Mouth ulcers form when your immune system attacks the thin lining of tissue inside your mouth, breaking it down and leaving an open sore. Between 5% and 25% of people experience recurrent mouth ulcers, with some studies putting that figure as high as 60% depending on the population. The process involves a chain reaction of immune cells, local triggers, and sometimes genetic vulnerability that together explain why these painful sores keep coming back for so many people.
What Happens Inside the Tissue
The lining of your mouth is only a few cell layers thick in many places, which makes it vulnerable to damage. When a mouth ulcer forms, a specific type of white blood cell called a T cell begins targeting and destroying the surface tissue. These immune cells are normally responsible for fighting infections, but in the case of mouth ulcers, they turn on healthy tissue instead.
Once the attack starts, the damaged cells release signaling molecules that recruit more immune cells to the area and keep the inflammatory cycle going. The tissue breaks down into a shallow crater, typically covered by a white or yellowish membrane with a red border. That red halo is the visible sign of increased blood flow and active inflammation surrounding the wound. The pain comes from exposed nerve endings in this raw, open tissue, which is why eating acidic or salty foods stings so intensely.
One theory for why the immune system misfires this way involves a common mouth bacterium. T cells may react to proteins on these bacteria that resemble proteins in your own tissue, essentially confusing “foreign” with “self.” This cross-reaction damages the mucosal lining and kicks off ulcer formation. People who get frequent ulcers also show abnormalities in a key immune signaling pathway that tilts the immune response toward an overly aggressive, inflammatory state.
The Stages of an Ulcer
Most people notice a tingling or burning sensation a day or two before the ulcer appears. This prodromal phase is your first clue that the immune process has already started beneath the surface. Next, a small raised spot appears, usually round and yellowish with a red rim. Within a day or so, this breaks open into the characteristic punched-out sore.
About 80% of mouth ulcers are the minor type, less than 5 mm across. These heal on their own within one to two weeks without scarring. Major ulcers are larger, often exceeding 10 mm, and can take weeks or even months to fully heal, sometimes leaving a scar behind. A third type, called herpetiform ulcers, shows up as clusters of tiny pinpoint sores that can merge together and typically resolve within a month.
Common Triggers That Start the Process
Physical trauma is one of the most straightforward triggers. Biting your cheek, poking your gum with a sharp chip, or irritation from braces can all breach the mucosal barrier just enough to set off the immune response in someone who’s prone to ulcers. But the trigger doesn’t have to be mechanical.
Your toothpaste may play a role. Sodium lauryl sulfate (SLS), the foaming agent in most toothpastes, strips protective oils from the mouth’s lining and can make tissue more vulnerable. In one study, people who switched from a standard SLS toothpaste to an SLS-free version saw their ulcer count drop by roughly 60 to 70%. If you get frequent ulcers, this is one of the simplest changes worth trying.
Nutritional deficiencies also lower your mouth’s defenses. Low levels of iron, vitamin B12, and folate are all linked to more frequent ulcers, likely because these nutrients are essential for maintaining healthy mucosal tissue and a properly functioning immune system.
Why Stress and Hormones Matter
Stress is one of the most commonly reported triggers, and research supports the connection, though not in the way you might expect. A study measuring cortisol (the body’s main stress hormone) in people with active ulcers found that cortisol levels didn’t actually rise during flare-ups. Instead, the association appears to run through anxiety and depression, which were significantly linked to ulcer occurrence. Chronic psychological distress may alter immune regulation in ways that make the mouth’s lining more susceptible, even without a spike in cortisol.
Hormonal shifts during the menstrual cycle are another well-documented trigger. Some women find ulcers are most troublesome during the luteal phase, the stretch between ovulation and menstruation. During this window, progesterone acts on tissue that has already been primed by estrogen, and fluctuations in these hormones alter immune activity in the mouth’s lining. For women who notice a clear monthly pattern, this hormonal influence is likely the driving factor.
The Genetic Component
If your parents dealt with frequent mouth ulcers, you’re more likely to get them too. Researchers have identified specific genetic markers in the immune system that influence susceptibility. Certain variations in the HLA system, the set of genes that helps your immune cells distinguish your own tissue from foreign invaders, are associated with higher ulcer risk. Other HLA variants appear to be protective, significantly reducing the likelihood of developing recurrent ulcers.
This genetic wiring helps explain why some people get ulcers after the slightest provocation while others rarely or never do, even when exposed to the same triggers. Your baseline immune programming determines how aggressively your T cells respond to minor tissue disruptions in the mouth.
Links to Broader Health Conditions
Mouth ulcers are usually a standalone nuisance, but persistent or unusually severe ulcers can sometimes signal a systemic condition. Inflammatory bowel diseases like Crohn’s disease are among the most notable. Oral symptoms, including ulcers, cobblestone-textured patches on the inner cheeks, and swollen lips, can appear as a direct manifestation of the disease in the mouth, sometimes even before gut symptoms develop.
Celiac disease is another condition where mouth ulcers serve as an early clue. The nutritional deficiencies caused by poor absorption in the gut, particularly iron and B vitamins, create conditions that favor ulcer formation. In these cases, the ulcers aren’t a separate problem but rather a visible consequence of what’s happening deeper in the body. Frequent, hard-to-explain mouth ulcers that don’t respond to typical management are worth investigating for these underlying causes.