Most mouth ulcers are canker sores, small round sores that form on the soft tissues inside your mouth. They’re not contagious, and in most cases they heal on their own within one to two weeks. But if you keep getting them, something is usually driving the cycle, whether it’s a nutritional gap, a reaction to your toothpaste, hormonal shifts, or chronic stress. Understanding your specific triggers is the key to getting fewer of them.
How Mouth Ulcers Form
A canker sore develops when your immune system damages a small patch of the tissue lining your mouth. Certain immune cells attack the surface layer of tissue, and the process is fueled by inflammatory signaling molecules that sustain the damage and prevent quick healing. The result is that familiar shallow crater: a white or yellow center surrounded by a red, inflamed border.
People who get frequent canker sores appear to have an overactive immune response in their oral tissue. Their immune systems react more aggressively to otherwise minor irritants, like a small bite wound or a bacterial protein already present in the mouth. This is why the same cheek bite that heals uneventfully for one person turns into a full ulcer for someone else.
The Most Common Triggers
Local trauma is one of the most reliable triggers. Biting the inside of your cheek, scraping your gums with a chip, getting poked by a toothbrush bristle, or irritation from braces can all set off an ulcer in someone who’s prone to them. The injury itself is minor, but it’s enough to kick off that exaggerated immune response.
Beyond physical damage, several other factors are well established:
- Stress. Emotional or physical stress increases the likelihood of a flare. Many people notice ulcers during exams, work deadlines, or periods of poor sleep.
- Food sensitivities. Acidic fruits, spicy foods, and cinnamon oil are common culprits. These don’t cause ulcers on their own but can irritate tissue enough to trigger one.
- Hormonal changes. Some women develop ulcers in the days just before their period, when hormone levels shift. This pattern, sometimes called menstruation gingivitis, can include swollen gums and canker sores on the inner cheeks.
- Certain medications. Some drugs can provoke oral ulcers as a side effect. If your ulcers started or worsened after beginning a new medication, that’s worth flagging with your prescriber.
Your Toothpaste Could Be Part of the Problem
Sodium lauryl sulfate (SLS) is a foaming agent found in most mainstream toothpastes. It can strip away the delicate protective layers inside your mouth, and for people prone to canker sores, it measurably increases how often ulcers come back. One clinical study found four times fewer soft tissue lesions after using an SLS-free toothpaste compared to one containing SLS.
If you get frequent ulcers, switching to an SLS-free toothpaste is one of the simplest changes you can make. Several brands market themselves as SLS-free, and the ingredient list on the back of the tube will confirm it. Many people notice a real reduction in flare-ups within a few weeks of switching.
Nutritional Deficiencies That Cause Ulcers
Recurring mouth ulcers are a recognized symptom of deficiencies in iron, vitamin B12, and folate. These nutrients play essential roles in maintaining the tissue that lines your mouth. When levels drop low enough, that tissue becomes fragile and more vulnerable to breakdown.
B12 and folate deficiency can also cause a sore, red tongue alongside the ulcers, sometimes with fatigue, weakness, or a pins-and-needles sensation in the hands and feet. If your ulcers are frequent and you also feel unusually tired or have a pale complexion, a simple blood test can check these levels. Correcting the deficiency, through diet changes or supplements, often reduces or eliminates the ulcers entirely.
Medical Conditions Linked to Mouth Ulcers
For most people, canker sores are a nuisance with no deeper cause. But persistent or unusually severe mouth ulcers can be an early sign of several systemic conditions, sometimes appearing before other symptoms develop.
Crohn’s disease can cause oral lesions that actually precede any abdominal symptoms. These may appear as deep linear ulcers or a cobblestone texture on the inner cheeks. Celiac disease, an autoimmune reaction to gluten, is another condition where recurrent mouth ulcers can be an early clue.
Behçet syndrome produces painful ulcers that tend to be numerous and favor the soft palate and back of the throat. Oral ulcers are the most common feature of this condition and are often its first sign. Lupus can cause irregularly shaped ulcers or areas of redness and tissue breakdown surrounded by white, radiating streaks. Pemphigus vulgaris, an autoimmune blistering disease, begins with painful oral ulcers in 50 to 80 percent of cases, sometimes a full year or more before any skin involvement appears.
None of this means your canker sores indicate a serious illness. The vast majority don’t. But if your ulcers are unusually large, heal very slowly, come in clusters, or appear alongside other symptoms like joint pain, digestive issues, skin rashes, or unexplained fatigue, those patterns are worth investigating.
Canker Sores vs. Cold Sores
These are completely different conditions, though people often confuse them. Canker sores form inside the mouth, on the cheeks, lips, tongue, or soft palate. They appear as single round sores with a white or yellow center. They are not caused by a virus and are not contagious.
Cold sores (fever blisters) form outside the mouth, typically on or around the border of the lips. They appear as clusters of small fluid-filled blisters rather than a single sore, and they’re caused by herpes simplex virus type 1. Cold sores are very contagious, especially when blisters are present. If your sore is inside your mouth and looks like a single shallow crater, it’s almost certainly a canker sore.
When an Ulcer Needs Attention
A standard canker sore heals within one to two weeks without treatment. If a mouth ulcer persists beyond two weeks without showing clear improvement, current guidelines from the American Dental Association recommend that it be biopsied or evaluated by a specialist within 10 to 14 days of that point. A non-healing ulcer can occasionally indicate oral cancer or another condition that requires a tissue sample to diagnose.
Size also matters. Minor canker sores, the most common type, are small and heal without scarring. Major canker sores are larger, deeper, and can take weeks to resolve, sometimes leaving a scar. A third type, called herpetiform ulcers, appear as clusters of many tiny sores that can merge together. Despite the name, these are not caused by the herpes virus.
How to Reduce Flare-Ups
Managing mouth ulcers is mostly about identifying and avoiding your personal triggers. Start with the most actionable changes: switch to an SLS-free toothpaste, reduce acidic and spicy foods during active flares, and use a soft-bristled toothbrush to minimize tissue trauma. If stress is a clear trigger for you, that’s useful information even if it’s harder to act on immediately.
For pain relief during an active ulcer, over-the-counter oral gels that contain a numbing agent can make eating and drinking more comfortable. Rinsing with warm salt water several times a day helps keep the area clean and can speed healing slightly. Avoid mouthwashes containing alcohol, which can sting and further irritate the sore.
If your ulcers are frequent or severe, a prescription topical steroid paste applied directly to the sore can reduce inflammation and shorten healing time. These are typically used once or twice a day during a flare. For people who get ulcers repeatedly despite trigger avoidance, checking blood levels of B12, folate, and iron is a practical next step that can reveal a correctable cause.