Canker sores don’t have a single cause. They result from a combination of triggers, including physical injury to the mouth, nutritional gaps, stress, and genetic susceptibility. Unlike cold sores, canker sores are not caused by a virus and are not contagious. About 8 to 23 percent of the population gets them repeatedly, and understanding your personal triggers is the best way to reduce how often they show up.
Physical Injury to the Mouth
The most straightforward trigger is mechanical trauma. Biting the inside of your cheek, brushing too aggressively, eating a sharp chip that scrapes your gums, or irritation from braces or ill-fitting dentures can all damage the soft tissue inside your mouth. In people who are already prone to canker sores, even minor injuries like these are enough to set one off. The sore typically appears a day or two after the injury, right at the spot where the tissue was damaged.
Certain Foods
Spicy, acidic, and salty foods are well-established triggers. Citrus fruits, tomatoes, pineapple, vinegar-based sauces, and heavily salted snacks can irritate the delicate lining of your mouth or aggravate a sore that’s already forming. These foods don’t cause canker sores in everyone, but if you notice a pattern after eating them, that connection is real and worth paying attention to.
Toothpaste Ingredients
A foaming agent called sodium lauryl sulfate (SLS), found in most commercial toothpastes, strips away the protective mucus layer inside your mouth. Without that barrier, the underlying tissue is exposed to irritants and more vulnerable to ulceration. SLS also breaks down proteins in the mouth’s lining, which can further weaken it.
A meta-analysis of four randomized controlled trials found that people with recurring canker sores who switched to SLS-free toothpaste experienced shorter healing times and less pain. The number of ulcers didn’t always drop dramatically, but the sores they did get were milder. SLS-free toothpastes are widely available and worth trying if you get canker sores frequently.
Vitamin and Mineral Deficiencies
Low levels of vitamin B12, folate, and iron are consistently linked to recurring canker sores. One study comparing people with frequent canker sores to a national health survey sample found that those with sores consumed about 7 percent less of the recommended daily intake of B12 and 20 percent less folate than the general population. These aren’t dramatic shortfalls, but they appear to be enough to make a difference in people who are susceptible.
Correcting these deficiencies through diet or supplements often leads to improvement. Foods rich in B12 include meat, fish, eggs, and dairy. Folate is found in leafy greens, beans, and fortified grains. Iron comes from red meat, lentils, and spinach. If your canker sores are persistent and you suspect a nutritional gap, a simple blood test can confirm whether a deficiency is playing a role.
Stress and Fatigue
Stress is one of the most commonly reported triggers, and there’s a biological explanation for why. When you’re under stress, your body ramps up production of stress hormones that alter how your immune system behaves. Specifically, these hormones increase the concentration of immune cells at sites of inflammation in the mouth, which can intensify the tissue breakdown that creates an ulcer. Fatigue, poor sleep, and overwork compound this effect. Many people notice canker sores flaring during exam periods, major deadlines, or stretches of poor rest.
Genetics and Family History
If your parents get canker sores, you’re significantly more likely to get them too. A hereditary predisposition is one of the strongest risk factors. This doesn’t mean a specific “canker sore gene” has been identified, but the tendency clearly runs in families and likely involves inherited differences in how the immune system responds to minor oral injuries and irritants.
Underlying Health Conditions
Frequent or unusually severe canker sores can sometimes signal a systemic condition. Celiac disease, inflammatory bowel diseases like Crohn’s and ulcerative colitis, and Behçet’s disease all feature recurrent oral ulcers as a symptom. In these cases, the canker sores are driven by broader immune dysfunction rather than local triggers. If you’re getting large or persistent sores alongside other symptoms like digestive problems, joint pain, or skin lesions, the sores may be pointing to something deeper.
Three Types of Canker Sores
Not all canker sores are the same. The type you get affects how long it lasts and how much it hurts.
- Minor canker sores are the most common. They’re typically 2 to 5 millimeters across, round or oval with a white or yellow center and red border, and heal on their own within 4 to 14 days without scarring.
- Major canker sores are 1 to 3 centimeters in diameter, deeper, and more painful. They can take anywhere from 10 days to 6 weeks to heal and sometimes leave scars.
- Herpetiform canker sores appear as clusters of very small ulcers (1 to 2 millimeters each) that can merge together. Despite the name, they have nothing to do with the herpes virus. They typically resolve in 7 to 10 days.
Canker Sores vs. Cold Sores
These two are often confused, but they’re entirely different conditions. Canker sores form inside the mouth, on the soft tissue of the cheeks, gums, tongue, or soft palate. Cold sores (fever blisters) appear outside the mouth, typically around the border of the lips. Canker sores look like single round white or yellow sores with a red edge. Cold sores are clusters of small fluid-filled blisters. The most important distinction: cold sores are caused by herpes simplex virus type 1 and are contagious. Canker sores are not viral and cannot spread from person to person.