Canker sores form when your immune system attacks the thin tissue lining your mouth. Unlike cold sores, which are caused by a virus, canker sores aren’t infections. They’re an overreaction by your own body, and anywhere from 5 to 25% of people get them repeatedly. The triggers range from something as simple as biting your cheek to underlying nutritional gaps or chronic stress.
What’s Actually Happening Inside Your Mouth
Canker sores are driven by your immune system’s T-cells, the same white blood cells that normally fight off infections. For reasons that aren’t fully understood, these cells sometimes target healthy tissue on the inside of your lips, cheeks, or tongue. Once activated, they release inflammatory signaling molecules (especially one called TNF-alpha) that break down the protective surface layer of your mouth, leaving behind a painful, shallow crater.
This is why canker sores look the way they do: a white or yellowish center surrounded by a red border. The white area is exposed tissue. The red ring is inflammation. Your mouth is one of the fastest-healing parts of your body, so most of these ulcers resolve on their own within one to two weeks. But if you’re prone to them, new ones can appear before old ones finish healing.
The Most Common Triggers
Most people notice canker sores after one of a few predictable triggers. Physical injury is the most straightforward: biting the inside of your cheek, scraping your gums with a sharp chip, or irritation from braces or dental work. Even vigorous toothbrushing can be enough.
Acidic and irritating foods are another major category. Citrus fruits, tomatoes, strawberries, fizzy drinks, and alcohol all lower the pH inside your mouth, which disrupts the protective proteins coating your oral tissue and leaves it vulnerable to ulceration. Spicy and salty foods irritate the delicate lining directly. Hard, crunchy foods like crusty bread or chips can cause enough mechanical friction to start a sore.
Your toothpaste may also play a role. Sodium lauryl sulfate (SLS), the foaming agent in most toothpastes, has been linked to more frequent outbreaks. A meta-analysis found that people who switched to SLS-free toothpaste developed roughly one fewer ulcer per outbreak cycle, and their sores healed about two days faster. They also experienced fewer total episodes and less pain. If you get canker sores regularly, switching toothpaste is one of the simplest changes you can make.
Stress, Anxiety, and Mood
People often notice canker sores flaring during stressful periods, and the research supports that connection, though not in the way you might expect. A study measuring cortisol (the body’s primary stress hormone) in saliva found no significant difference in cortisol levels between people with active canker sores and healthy controls. The hormone itself doesn’t appear to be the direct trigger.
What the study did find was that anxiety and depression levels were significantly higher in people who get recurrent canker sores. Canker sores 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. The link between psychological distress and outbreaks is real, even if the exact biological pathway isn’t a simple cortisol spike.
Nutritional Deficiencies
Running low on certain vitamins and minerals makes your mouth lining more fragile and slower to repair itself. The nutrients most consistently linked to canker sore outbreaks are iron, vitamin B12, folic acid, and zinc. You don’t necessarily need to be severely deficient. Even borderline-low levels can be enough to tip the balance if you’re already predisposed.
This is worth paying attention to if your diet is restrictive, you follow a plant-based diet (B12 is found almost exclusively in animal products), or you have heavy menstrual periods (a common cause of iron depletion). A basic blood panel can check these levels, and correcting a deficiency often reduces how frequently sores appear.
Hormonal Changes in Women
Some women notice canker sores appearing on a monthly schedule, typically a day or two before their period starts. This timing lines up with the rise in progesterone during the second half of the menstrual cycle. That hormonal shift changes blood flow to oral tissues, increases gum sensitivity, and can trigger canker sore formation. The sores usually clear up shortly after the period begins and hormone levels drop.
Underlying Health Conditions
Frequent or unusually persistent canker sores can sometimes signal a deeper issue. Celiac disease and inflammatory bowel diseases like Crohn’s are the most well-established connections. In an Italian survey of patients with these conditions, about 66% reported oral symptoms during active flares of their intestinal disease, and canker sores were by far the most common complaint, affecting 63% of those with oral involvement.
These sores look identical to ordinary canker sores and can’t be distinguished without a biopsy. But they tend to recur more stubbornly and may not respond well to the usual remedies. If you get canker sores frequently and also have digestive symptoms like chronic bloating, diarrhea, or unexplained weight loss, the mouth sores could be an early clue pointing toward one of these conditions.
Three Types, Different Severity
Not all canker sores behave the same way. Most people get minor aphthous ulcers, which are less than 1 centimeter across and shallow. These are painful but heal without scarring in a week or two.
Major aphthous ulcers are larger and dig deeper into the tissue. They take longer to heal, sometimes weeks, and can leave scars. These are less common but significantly more disruptive.
Herpetiform ulcers (despite the name, they’re not caused by herpes) appear as clusters of many tiny sores that can merge together. They look different from the typical single round ulcer but follow the same immune-driven process.
Why Some People Get Them and Others Don’t
Canker sores peak in the second decade of life, with most people experiencing their first outbreak during childhood or adolescence. If you’ve never had one by your mid-twenties, you’re unlikely to start. There’s a strong genetic component: if both your parents get canker sores, your risk is substantially higher.
The reason some people’s immune systems react this way and others don’t likely comes down to a combination of genetic susceptibility, the specific makeup of oral bacteria, and how reactive your T-cells are to minor tissue damage. Two people can bite their cheek in the same spot, and one develops a canker sore while the other heals without incident. That difference is mostly written into your biology, but the triggers that set off each individual episode are often things you can manage: diet, stress, toothpaste choice, and nutritional status.