Mouth ulcers are usually caused by a combination of factors rather than one single thing, and the exact mechanism still isn’t fully understood. The most common triggers include minor injuries inside the mouth, stress, certain foods, and nutritional gaps. Most ulcers are harmless and heal on their own within 10 to 14 days, but frequent or long-lasting ones can signal something deeper going on.
The Most Common Triggers
The everyday mouth ulcer, known medically as an aphthous ulcer or canker sore, tends to show up after one or more of these triggers:
- Physical injury: Biting the inside of your cheek, brushing too aggressively, a sharp edge on a tooth, or irritation from braces or dental work. Braces are a particularly common culprit because the metal brackets rub against the inner lips and cheeks, creating small abrasions that turn into sores.
- Certain foods: Chocolate, coffee, strawberries, eggs, nuts, cheese, and anything very spicy or acidic. These foods don’t cause ulcers on their own, but they can trigger an outbreak in people who are sensitive to them, and they’ll make an existing ulcer hurt more.
- Toothpaste ingredients: Products containing sodium lauryl sulfate, a foaming agent found in many toothpastes and mouthwashes, are linked to more frequent outbreaks. Switching to an SLS-free toothpaste is one of the simplest things you can try.
- Stress: Psychological stress triggers your body to release cortisol and activates parts of the immune system in ways that can initiate or worsen ulcers. Stress also shifts the balance of immune cells like T cells and natural killer cells, which may explain why ulcers tend to cluster during high-pressure periods like exams or work deadlines.
- Hormonal changes: Some people notice ulcers flaring around menstruation, suggesting that hormonal shifts play a role in outbreaks.
Nutritional Deficiencies That Play a Role
If you’re getting mouth ulcers repeatedly, your diet may be part of the picture. Deficiencies in vitamin B12, iron, zinc, and folate are all associated with recurrent outbreaks. A B12 or folate deficiency severe enough to cause anemia often shows up as a sore, red tongue alongside mouth ulcers. You don’t necessarily need to be severely deficient for it to matter. Even borderline-low levels of these nutrients can leave the lining of your mouth more vulnerable to breaking down.
This is worth paying attention to if you follow a restricted diet, whether vegetarian, vegan, or simply limited in variety. A blood test from your doctor can check these levels and give you a clear answer about whether supplementation would help.
Genetics and Family History
Some people are simply more prone to mouth ulcers than others, and genetics is a significant reason why. A positive family history is found in up to 40% of people with recurrent aphthous ulcers. If your parents dealt with frequent mouth ulcers, you’re considerably more likely to as well. This genetic predisposition likely affects how your immune system responds to minor irritation inside the mouth, essentially lowering the threshold for an ulcer to form.
When Ulcers Signal a Bigger Health Issue
In most cases, mouth ulcers are just a nuisance. But recurrent or unusually severe ulcers can be an early sign of a systemic condition, sometimes appearing before other symptoms do.
Celiac disease, an autoimmune reaction to gluten, is one of the more common underlying causes. People with undiagnosed celiac disease often get mouth ulcers as one of their first noticeable symptoms, sometimes well before any digestive problems appear. Inflammatory bowel diseases like Crohn’s disease can also produce oral ulcers. The reported prevalence of mouth lesions in Crohn’s ranges from 0.5 to 20 percent of patients, and these ulcers can show up before any abdominal symptoms develop.
Behçet’s disease, a rare condition that causes widespread inflammation, frequently starts with painful mouth ulcers, often numerous and concentrated on the soft palate. Oral lesions are the most common feature of this disease and may be its first visible sign. Lupus produces mouth ulcers in roughly 8 to 45 percent of people with the systemic form. Pemphigus vulgaris, another autoimmune condition, begins with painful oral ulceration in 50 to 80 percent of cases, sometimes a year or more before skin symptoms appear.
An immune system that isn’t functioning normally, whether from HIV/AIDS or other causes, also raises the risk of persistent or severe ulcers.
Mouth Ulcers vs. Cold Sores
These two get confused constantly, but they’re different conditions. The easiest way to tell them apart is location. Mouth ulcers (canker sores) occur inside your mouth, on the inner cheeks, lips, or tongue. Cold sores (fever blisters) appear outside the mouth, typically around the border of the lips. They also look different: cold sores are clusters of small fluid-filled blisters, while canker sores are usually single, round, white or yellow sores with a red border.
The other key difference is that cold sores are caused by the herpes simplex virus and are highly contagious. Canker sores are not contagious at all, and they aren’t caused by a virus.
How Long They Last
A typical minor mouth ulcer resolves on its own within 10 to 14 days without treatment. During that window, it will generally hurt most in the first few days, then gradually become less painful as it heals. Ulcers that haven’t cleared up within two weeks deserve attention from a dentist or doctor, especially if you use tobacco or drink alcohol regularly, since a non-healing ulcer in that context needs to be evaluated for oral cancer.
Frequent recurrence is also worth investigating. If you’re getting ulcers every few weeks or multiple times a month, that pattern suggests an underlying trigger, whether nutritional, immune-related, or tied to a systemic condition, that’s worth identifying rather than just waiting out each episode.
Reducing Outbreaks
Since mouth ulcers result from overlapping triggers, reducing them usually means addressing several factors at once. Switch to an SLS-free toothpaste and use a soft-bristled brush. Pay attention to whether specific foods seem to precede your outbreaks and cut back on them for a trial period. If you wear braces, orthodontic wax applied over the brackets can reduce the friction that causes sores.
On the nutritional side, make sure your diet includes adequate sources of B12, iron, folate, and zinc. If you suspect a deficiency, a blood test can confirm it and guide whether you need supplements. Managing stress through whatever works for you, whether exercise, sleep, or simply reducing your commitments, can also lower the frequency of outbreaks. None of these steps guarantees you’ll never get another ulcer, but together they meaningfully reduce how often they show up and how severe they are.