Mouth ulcers are usually caused by your immune system overreacting to minor triggers, from physical trauma to stress to nutritional gaps. The small, painful sores that form on the soft tissue inside your mouth (called aphthous ulcers or canker sores) are one of the most common oral complaints, and for most people they heal on their own within two weeks. But if yours keep coming back, there are specific reasons worth investigating.
What Actually Happens Inside Your Mouth
Mouth ulcers aren’t infections. They’re the result of your own immune system turning on the lining of your mouth. The process starts when certain immune cells release an inflammatory signaling molecule called TNF-alpha, which triggers a cascade of white blood cells rushing to a spot on the soft tissue. Those immune cells then destroy the surface cells of the mouth lining, leaving behind the raw, exposed crater you feel with your tongue.
This is why mouth ulcers hurt so much. The tissue that normally protects the nerve endings underneath is gone, leaving them directly exposed to food, saliva, and air. It’s also why acidic or salty foods sting so sharply when they hit the sore.
The Most Common Triggers
For most people, mouth ulcers are set off by one or more everyday triggers rather than a single underlying disease. The most frequent culprits include:
- Minor mouth injuries: Biting the inside of your cheek, brushing too hard, rough dental work, or sharp edges on braces or dentures can all start the immune cascade that leads to an ulcer.
- Stress and poor sleep: Emotional or physical stress is one of the most reliable triggers for recurrent ulcers. The exact link isn’t fully understood, but stress hormones are known to alter immune function in ways that make these flare-ups more likely.
- Hormonal changes: Some women notice ulcers appearing at specific points in their menstrual cycle, suggesting hormonal shifts play a role.
- Certain foods: Acidic fruits (citrus, tomatoes), spicy dishes, and chocolate are frequently reported triggers, though these vary widely from person to person.
Your Toothpaste Could Be a Factor
A foaming agent called sodium lauryl sulfate (SLS), found in most commercial toothpastes, has a measurable connection to recurrent mouth ulcers. 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 experienced. All four measures improved consistently across the studies.
SLS strips away the protective mucus layer inside the mouth, which may leave the tissue more vulnerable to irritation and immune attack. If you get ulcers frequently, switching toothpaste is one of the simplest changes to try first. SLS-free options are widely available in most pharmacies.
Nutritional Deficiencies That Cause Ulcers
Recurrent mouth ulcers can be a sign that your body is short on specific vitamins or minerals. The deficiencies most strongly linked to mouth ulcers are iron, vitamin B12, folic acid, vitamin B3, and vitamin C. Of these, B12 and iron are the most commonly identified in people with chronic ulcers.
In one documented case, a patient’s recurrent ulcers resolved after B12 supplementation. Her blood level was 65 pmol/L, well below the normal range of 116 to 781 pmol/L. The ulcers had persisted for years before the deficiency was identified. This is worth noting because B12 deficiency doesn’t always produce obvious symptoms beyond fatigue, and it’s easily missed on routine exams unless specifically tested.
If your ulcers keep returning without a clear trigger, a simple blood test checking your iron, B12, and folate levels can rule out or confirm a nutritional cause. Vegetarians, vegans, and people with absorption issues are at higher risk for these deficiencies.
Three Types of Mouth Ulcers
Not all mouth ulcers are the same. They fall into three categories, and knowing which type you’re dealing with helps you understand what to expect.
Minor aphthous ulcers are by far the most common. They’re smaller than 10 mm across, usually appear in groups of one to five, and heal within 10 to 14 days without scarring. These are the standard canker sores most people get occasionally.
Major aphthous ulcers are larger than 10 mm, extend deeper into the tissue, and can persist for up to six weeks. They may appear in groups of one to ten. Major ulcers occasionally leave scars, though this is rare. They’re significantly more painful and can make eating and speaking difficult.
Herpetiform ulcers (despite the name, these are not caused by the herpes virus) appear as clusters of 10 to 100 tiny sores that can merge together into larger, irregular ulcers. When ulcers fuse like this, scarring becomes more likely.
When Mouth Ulcers Signal Something Bigger
In some cases, recurrent mouth ulcers are an early sign of a systemic health condition. Crohn’s disease is one of the most notable examples. Oral ulcers, redness, swelling, and pain can appear as the first symptom of Crohn’s, sometimes before any digestive symptoms develop. If you’re experiencing mouth ulcers alongside unexplained digestive issues like chronic diarrhea, abdominal pain, or weight loss, that combination is worth bringing up with a doctor.
Celiac disease is another condition where mouth ulcers can be a presenting feature. In celiac disease, the ulcers are thought to result from nutrient malabsorption (particularly iron and folate) caused by damage to the small intestine. Other conditions associated with oral ulceration include certain autoimmune diseases, inflammatory conditions, and, more rarely, blood disorders.
An ulcer that doesn’t heal within two weeks, or one that fails to improve after one to two weeks of treatment, warrants further evaluation. Clinical guidelines recommend a tissue biopsy in these cases to rule out more serious causes. A single, painless ulcer that persists for weeks is a different clinical picture from the typical recurrent canker sore and should be evaluated promptly.
What Helps Them Heal
Most minor mouth ulcers heal on their own within about two weeks. The goal of treatment is to reduce pain and, if possible, speed up that timeline.
Honey applied directly to the ulcer has surprisingly strong clinical evidence behind it. In a randomized controlled trial comparing honey to a standard prescription steroid paste, the honey group saw ulcer healing in an average of about 2.7 days, compared to nearly 6 days with the steroid and 7 days with a plain protective paste. Honey reduced ulcer size, redness, and pain more effectively than either comparison treatment. Raw, unprocessed honey applied several times a day appears to work best.
Saltwater rinses (half a teaspoon of salt in a cup of warm water) help keep the ulcer clean and can reduce bacterial irritation, though they won’t dramatically speed healing. Over-the-counter protective gels and pastes create a barrier over the ulcer, shielding it from further irritation while you eat and drink. For more severe or persistent ulcers, prescription-strength anti-inflammatory rinses or topical treatments are available.
Reducing How Often They Come Back
If you’re someone who gets ulcers regularly, prevention matters more than treatment. Start by switching to an SLS-free toothpaste. Track whether specific foods seem to precede your outbreaks and reduce those. Address obvious sources of mouth trauma: sharp tooth edges, poorly fitting dental appliances, or aggressive brushing habits.
Get your B12, iron, and folate levels checked, especially if you follow a restricted diet or have any reason to suspect absorption problems. Manage stress where you can, since it’s one of the most consistent triggers for recurrence. Sleep deprivation compounds the problem, so consistent rest matters.
For people who get ulcers despite all of these measures, the pattern itself is useful information. Frequent, severe, or unusually large ulcers that don’t respond to basic management deserve a closer look at potential underlying conditions.