Most mouth ulcers are canker sores, and they affect 5 to 25% of the general population at some point. The most common triggers are minor injuries inside the mouth, emotional stress, and certain foods, but the full list of causes runs deeper than that. Understanding what’s behind your ulcers can help you figure out whether they’re routine or worth investigating further.
The Most Common Triggers
Canker sores (the medical term is recurrent aphthous stomatitis) don’t have a single clear cause. Instead, several factors can set off an episode, sometimes in combination. The triggers that show up most often are:
- Minor mouth injuries: Biting your cheek, brushing too hard, a sharp edge on a tooth or braces, or irritation from dental work can all damage the lining of your mouth enough to start an ulcer.
- Stress: Emotional or physical stress is one of the most reliable triggers. College students, for example, commonly develop canker sores during final exams.
- Certain foods: Chocolate, peanuts, and eggs are among the foods most often linked to flare-ups. Acidic fruits like citrus and tomatoes can also irritate the mouth lining and provoke sores in people who are prone to them.
There’s a popular theory that sodium lauryl sulfate (SLS), the foaming agent in most toothpaste, causes mouth ulcers. The evidence for this is weak. A double-blind study found that switching to SLS-free toothpaste didn’t significantly change how often ulcers appeared, how long they lasted, or how much they hurt. A 2019 review concluded there simply isn’t enough data to recommend SLS-free products for ulcer prevention. That said, if you notice your ulcers tend to flare up after switching toothpaste brands, it’s a low-cost experiment to try one without SLS.
What’s Happening Inside Your Mouth
When a canker sore forms, your immune system is attacking the thin tissue lining your mouth. Certain immune cells, particularly a type of white blood cell involved in killing damaged or “foreign-looking” cells, appear to destroy the surface tissue. Once they start, they release signaling molecules that sustain the inflammation, keeping the sore open and painful longer than a simple scratch would last. This is why canker sores hurt so much more than their small size would suggest, and why they take days or weeks to close up rather than hours.
Researchers have found that in people who get frequent canker sores, the immune system may be reacting to proteins from common mouth bacteria that resemble proteins in the body’s own cells. In other words, the immune system gets confused, mistakes the mouth’s lining for something harmful, and attacks it. This helps explain why some people get ulcers repeatedly while others almost never do: the tendency is partly built into how your immune system is wired.
Nutritional Deficiencies
If you’re getting mouth ulcers frequently and can’t pin them on an obvious trigger, a nutritional gap may be involved. Three deficiencies are most closely linked to recurrent oral ulcers:
- Iron
- Vitamin B12
- Folate (vitamin B9)
All three play roles in maintaining healthy mucosal tissue and producing red blood cells. When levels drop low enough to cause anemia, mouth ulcers and a sore, red tongue are among the earliest symptoms. You might also notice fatigue, pale skin, or tingling in your hands and feet. A simple blood test can check for these deficiencies, and correcting them often reduces ulcer frequency significantly.
Medical Conditions That Cause Mouth Ulcers
Sometimes recurring mouth ulcers are a sign of something systemic. Celiac disease is one of the more commonly overlooked causes. People with undiagnosed celiac disease may develop mouth ulcers as one of their only noticeable symptoms, sometimes years before digestive problems appear. Crohn’s disease, another condition affecting the digestive tract, can also produce oral ulcers because the inflammation it causes isn’t limited to the intestines.
Behçet’s syndrome is a rarer condition where ulcers form in the mouth and on the genitals along with eye inflammation. HIV and other conditions that weaken the immune system can make ulcers more frequent and slower to heal. In children, a condition called PFAPA syndrome causes recurring episodes of fever, sore throat, swollen neck glands, and mouth ulcers on a predictable cycle, often every few weeks.
If you’re getting ulcers frequently (more than a few times a year), they’re unusually large, or they come with other symptoms like joint pain, fatigue, digestive trouble, or genital sores, it’s worth looking into whether an underlying condition is driving them.
Medications That Trigger Ulcers
Several common medications can cause mouth ulcers as a side effect. Anti-inflammatory painkillers like aspirin and naproxen can irritate and damage the mouth’s lining, sometimes causing white or yellowish lesions. Certain blood pressure medications, including some in the ACE inhibitor and ARB classes, carry the same risk. Some anti-seizure medications and the gout drug allopurinol can trigger more serious mouth reactions in rare cases, where widespread ulcers develop alongside skin blistering. If you started a new medication recently and mouth ulcers followed, the timing may not be a coincidence.
Canker Sores vs. Cold Sores
People often confuse canker sores with cold sores, but they’re completely different. Cold sores (fever blisters) are caused by the herpes simplex virus and appear on the outside of your mouth, typically around the lips. They look like clusters of small fluid-filled blisters and are contagious. Canker sores appear inside the mouth, on the inner cheeks, lips, or tongue. They’re single round sores, usually white or yellow with a red border, and they are not contagious or caused by a virus.
How Long Mouth Ulcers Take to Heal
Healing time depends on the type of ulcer. Minor canker sores, which are the most common and smaller than a pea, typically heal within a few weeks and don’t leave scars. Major canker sores are larger than one centimeter, extremely painful, and can take months to heal. They often leave scars. A third, rarer type called herpetiform canker sores (despite the name, these are not related to herpes) appear as clusters of tiny pinpoint sores and usually clear up within about two weeks.
When a Mouth Ulcer Might Be Something Else
The vast majority of mouth ulcers are harmless canker sores. But oral cancer can start as a small sore that doesn’t heal, so it’s worth knowing the differences. Canker sores are painful from the start and gradually hurt less as they heal. Early oral cancers are usually painless. Canker sores tend to be flat with red, inflamed edges. Oral cancers often have a small lump or bump beneath the surface that you can feel with your tongue or finger.
The clearest red flag is duration. If a mouth sore hasn’t healed within two to three weeks, that’s reason to get it checked. Other warning signs include a white spot that turns red, a small spot that keeps growing larger, or a lesion that starts bleeding when it didn’t before. Catching oral cancer early makes a dramatic difference in outcomes, so a sore that breaks the normal pattern is always worth a professional look.