Mouth ulcers form when your immune system mistakenly attacks the thin lining inside your mouth, breaking down tissue and leaving a painful open sore. Most are small, heal within two weeks, and aren’t dangerous. But the reasons they appear in the first place involve a surprisingly complex chain reaction of immune activity, and certain people are far more prone to them than others.
The Immune Reaction Behind the Sore
The inside of your mouth is lined with a delicate layer of cells called mucosal epithelium. When something triggers the immune system in that area, a specific type of white blood cell (T lymphocytes) becomes activated and begins attacking those lining cells as though they were a threat. This sets off a cascade: the activated immune cells release a flood of inflammatory signaling molecules, which recruit even more immune cells to the site. The result is localized tissue destruction, and that raw, open crater is the ulcer itself.
What makes this process self-reinforcing is that the inflammatory signals don’t just cause damage directly. They also stimulate the production of additional immune cells and more signaling molecules, creating a feedback loop. At the same time, the body’s anti-inflammatory signals, which normally keep this kind of reaction in check, appear to be suppressed or out of balance in people who get frequent ulcers. A 2024 meta-analysis in PLOS One confirmed that people with recurrent mouth ulcers show elevated levels of several pro-inflammatory markers and altered levels of anti-inflammatory ones, pointing to a fundamental imbalance in how their immune system regulates itself inside the mouth.
Common Triggers That Start the Process
That immune cascade doesn’t fire randomly. It needs a trigger, and the list of known triggers is long. Some are physical: biting the inside of your cheek, a sharp edge on a tooth, aggressive brushing, or dental work that scrapes the lining. Others are chemical, dietary, or hormonal. What all triggers share is that they create some initial irritation or stress on the mucosal lining, which the immune system then overreacts to.
One well-studied chemical trigger is sodium lauryl sulfate (SLS), a foaming agent found in most toothpastes. SLS strips away the protective mucus layer that coats the inside of your mouth, leaving the tissue underneath exposed. In a clinical study of people with frequent ulcers, switching from an SLS-containing toothpaste to an SLS-free one reduced the average number of ulcers from 14.3 over three months to 5.1. That’s roughly a 64% reduction from changing toothpaste alone.
Why Stress Makes Ulcers Worse
If you’ve noticed ulcers appearing during stressful periods, there’s a direct physiological explanation. Psychological stress activates a hormonal chain reaction: your brain signals the adrenal glands to release cortisol, the body’s primary stress hormone. Cortisol, in turn, alters the activity of T cells, B cells, and natural killer cells. Stress also triggers the release of adrenaline and related compounds through the nervous system. Together, these changes can shift the immune system’s behavior enough to initiate or worsen the inflammatory process that causes ulcers.
Salivary stress markers are measurably elevated in people during active ulcer episodes, reinforcing that the connection between stress and ulcers isn’t just anecdotal. It’s a hormonal and immunological event.
Nutritional Deficiencies That Raise Your Risk
Three nutrient deficiencies are consistently linked to recurrent mouth ulcers: vitamin B12, folate, and iron. All three play essential roles in tissue repair and healthy immune function. When your body is low on any of them, the mucosal lining regenerates more slowly, and the immune system is more likely to behave abnormally.
B12 deficiency is particularly common and often goes undiagnosed. A cross-sectional study in Indian children found that 64% of participants were B12-deficient, with deficient children showing significantly higher rates of oral problems. If you get frequent ulcers and haven’t had your levels checked, a simple blood test can identify or rule out these deficiencies.
Genetics Play a Significant Role
Some people are genetically wired to get more mouth ulcers. Research has identified several specific genetic markers that increase susceptibility. Certain variations in HLA genes (the genes that help your immune system distinguish your own cells from invaders) appear at higher rates in people with recurrent ulcers than in the general population. Variations in genes involved in inflammation regulation, including one called NLRP3, also increase risk. One study found that a particular gene variant can nearly double the likelihood of developing recurrent ulcers.
This genetic component explains why mouth ulcers often run in families. If both your parents experienced frequent ulcers, your chances of having them are substantially higher, regardless of diet or lifestyle.
Three Types of Mouth Ulcers
Not all mouth ulcers are identical. They fall into three categories based on size and behavior:
- Minor ulcers are the most common type. They’re typically 2 to 3 mm across (always under 8 mm), heal within about 10 days, and leave no scarring.
- Major ulcers are larger than 1 cm in diameter and can last weeks to months. These are deeper, more painful, and may leave scars when they finally heal.
- Herpetiform ulcers appear as clusters of small sores (under 1 cm each) that can merge into larger, irregularly shaped ulcers. Despite the name, they have nothing to do with the herpes virus. They typically resolve within two weeks.
The vast majority of ulcers people experience are the minor type.
When Ulcers Signal Something Deeper
Occasional mouth ulcers are extremely common and rarely indicate a serious problem. But persistent or unusual ulcers can sometimes be the first visible sign of a systemic disease. Crohn’s disease, an inflammatory bowel condition, produces oral lesions in up to 20% of patients, sometimes appearing as deep linear ulcers or a cobblestone texture on the inner cheeks. Behçet syndrome, a blood vessel disorder, causes recurrent painful ulcers that often cluster on the soft palate and back of the throat. In pemphigus vulgaris, an autoimmune blistering disease, oral ulcers are the first symptom in 50 to 80% of patients and may appear a full year or more before any skin involvement.
Lupus can also produce oral ulcers, typically appearing as well-defined red or white patches that may ulcerate. Celiac disease is another condition where mouth ulcers can be an early clue.
Ulcers vs. Oral Cancer
The key distinction between a common mouth ulcer and something more concerning is time. A typical ulcer, even without treatment, resolves within two weeks. A sore that persists beyond two weeks needs professional evaluation. Oral cancer can present as a mouth sore that doesn’t heal, a white or reddish patch, a lump or growth, persistent mouth or ear pain, or difficulty swallowing. These lesions aren’t always painful, which is why lack of pain shouldn’t be reassuring if the sore isn’t going away. Any mouth sore lasting longer than two weeks warrants a visit to your doctor or dentist so they can rule out infection or malignancy.