Mouth ulcers are usually caused by minor tissue damage, stress, or an overactive immune response, not by infection. Most are what doctors call aphthous ulcers (canker sores), small painful sores on the soft tissues inside your mouth that heal on their own within one to three weeks. But the list of potential triggers is surprisingly long, ranging from the toothpaste you use every morning to underlying medical conditions you may not know you have.
How the Immune System Creates an Ulcer
Mouth ulcers aren’t simply wounds. They form when your immune system attacks the lining of your own mouth. Certain white blood cells become destructive to the oral tissue, and the process is sustained by a cascade of inflammatory signaling molecules. People who get recurrent mouth ulcers show higher levels of several inflammatory markers in their blood and an abnormal immune response that leans heavily toward inflammation. In other words, the ulcer isn’t caused by a single event. It’s a localized immune overreaction, which is why the same person tends to get ulcers again and again while others rarely do.
Physical Damage to Mouth Tissue
The most straightforward trigger is mechanical trauma. Biting the inside of your cheek, burning your mouth on hot food, or scraping your gums with a sharp chip can all start the process. Orthodontic braces are a common culprit: brackets and wires create friction against the lips, cheeks, and tongue, irritating the soft tissue enough to produce ulcers. Most heal within 7 to 10 days, but constant friction from braces can cause new ones to form before the old ones disappear.
Other physical triggers include jagged or broken teeth, poorly fitting dentures, and aggressive toothbrushing. If you notice ulcers repeatedly forming in the same spot, a sharp edge in your mouth is a likely cause.
Toothpaste Ingredients
Sodium lauryl sulfate (SLS), the foaming agent in most toothpastes, can irritate the mouth lining in sensitive people. SLS breaks down proteins in the outer layer of oral tissue, causing the protective surface to separate and peel. This leaves the tissue underneath exposed and vulnerable to ulcer formation. If you get frequent mouth ulcers, switching to an SLS-free toothpaste is one of the simplest changes you can make. Several brands market themselves specifically as SLS-free.
Stress, Anxiety, and Depression
Psychological stress has a real, measurable connection to mouth ulcers, though the relationship is more nuanced than “stress causes ulcers.” People with recurrent mouth ulcers consistently score higher on measures of anxiety and depression than people who don’t get them. One study found that ulcers were about six times more common in people with anxiety (12%) compared to controls (2.2%), and roughly twice as common in people with depression.
Interestingly, the link doesn’t appear to run through cortisol, the body’s primary stress hormone. Salivary cortisol levels in ulcer patients aren’t significantly different from those in healthy controls. The mechanism likely involves other stress-related changes in immune function that haven’t been fully mapped out yet. Regardless of the exact pathway, managing stress and mental health can reduce ulcer frequency for many people.
Nutritional Deficiencies
Your mouth lining turns over rapidly, meaning it constantly needs raw materials to rebuild. Shortfalls in iron, vitamin B12, folate, and zinc are all linked to recurrent mouth ulcers. These deficiencies impair the normal replacement of oral tissue and can also disrupt immune regulation. If you’re getting ulcers frequently without an obvious mechanical cause, a blood test checking these levels is a reasonable step. Correcting a deficiency often reduces or eliminates episodes entirely.
Food Sensitivities and Acidic Foods
Certain foods trigger ulcers in susceptible people. Citrus fruits, tomatoes, strawberries, and pineapple are common offenders, likely because their acidity irritates the mucosal lining. Chocolate, coffee, cheese, nuts, and spicy foods also appear on the list of frequent triggers, though the specific culprits vary from person to person. This isn’t a true food allergy in most cases. It’s a local tissue reaction. Keeping a food diary when ulcers flare up can help you identify your personal triggers.
Medications That Cause Oral Ulcers
Several types of medication can cause mouth ulcers as a side effect. Anti-inflammatory painkillers (NSAIDs), beta-blockers, and certain chemotherapy drugs are well-known triggers. One particularly notable example is nicorandil, a medication used for chest pain, which can cause serious ulceration of the mouth, throat, and digestive tract. In one reported case, a patient developed ulcers of the mouth and pharynx after less than a week on the drug. If mouth ulcers appear shortly after starting a new medication, that timing is worth mentioning to your prescriber.
Hormonal Changes
Many women notice mouth ulcers flaring in a pattern tied to their menstrual cycle, typically in the days just before a period. This suggests that fluctuations in estrogen and progesterone play a role in oral tissue vulnerability. Some women report improvement during pregnancy, when hormone levels stabilize at higher levels, and a return of ulcers after menopause. Hormonal contraceptives can sometimes reduce ulcer frequency for these individuals.
Underlying Medical Conditions
Recurrent or unusually severe mouth ulcers can be a sign of a systemic condition. Celiac disease, an autoimmune reaction to gluten, frequently causes mouth ulcers as one of its earliest or only symptoms. Some people with celiac disease have no digestive complaints at all, just persistent oral ulcers and possibly iron deficiency.
Crohn’s disease and other inflammatory bowel conditions can also produce mouth ulcers, sometimes before any gut symptoms appear. The ulcers in Crohn’s tend to be deeper and more irregular than typical canker sores.
Behcet’s disease is a rarer condition where painful mouth sores are the most common symptom. These begin as raised, round lesions that quickly become ulcers, typically healing in one to three weeks but frequently returning. Behcet’s also causes genital ulcers, eye inflammation, and skin problems, so mouth ulcers combined with any of those symptoms warrant further investigation.
Canker Sores vs. Cold Sores
Not every mouth sore is a canker sore, and confusing the two matters because cold sores are caused by herpes simplex virus and are contagious. The differences are reliable if you know what to look for.
- Location: Canker sores appear on soft, non-keratinized tissue like the inner cheeks, lips, floor of the mouth, and sides of the tongue. Cold sores favor harder tissue: the outer lip border, hard palate, and gums attached directly to bone.
- Appearance: A canker sore is typically a single oval ulcer with a white-to-yellow center and a smooth red border. A cold sore starts as a cluster of tiny blisters, each about 1 mm across, that burst and merge into a larger sore with a scalloped, irregular edge.
- Contagion: Canker sores are not contagious. Cold sores are highly contagious, especially when blisters are present.
When Ulcers Signal Something More Serious
A single mouth ulcer that heals within two to three weeks is almost always harmless. But an ulcer that lasts longer than three weeks without healing, keeps growing, or is painless deserves prompt evaluation. Oral cancers can initially look like a persistent ulcer, and early detection dramatically improves outcomes. Ulcers that come with unexplained weight loss, fever, or new ulcers appearing on the genitals or skin also point toward systemic conditions that need a proper workup.