Canker sores are caused by an overactive immune response that destroys small patches of the tissue lining your mouth. Unlike cold sores, they aren’t caused by a virus and aren’t contagious. The exact trigger varies from person to person, but genetics, stress, hormonal shifts, nutritional deficiencies, and certain foods all play a role.
Your Immune System Attacks Mouth Tissue
At the cellular level, canker sores form when specific immune cells turn against the soft lining of your mouth. Certain white blood cells, particularly a type called T cells, begin destroying the oral tissue while releasing inflammatory signaling molecules that keep the process going. People with active canker sores have a higher proportion of these aggressive T cells compared to people without them.
Researchers have also found that people prone to canker sores show an abnormal inflammatory response through one of the body’s key threat-detection pathways. This leads to a disproportionate immune reaction to triggers that wouldn’t bother most people. Blood tests consistently show higher levels of inflammatory markers in canker sore patients, and the ratio of certain white blood cells is measurably elevated, all pointing to an immune system that overreacts to minor irritation.
Genetics Play a Major Role
If both of your parents get canker sores, you have about a 90% chance of getting them too. If neither parent is affected, that drops to around 20%. This strong hereditary pattern is one of the clearest risk factors. It likely explains why some people get canker sores repeatedly throughout their lives while others never get a single one, even when exposed to the same dietary and environmental triggers.
Common Triggers
Even with an underlying genetic or immune predisposition, canker sores usually need a trigger to appear. These are the most well-established ones:
- Mouth injuries: Biting your cheek, aggressive tooth brushing, dental work, or sharp food like chips can break the mucosal barrier and set off the immune cascade.
- Stress: Both emotional stress and physical stress on the body (illness, sleep deprivation, exhaustion) are reliably linked to outbreaks.
- Certain foods: Citrus fruits, tomatoes, chocolate, coffee, and spicy foods are frequent culprits. Acidic foods irritate the lining directly, while others may provoke an immune response in susceptible people.
- Nutritional deficiencies: Low levels of iron, zinc, folate, or vitamin B12 are commonly found in people with recurrent canker sores. Correcting these deficiencies often reduces how frequently sores appear.
- Hormonal changes: Some women notice canker sores in the days before their period. The American Dental Association notes that hormonal fluctuations during the menstrual cycle can trigger canker sores, along with gum swelling and other oral changes, though most women don’t experience this.
- Toothpaste ingredients: Sodium lauryl sulfate (SLS), a foaming agent in many major toothpaste brands, is a known soft tissue irritant. It’s the same compound used in shampoos and household cleaners. Switching to an SLS-free toothpaste is one of the simplest changes people make to reduce outbreaks.
Linked Health Conditions
For most people, canker sores are a nuisance with no deeper medical meaning. But frequent or unusually severe sores can sometimes signal an underlying condition. Celiac disease is one of the most common associations. People with undiagnosed celiac disease often get recurrent mouth ulcers as one of their first symptoms, likely because the condition impairs nutrient absorption (particularly iron and B vitamins) and drives widespread immune dysfunction. Canker sores sometimes resolve entirely once gluten is removed from the diet.
Inflammatory bowel diseases like Crohn’s disease and ulcerative colitis are also associated with recurrent canker sores. The inflammation that affects the digestive tract in these conditions can extend to the mouth. Behçet’s disease, a rarer condition involving blood vessel inflammation throughout the body, features recurrent oral ulcers as one of its hallmark symptoms. HIV and other immune-suppressing conditions can also cause frequent, severe outbreaks.
Three Types, Different Severity
Not all canker sores are the same. Minor canker sores are by far the most common. They’re small (under 1 cm), round or oval with a grayish-white center and a red border, and they show up on the inner lips, cheeks, or underside of the tongue. They heal on their own within 7 to 10 days without scarring.
Major canker sores are larger than 1 cm, can persist for more than 4 weeks, and often leave scars. They’re significantly more painful and can make eating and speaking difficult for weeks. Herpetiform canker sores are a third type: clusters of very small ulcers that merge into larger, irregularly shaped sores. Despite the name, they have nothing to do with herpes. They typically heal within 7 to 14 days without scarring.
What You Can Do About Them
Because the underlying cause is immune-driven, there’s no way to cure canker sores permanently. But you can reduce how often they appear and how much they hurt. Switching to an SLS-free toothpaste is a good starting point. Avoiding your personal food triggers (keep a log if you’re not sure which foods set you off) and managing stress both help over time.
If you suspect a nutritional deficiency, a blood test for iron, B12, folate, and zinc can identify gaps worth correcting through diet or supplements. Over-the-counter topical gels and mouth rinses can numb pain and protect the sore while it heals. For major canker sores or outbreaks that happen more than a few times a year, prescription options exist that target the inflammatory response more directly.
If your canker sores are unusually large, heal slowly, come with other symptoms like digestive issues or joint pain, or you’re getting them constantly, that pattern is worth investigating for an underlying condition like celiac disease or an inflammatory bowel disorder.