Canker sores form when your immune system mistakenly attacks the cells lining the inside of your mouth. Unlike cold sores, which are caused by a virus and appear on the outside of your lips, canker sores are not contagious and not caused by an infection. They develop inside your mouth, on your cheeks, lips, tongue, or the soft tissue at the base of your gums, and they result from a combination of immune triggers, nutritional gaps, and physical irritation.
What Happens Inside Your Mouth
In people prone to canker sores, the body’s immune cells overreact to something, whether it’s minor tissue damage, a food, or stress. The process starts when white blood cells release a signaling protein called TNF-alpha, which triggers inflammation. That inflammation ramps up even further, eventually directing a specific type of immune cell (called CD8+ T cells) to attack and destroy the surface tissue of your mouth. The result is that painful, shallow crater with a white or yellowish center and a red border.
This is why canker sores aren’t random skin damage. They’re a localized immune event. Your body essentially treats healthy mouth tissue as a threat, breaks it down, and leaves behind an open ulcer that’s exposed to everything you eat, drink, and breathe.
The Most Common Triggers
There’s rarely one single cause. Most outbreaks are the result of several overlapping factors.
Physical injury is one of the most straightforward triggers. Biting your cheek, scraping your gums with a sharp chip, brushing too aggressively, or getting poked by dental work can all create the initial tissue damage that kicks off the immune response. Rough-textured foods like nuts, seeds, and pretzels are common culprits because they create tiny abrasions you might not even notice.
Acidic and irritating foods don’t necessarily cause canker sores on their own, but they can irritate already-vulnerable tissue enough to trigger one. Citrus fruits, tomatoes and tomato-based sauces, spicy foods, coffee, and carbonated drinks are frequently reported triggers. Strawberries, despite their health benefits, contain enough natural acid to bother people who are prone to outbreaks.
Stress is one of the most reliable triggers. When you’re under sustained emotional or physical stress, elevated cortisol weakens your immune defenses and makes your mouth lining more susceptible to breakdown. Some researchers also note that stressed people are more likely to unconsciously bite or chew on their lips and cheeks, combining the physical and immune triggers at once.
Hormonal shifts play a role as well. Women are more likely to develop canker sores during their menstrual cycles, likely due to fluctuations in estrogen and progesterone that influence immune activity.
Nutritional Deficiencies That Fuel Recurrences
If you get canker sores frequently, a nutritional deficiency could be an underlying factor. Studies have found a surprisingly high rate of iron and B vitamin deficiency among people with recurring outbreaks, and correcting those deficiencies has been shown to reduce or eliminate recurrences in most cases.
The nutrients most strongly linked to canker sores include:
- Vitamin B12: Even people who aren’t technically deficient may benefit from supplementation. A double-blind study found that B12 supplements prevented recurrences regardless of whether participants had low levels to begin with.
- Iron: Iron-deficiency anemia is commonly associated with recurring mouth ulcers. This one requires a blood test to confirm before supplementing.
- Zinc: Zinc deficiency has been linked to recurrent canker sores in multiple studies. One preliminary trial found that zinc supplementation reduced recurrences by 50 to 100%, with the greatest benefit in people who were genuinely deficient.
- Thiamine (B1): A deficiency in this specific B vitamin has been independently linked to increased risk.
- Folate: Low folic acid levels are another recognized trigger.
If your canker sores keep coming back, a simple blood panel checking these levels can reveal whether a correctable deficiency is driving the cycle.
Your Toothpaste Might Be Making It Worse
Sodium lauryl sulfate (SLS) is a foaming agent found in most commercial toothpastes, and it’s a well-documented irritant for people prone to canker sores. A meta-analysis of clinical trials found that switching to an SLS-free toothpaste reduced the number of ulcers by about one per cycle, shortened healing time by roughly two days, and significantly decreased pain. The number of episodes also dropped. This is one of the simplest changes you can make if you deal with frequent outbreaks. SLS-free toothpastes are widely available at most drugstores.
Three Types, Three Timelines
Not all canker sores are identical. They fall into three categories that differ in size, severity, and how long they stick around.
Minor canker sores are the most common type. They measure less than one centimeter across (smaller than a pea), heal within a few weeks, and don’t leave scars. These are the ones most people picture when they think of a canker sore.
Major canker sores are larger than one centimeter, extremely painful, and can take months to heal. They often leave scars behind. If you have an ulcer that’s unusually large or hasn’t healed after several weeks, this is likely what you’re dealing with.
Herpetiform canker sores are clusters of tiny, pinpoint-sized ulcers that group together. Despite the name, they have nothing to do with the herpes virus. They typically heal within about two weeks without scarring.
Canker Sores vs. Cold Sores
People frequently 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. Cold sores (fever blisters) appear on the outside of the mouth, around the lips and sometimes on the nose or chin. Cold sores are caused by the herpes simplex virus (usually HSV-1) and are highly contagious. Canker sores have no viral cause and cannot be spread to another person through kissing, sharing utensils, or any other contact.
When Canker Sores Signal Something Bigger
Occasional canker sores are extremely common and usually harmless. But frequent or severe outbreaks can sometimes point to an underlying condition. Celiac disease, an autoimmune reaction to gluten, is one of the most recognized associations. People with undiagnosed celiac disease often develop recurrent mouth ulcers as one of their earliest symptoms, sometimes before any digestive issues appear.
Crohn’s disease and other inflammatory bowel conditions can also cause oral ulcers as part of their broader pattern of inflammation. Behçet’s disease, a rarer condition involving blood vessel inflammation throughout the body, lists recurrent mouth ulcers as one of its hallmark symptoms. Immune system disorders that affect white blood cell counts or function can similarly lead to persistent outbreaks.
A single canker sore after biting your cheek is nothing to worry about. But if you’re getting large or numerous ulcers several times a year, especially alongside other symptoms like fatigue, digestive problems, joint pain, or skin lesions, the sores may be a signal worth investigating rather than just a nuisance to manage.