Most mouth ulcers, commonly called canker sores, result from a combination of immune system quirks, minor injuries to the tissue inside your mouth, stress, and dietary triggers. They’re not contagious and they’re rarely dangerous, but they can be surprisingly painful and frustratingly repetitive. Understanding what’s behind yours is the first step to getting fewer of them.
The Most Common Triggers
The exact cause of canker sores isn’t fully understood, but the pattern is consistent: something irritates or disrupts the delicate lining inside your mouth, and your immune system overreacts. The result is a round, white or yellow sore with a red border, usually on the inside of your cheeks, lips, or tongue.
The triggers that set off an episode tend to fall into a few predictable categories:
- Physical injury: Biting your cheek, brushing too hard, sharp edges on braces or dental work, or even crunchy food scraping the inside of your mouth can all kick-start an ulcer.
- Stress: Emotional stress is one of the most reliable triggers. College students, for example, commonly develop canker sores during finals week.
- Certain foods: Chocolate, peanuts, eggs, and highly acidic foods like citrus or tomatoes are frequent offenders. These don’t cause ulcers in everyone, but if you’re prone to them, these foods can make outbreaks worse.
- Hormonal shifts: Some people notice ulcers appearing at specific points in their menstrual cycle.
- Genetics: If your parents got canker sores regularly, you’re more likely to as well. There’s a clear hereditary component.
If you’re getting ulcers repeatedly but they heal within two to three weeks each time, you likely have what’s called recurrent aphthous stomatitis. It’s the most common type of oral ulceration and, while annoying, it’s generally harmless.
Your Toothpaste Might Be Part of the Problem
One overlooked trigger is sodium lauryl sulfate (SLS), a foaming agent found in most mainstream toothpastes. SLS can irritate the mucous membrane inside your mouth, potentially worsening existing ulcers or even triggering new ones in people who are sensitive to it. If you’re getting ulcers frequently and can’t identify another cause, switching to an SLS-free toothpaste is a simple, low-risk experiment worth trying. Several brands now market SLS-free options specifically for people prone to canker sores.
Nutrient Deficiencies That Cause Ulcers
Recurring mouth ulcers are sometimes your body’s way of signaling a nutritional gap. The nutrients most closely linked to oral ulcers are iron, vitamin B12, and folate. When you’re deficient in any of these, the rapidly dividing cells that line your mouth don’t regenerate properly, leaving the tissue more fragile and prone to breaking down into sores. You might also notice a sore or unusually red tongue alongside the ulcers.
These deficiencies are especially common in people with restrictive diets, heavy menstrual periods, or conditions that impair nutrient absorption (more on that below). A simple blood test can check your levels, and correcting the deficiency often reduces or eliminates the ulcers entirely.
When Mouth Ulcers Signal Something Deeper
For most people, canker sores are a standalone nuisance. But persistent or unusually severe mouth ulcers can be a symptom of an underlying condition that affects the whole body.
Celiac disease is one of the more common culprits. When someone with undiagnosed celiac disease eats gluten, the immune reaction damages the lining of the small intestine, impairing absorption of iron, B12, and folate. Mouth ulcers may be the most visible symptom, sometimes appearing before any digestive complaints do.
Crohn’s disease can also cause oral ulcers. Somewhere between 0.5 and 20 percent of people with Crohn’s develop lesions in their mouth, ranging from shallow sores to deep, linear ulcers along the inside of the cheeks. These ulcers tend to look different from typical canker sores: the surrounding tissue may appear swollen or cobblestoned, and the ulcers themselves can have raised, thickened edges.
Other conditions linked to mouth ulceration include lupus, Behçet’s disease (which causes ulcers in the mouth and genitals along with eye inflammation), and certain autoimmune blistering disorders where the immune system attacks the mucous membranes. These are less common but worth considering if your ulcers are large, frequent, slow to heal, or accompanied by symptoms elsewhere in your body.
Canker Sores vs. Cold Sores
People often confuse these two, but they’re completely different conditions. Canker sores appear inside the mouth, on soft tissue like the inner cheeks, lips, and tongue. They’re single round sores, white or yellow with a red border, and they are not contagious. No virus causes them.
Cold sores (fever blisters) appear outside the mouth, typically around the border of the lips. They look like clusters of small fluid-filled blisters, and they’re caused by herpes simplex virus type 1. Cold sores are highly contagious, especially when the blisters are open.
The location is the quickest way to tell them apart: inside the mouth means canker sore, outside the mouth means cold sore. Treatment is different for each, so the distinction matters.
What Helps Them Heal
Most canker sores heal on their own within two to three weeks without any treatment. But you don’t have to just wait it out in pain.
A simple salt water rinse acts as a natural disinfectant and can reduce swelling. Dissolve about half a teaspoon of salt in a cup of warm water, swish gently for 30 seconds, and spit. A baking soda rinse works similarly: dissolve one teaspoon of baking soda in half a cup of warm water. Either option can be used several times a day and tends to take the edge off the pain.
For sores that are especially painful or large, over-the-counter oral gels that coat the ulcer and protect exposed nerve endings can provide temporary relief. These bioadhesive products form a barrier over the sore so eating and drinking hurt less.
When ulcers are severe or keep coming back, prescription topical treatments that reduce inflammation can shorten each episode and ease pain, though they don’t prevent future outbreaks. Medicated mouth rinses can also reduce the severity and pain of each episode. For the small number of people with truly debilitating, frequent ulcers, stronger systemic medications exist, but they come with significant side effects and are reserved for cases where nothing else works.
Signs That Need Attention
A typical canker sore, even a painful one, will heal within two to three weeks. If yours doesn’t, that timeline alone is reason enough to get it checked. MD Anderson Cancer Center specifically notes a few warning signs that distinguish a harmless sore from something more concerning: a small spot that keeps growing larger, a white patch that turns red, a lesion that starts bleeding when it previously didn’t, or a sore with a tiny lump or bump underneath that you can feel with your tongue or finger. Oral cancers often present with that telltale firm bump beneath what looks like an ordinary ulcer on the surface.
You should also pay attention if your ulcers are unusually large (bigger than a centimeter across), appear in clusters, come with a fever, or make it difficult to eat or drink enough to stay hydrated. Any of these patterns warrants a closer look from a dentist or doctor who can examine the tissue directly and, if needed, take a small sample to rule out something more serious.