes. A single crop typically lasts 7 to 14 days.
Canker Sores vs. Cold Sores
These two get confused constantly, but they’re completely different conditions. Canker sores form inside the mouth, appear as a single round sore (white or yellow center, red border), and are not contagious. Cold sores (fever blisters) form on the outside of the mouth around the lips, look like clusters of small fluid-filled blisters, and are caused by the herpes simplex virus, making them contagious. If your sore is inside your mouth and isn’t a blister, it’s almost certainly a canker sore.
One important note: any mouth ulcer that doesn’t heal within four weeks should be evaluated, as persistent sores can occasionally indicate oral cancer or another condition that needs attention.