Canker sores are caused by an overactive immune response that attacks the lining of your mouth, but the triggers that set off that response vary widely from person to person. Stress, nutritional deficiencies, hormonal shifts, genetics, certain foods, and even your toothpaste can all play a role. Most people get the minor type, which accounts for 75% to 85% of all cases, but understanding what’s behind them can help you reduce how often they come back.
Your Immune System Is the Direct Cause
A canker sore isn’t an infection. It’s your own immune system damaging the soft tissue inside your mouth. Before a sore even appears, immune cells (primarily a type of white blood cell called T-lymphocytes) begin gathering just beneath the surface layer of the oral lining. As the process ramps up, these cells release inflammatory signaling molecules that recruit even more immune cells to the area, creating a cascade that breaks down the tissue and forms the ulcer you eventually see and feel.
People who get recurrent canker sores show consistently higher levels of these inflammatory signals, not just during an active outbreak but even between episodes. At the same time, they tend to have lower levels of the molecules that normally calm inflammation down. This imbalance means their mouth lining is essentially primed to overreact. The open question, and the reason canker sores are so frustrating, is what flips the switch each time. That’s where the triggers below come in.
Genetics Play a Major Role
If both of your parents get canker sores, your chance of getting them is roughly 90%. If neither parent is affected, that drops to about 20%. This strong hereditary pattern suggests that much of the susceptibility is built into how your immune system is wired from birth. You can’t change your genetic predisposition, but knowing it exists helps explain why some people get canker sores constantly while others never do, even when exposed to the same triggers.
Nutritional Deficiencies
Screening studies have found that roughly 14% to 18% of people with recurrent canker sores are deficient in vitamin B12, iron, or folate. These nutrients are essential for maintaining healthy mucosal tissue and supporting normal immune function. When levels drop, the mouth lining becomes more fragile and slower to repair, making it easier for ulcers to form.
B12 deficiency is particularly common in vegetarians and vegans, while iron deficiency is more prevalent in women of reproductive age. Folate can be low in people who eat few leafy greens or whose gut doesn’t absorb nutrients well. If you get canker sores frequently, a simple blood test can check whether a deficiency is contributing. In many cases, correcting the deficiency reduces the frequency of outbreaks.
Stress and Cortisol
Psychological stress is one of the most commonly reported triggers, and the mechanism is physiological, not just “in your head.” Stress raises cortisol levels, and elevated cortisol disrupts the balance of inflammatory signaling molecules in your body. This imbalance can tip the immune system toward the kind of overreaction that produces canker sores. Many people notice outbreaks cluster around exams, deadlines, major life changes, or periods of poor sleep, all of which elevate cortisol.
Hormonal Shifts in Women
Some women notice canker sores appearing at the same point in every menstrual cycle. This pattern is linked to the rise in progesterone that occurs after ovulation. In a summary of reported cases, the menstrual cycle accounted for 73% of hormonally triggered oral ulcers, with pregnancy and hormonal therapy making up the remainder. If your canker sores reliably show up in the second half of your cycle, hormonal sensitivity is likely involved.
Toothpaste With SLS
Sodium lauryl sulfate (SLS) is a foaming agent found in most conventional toothpastes, and it’s a well-documented canker sore trigger. A systematic review of clinical trials found that switching to an SLS-free toothpaste significantly reduced the number of ulcers, the duration of each ulcer, the number of episodes, and pain levels. This is one of the simplest changes you can make. SLS-free toothpastes are widely available, and many people see a noticeable drop in outbreaks within a few weeks of switching.
Foods That Trigger Outbreaks
Certain foods can irritate the oral lining or provoke an immune reaction that leads to canker sores. The most common culprits fall into a few categories:
- Acidic foods: Citrus fruits, tomatoes, pineapple, and soft drinks. The acid directly irritates the delicate tissue inside your mouth.
- Spicy foods: Hot peppers and heavily spiced dishes can inflame already-sensitive tissue.
- Coffee: Contains salicylic acid, which can irritate the gums, cheeks, and tongue.
- Chocolate: A common sensitivity trigger for some people, possibly due to its mild acidity and specific proteins.
- Sharp or crunchy foods: Chips, pretzels, crusty bread, and nuts can physically scratch the lining of your mouth, creating a wound that develops into a canker sore.
Gluten sensitivity deserves a separate mention. People with celiac disease or non-celiac gluten sensitivity develop canker sores at higher-than-normal rates. The connection likely involves the intestinal damage that gluten causes in these individuals, which impairs nutrient absorption and creates the same kind of deficiencies (B12, iron, folate) that independently trigger canker sores.
Physical Trauma to the Mouth
Biting your cheek, brushing too aggressively, dental work, braces, and ill-fitting dentures can all create small injuries that become canker sores in susceptible people. The trauma itself doesn’t cause the sore in the way a cut causes a scab. Instead, the damaged tissue triggers a localized immune response that, in someone already predisposed, spirals into a full ulcer rather than healing normally. If you notice sores consistently forming where a bracket rubs or where you tend to bite, the mechanical irritation is likely the trigger.
The Three Types and How They Differ
Not all canker sores are the same. Minor aphthous ulcers are the most common (75% to 85% of cases), typically under a centimeter across, and heal on their own within one to two weeks without scarring. Major aphthous ulcers account for 5% to 10% of cases, are larger, deeper, and can take weeks or even months to heal, sometimes leaving scars. The herpetiform type, also 5% to 10% of cases, appears as clusters of tiny ulcers that can merge into larger irregular sores. Despite the name, herpetiform canker sores have nothing to do with the herpes virus.
Knowing which type you’re dealing with matters because major and herpetiform ulcers are more likely to signal an underlying condition like an immune disorder, celiac disease, or a significant nutritional deficiency. Recurrent major ulcers, or canker sores that appear in unusually high numbers, often warrant blood work to check for contributing factors that can be treated.