Mouth ulcers are most often caused by minor tissue damage, immune system overreaction, or nutritional deficiencies, though stress, hormonal shifts, and certain systemic diseases can also trigger them. About 20% of the general population experiences recurrent mouth ulcers at some point, making them one of the most common oral health complaints worldwide.
Physical Damage to the Mouth Lining
The simplest and most common cause is direct injury to the soft tissue inside your mouth. Biting your cheek, brushing too aggressively, eating something sharp or crunchy, or irritation from braces or ill-fitting dentures can all break the delicate lining of the mouth and create an ulcer. These traumatic ulcers typically appear as a single sore with a slightly raised, reddish border, most often on the tongue, inner lips, or inner cheek. They usually heal on their own within a week or two.
Your Immune System Attacks the Tissue
Recurrent aphthous stomatitis, the clinical name for the common mouth ulcer that keeps coming back, is driven by an immune response rather than infection. In people who are genetically predisposed, certain triggers set off a cascade of inflammatory signals directed at specific spots on the oral lining. White blood cells, particularly a type called T cells, accumulate beneath the surface layer of tissue. As the immune response intensifies, these cells destroy the tissue above them, creating the characteristic painful, round, white-or-yellow ulcer ringed with red.
This is why mouth ulcers aren’t contagious. Unlike cold sores, which are caused by the herpes simplex virus and typically appear as clusters of blisters on the lips or hard palate, common aphthous ulcers are your own immune system misfiring against your own tissue.
Stress and Cortisol
Psychological stress is one of the most frequently reported triggers, and there’s measurable biology behind it. In one study comparing people with recurrent ulcers to healthy controls, salivary cortisol levels in the ulcer group were more than three times higher than in people without ulcers. Anxiety scores tracked almost perfectly with cortisol levels.
The connection works through your body’s stress-response system. When you’re under sustained stress, elevated cortisol and other stress hormones alter how your immune system operates, increasing inflammatory activity at the oral lining. This is why ulcers often appear during exam periods, job changes, or prolonged emotional strain rather than during the stressful event itself. The immune disruption takes time to produce a visible sore.
Nutritional Deficiencies
Low levels of iron, vitamin B12, and folate are well-established triggers. In a study of 130 patients with recurrent mouth ulcers, nearly 18% had a deficiency in at least one of these nutrients. Iron deficiency was the most common, found in 15 of the 23 deficient patients, followed by folate (7 patients) and B12 (5 patients). Some had more than one deficiency at the same time.
These nutrients play essential roles in maintaining healthy mucosal tissue and supporting normal immune function. When levels drop, the lining of the mouth becomes more fragile and the immune system more prone to the kind of overreaction that produces ulcers. If you get ulcers frequently without an obvious trigger, a blood test checking these levels is a reasonable step.
Foods and Drinks That Trigger Ulcers
Certain foods can irritate the mouth lining enough to start an ulcer or worsen an existing one. Citrus fruits, alcohol, and fizzy drinks are common culprits because they lower the pH inside your mouth, creating an acidic environment. Strawberries and tomatoes are less obvious triggers that catch many people off guard, since they’re also quite acidic. Spicy and salty foods irritate the delicate tissue directly.
If you already have an ulcer, the texture of what you eat matters too. Hard, crunchy, or sharp-edged foods like crusty bread or chips can reinjure the site and delay healing. Softer foods and cooler temperatures are easier on an open sore.
Toothpaste Ingredient: Sodium Lauryl Sulfate
Sodium lauryl sulfate (SLS) is a foaming agent found in many toothpastes, and it can contribute to ulcer formation by stripping away the protective mucus layer inside the mouth. 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 the pain associated with them. If you’re prone to mouth ulcers, checking your toothpaste label is one of the easiest changes you can make.
Hormonal Fluctuations
Some women notice mouth ulcers appearing at predictable points in their menstrual cycle, typically in the days just before or during their period when progesterone levels shift. In rare cases, this pattern reflects a true hypersensitivity to progesterone, a condition most commonly triggered by the menstrual cycle (about 73% of cases), followed by pregnancy (15%) and hormonal therapy (9%). The immune system essentially reacts to the body’s own progesterone as though it were a foreign substance, producing inflammation that can show up as oral ulcers among other symptoms.
Even without a full hypersensitivity reaction, hormonal changes around menstruation and menopause can make the oral lining more vulnerable to ulceration. If you notice a clear cyclical pattern, tracking your ulcers alongside your cycle can help confirm the connection.
Underlying Medical Conditions
Mouth ulcers that are unusually persistent, severe, or frequent can sometimes signal a broader health condition. Several systemic diseases use the mouth as one of their first calling cards.
Inflammatory bowel disease: Both Crohn’s disease and ulcerative colitis can cause oral ulcers, sometimes appearing months before any gut symptoms develop. Crohn’s disease tends to produce deep, linear ulcers with rolled edges in the inner cheeks, while ulcerative colitis more often causes shallow ulcers resembling typical aphthous sores.
Celiac disease: Damage to the intestinal lining from gluten intolerance impairs nutrient absorption, particularly iron, folate, and B12, which circles back to the nutritional deficiency pathway.
Behçet’s disease: This inflammatory condition causes recurrent ulcers that look similar to common aphthous ulcers but tend to be more numerous, longer-lasting, and more painful. Ulcers in the genital area, eye inflammation, and joint pain alongside oral ulcers are key distinguishing signs.
Immune-related conditions: Diseases like pemphigus vulgaris, where the immune system attacks the cells that hold skin layers together, produce blistering that frequently starts in the mouth. Oral lesions are the first sign of disease in about half of pemphigus cases. Cyclic drops in white blood cell counts, a condition called cyclic neutropenia, can also cause recurring painful ulcers that leave scars.
When Ulcers Signal Something Serious
A single mouth ulcer that doesn’t heal within two to three weeks warrants attention. Chronic, non-healing ulcers, particularly those with hardened or rolled borders, can indicate oral squamous cell carcinoma. These tend to appear on the lower lip, the floor of the mouth, or the sides of the tongue. They’re usually painless in early stages, which is part of what makes them easy to overlook. Any ulcer that persists beyond three weeks, grows rather than shrinks, or feels firm around the edges should be examined by a dentist or doctor.
Viral infections also cause mouth ulcers, though these look distinct from the common aphthous type. A primary herpes infection produces multiple small, symmetrical ulcers that can appear across both the gums and the inner cheeks, often accompanied by fever. Herpes reactivations (cold sores appearing inside the mouth) typically show up as clusters of tiny ulcers on the hard palate or gums, usually on just one side.