Canker sores don’t have a single known cause. They result from an overactive immune response that attacks the lining of your mouth, but the exact reason this happens varies from person to person. Genetics, stress, minor injuries, nutritional gaps, and hormonal shifts can all play a role, and most people who get them regularly have more than one trigger at work.
What Happens Inside Your Mouth
A canker sore forms when your immune system mistakenly damages the soft tissue lining your cheeks, lips, tongue, or the floor of your mouth. Specific immune cells called T cells attack the oral tissue, and the damage is fueled by a cascade of inflammatory signaling molecules that keep the process going even after the initial trigger is gone. In people who get canker sores repeatedly, researchers have found unusually high levels of certain types of T cells compared to people who never get them.
One theory is that bacteria naturally living in the mouth share structural similarities with proteins in the cells of your oral lining. Your immune system may respond to those bacteria and, in the process, accidentally target healthy tissue. This kind of friendly-fire reaction helps explain why canker sores can appear without any obvious injury or illness.
Genetics Are a Major Factor
If your parents get canker sores, you’re far more likely to get them too. When both parents have a history of recurrent canker sores, their child has roughly a 90% chance of developing them. When neither parent is affected, that number drops to about 20%. This strong hereditary pattern suggests that the underlying immune quirk responsible for canker sores is largely inherited, not something you pick up from your environment.
Common Triggers
Even with a genetic predisposition, canker sores typically need a trigger to flare up. The most frequently reported ones include:
- Minor mouth injuries: Biting your cheek, brushing too hard, dental work, braces, or rough-edged foods like chips and crusty bread can all nick the tissue enough to start an ulcer.
- Stress and lack of sleep: Emotional stress is one of the most consistent triggers people report. It likely works by shifting immune activity in ways that make the oral lining more vulnerable.
- Nutritional deficiencies: Low levels of iron, folic acid, or vitamin B12 are linked to more frequent outbreaks. Zinc and vitamin D deficiencies may also contribute.
- Certain foods: Acidic or spicy foods, including citrus fruits, tomatoes, and chocolate, are common culprits. These don’t cause canker sores on their own but can irritate tissue that’s already prone to ulceration.
- Hormonal changes: Some women develop canker sores in the second half of their menstrual cycle, when progesterone levels shift. This pattern is consistent enough that it’s been studied as a distinct subtype of recurrent canker sores.
- Smoking cessation: Oddly, some people develop canker sores after quitting smoking. Nicotine thickens the oral lining, so removing that protective effect can leave the tissue more exposed.
What About Toothpaste?
You may have heard that sodium lauryl sulfate (SLS), a foaming agent in most toothpastes, causes canker sores. This idea has been around for decades, but the evidence is thin. A 2019 review of the available studies found there wasn’t enough data to confirm that switching to SLS-free toothpaste reduces how often canker sores appear, how long they last, or how much they hurt. Some individuals do report improvement after switching, so it may be worth trying if you get frequent sores, but it’s not a reliable fix for most people.
Medical Conditions Linked to Canker Sores
Occasional canker sores are extremely common and usually harmless. But frequent or unusually severe outbreaks can sometimes signal an underlying health issue. Celiac disease, inflammatory bowel diseases like Crohn’s, and immune deficiencies are all associated with recurrent oral ulcers. In these cases, the canker sores are a symptom of broader immune dysfunction, not a standalone problem. Certain medications, particularly anti-inflammatory drugs and some blood pressure medications, can also trigger them.
If you’re getting large, slow-healing sores or outbreaks that happen almost continuously, a blood test checking for nutritional deficiencies and autoimmune markers can help rule out these conditions.
The Three Types of Canker Sores
Not all canker sores are the same. They fall into three categories based on size and behavior:
Minor canker sores account for 75% to 85% of all cases. They’re less than 1 centimeter across, heal within one to two weeks, and don’t leave scars. These are the small, round, white or yellow sores most people picture when they think of a canker sore.
Major canker sores make up 5% to 10% of cases and are larger than 1 centimeter. They’re deeper, significantly more painful, and can take up to six weeks to heal. These often leave scars on the oral tissue.
Herpetiform canker sores also represent 5% to 10% of cases. Despite the name, they have nothing to do with the herpes virus. They appear as clusters of tiny sores, sometimes as many as 100 at once, each only 1 to 3 millimeters wide. They typically heal within two weeks.
Canker Sores vs. Cold Sores
People often confuse canker sores with cold sores, but they’re completely different conditions. Cold sores are caused by herpes simplex virus type 1 (HSV-1). They appear as fluid-filled blisters on the outside of the mouth, usually around the lips, and they’re contagious. Canker sores have no viral cause, form only inside the mouth, and cannot be spread to another person.
The location is the easiest way to tell them apart. If the sore is on the inner surface of your cheek, lip, or tongue, it’s almost certainly a canker sore. If it’s on or around your lips with a blister-like appearance, it’s likely a cold sore.
Why Some People Get Them Repeatedly
About 20% of the general population gets canker sores at some point, but a smaller subset deals with them on a recurring basis, sometimes monthly or even more often. In these people, the immune system’s overreaction to oral tissue appears to be a persistent trait rather than a one-time event. Researchers have found that people with recurrent canker sores show abnormalities in a specific immune signaling pathway that promotes an exaggerated inflammatory response in the mouth.
Recurrence tends to be worst in adolescence and young adulthood, then gradually decreases with age. Women are affected slightly more often than men, which aligns with the hormonal connection. For most people, the frequency and severity of outbreaks improve over time, even without treatment, though the tendency never fully disappears.
Reducing Flare-Ups
Since there’s no cure for canker sores, management focuses on avoiding known triggers and shortening the duration of outbreaks when they happen. Keeping a food diary can help you identify whether specific foods consistently precede a sore. Managing stress through sleep, exercise, or other strategies can reduce frequency for people whose outbreaks are stress-driven.
Correcting a nutritional deficiency, if one exists, is one of the most effective long-term strategies. A simple blood panel can check your iron, B12, and folate levels. For the sores themselves, over-the-counter topical gels that form a protective coating over the ulcer can reduce pain and help it heal faster. Rinsing with a saltwater or baking soda solution a few times a day also soothes irritation without disrupting the healing process.