Canker sores develop when your immune system overreacts to a trigger, attacking the thin lining inside your mouth and creating a small, painful ulcer. The triggers range from something as simple as biting your cheek to deeper factors like genetics, stress, and diet. Understanding what sets them off is the key to figuring out why you keep getting them.
Your Immune System Drives the Process
A canker sore isn’t an infection. It forms when your body’s own immune cells mistakenly target the soft tissue inside your mouth. People who get recurrent canker sores have significantly higher levels of certain inflammatory signaling molecules in their blood compared to people who don’t get them. These signals activate immune cells that damage the protective surface layer of the mouth, creating the characteristic shallow, round ulcer with a white or yellowish center.
This is why canker sores aren’t contagious. Unlike cold sores, which are caused by a virus and appear on the outside of the lips, canker sores are an internal immune response. They show up on the inner cheeks, gums, tongue, or soft palate, and they heal on their own as the immune response winds down, typically within one to two weeks.
Genetics Play a Larger Role Than Most People Realize
If your parents get canker sores, you’re almost certainly going to get them too. More than 40% of people with recurrent canker sores have a family history of them. When both parents are affected, their children have roughly a 90% chance of developing them as well. People with a family history also tend to get their first outbreak earlier in life and experience more severe symptoms. This strong hereditary pattern suggests that certain people inherit immune characteristics that make their oral tissue more reactive to everyday triggers.
Physical Injury to the Mouth
Mechanical trauma is one of the most common and straightforward triggers. Braces are a frequent culprit: brackets and wires rub against the inner cheeks and lips, creating friction that breaks down the tissue and gives a canker sore a place to form. Accidentally biting the inside of your mouth, poking your gums with a toothbrush, or eating sharp, crunchy foods like chips and crusty bread can do the same thing. Even a rough edge on a dental filling or a poorly fitting retainer can cause repeated irritation that leads to ulcers.
Foods That Trigger Outbreaks
Certain foods are well-known for either triggering new canker sores or making existing ones worse. Acidic foods top the list: coffee, oranges, grapefruit, and pineapple irritate the sensitive tissue inside the mouth. Spicy foods do the same. But the list extends beyond the obvious. Chocolate, cheese, nuts, strawberries, salty pretzels, and chips are all associated with outbreaks in some people. The pattern varies from person to person, so tracking which foods precede your outbreaks can help you identify your specific triggers.
Your Toothpaste May Be a Factor
Sodium lauryl sulfate, a foaming agent found in most mainstream toothpastes, is a known soft tissue irritant. The same ingredient is used in shampoos, soaps, and household cleaning products. For people prone to canker sores, switching to an SLS-free toothpaste can reduce the frequency and severity of outbreaks. It’s one of the simplest changes you can make, and several brands now specifically market SLS-free formulas.
Stress Changes Your Mouth’s Defenses
Psychological stress doesn’t just make you feel run down. It triggers a hormonal chain reaction that directly weakens your mouth’s ability to protect itself. When you’re stressed, your brain signals the adrenal glands to release cortisol. Elevated cortisol levels alter the activity of your immune cells, including the T cells and natural killer cells that normally keep your oral tissue healthy. At the same time, stress increases levels of a digestive enzyme in your saliva that, alongside the immune disruption, can create the conditions for an ulcer to form.
This explains why canker sore outbreaks cluster around exams, work deadlines, and emotionally difficult periods. The stress doesn’t directly create the sore. It lowers the threshold at which other triggers, like a minor bite or an acidic meal, can tip the balance.
Hormonal Shifts During the Menstrual Cycle
Some women notice their canker sores follow a predictable monthly pattern, with outbreaks recurring during every menstrual cycle. The hormonal fluctuations that occur, particularly shifts in progesterone and estrogen, appear to influence the oral tissue’s vulnerability in a way that mirrors the stress response. If you notice this kind of timing, it’s worth noting because it confirms the outbreaks are hormonally driven rather than caused by diet or other factors.
Underlying Health Conditions
Frequent and severe canker sores can be a sign of an underlying condition, particularly one that affects the immune system or the gut. Celiac disease is one of the strongest associations. People with celiac disease experience significantly more frequent and more severe outbreaks than the general population, and following a strict gluten-free diet has been shown to reduce both the frequency and severity of their sores. Crohn’s disease and Behçet’s disease are also linked to recurrent oral ulcers.
Nutritional deficiencies can play a role too. Low levels of iron, zinc, folate, or B vitamins are associated with canker sore outbreaks, likely because these nutrients are essential for maintaining healthy mucosal tissue. In some cases, correcting the deficiency is enough to stop the cycle.
Why Some People Get Them Repeatedly
For most people, canker sores result from a combination of factors rather than a single cause. You might have the genetic predisposition, use an SLS toothpaste, eat acidic foods regularly, and go through a stressful period, all at once. Each factor lowers the threshold a little more. This layered nature is why outbreaks can feel so unpredictable: the same food that caused no problems last month might trigger a sore this month because your stress levels are higher or you bit your cheek without noticing.
Keeping a simple log of your outbreaks, noting your stress levels, recent foods, menstrual cycle timing, and any mouth injuries, can reveal patterns that are invisible otherwise. Once you identify your primary triggers, reducing even one or two of them often decreases how often sores appear.