Canker sores form when your immune system attacks the thin lining of tissue inside your mouth, creating small, painful ulcers. Unlike cold sores, which are caused by a virus and appear on the outside of your lips, canker sores are not infections and are not contagious. They result from an overactive immune response that breaks down your own oral tissue, and a combination of genetics, stress, diet, and physical irritation determines who gets them and how often.
What’s Actually Happening Inside Your Mouth
The inside of your mouth is lined with a thin, delicate layer of cells called the oral mucosa. When something triggers your immune system to misfire, a specific type of white blood cell begins attacking this lining as though it were a foreign invader. The immune cells release inflammatory chemicals that destroy the surface tissue, leaving behind a shallow crater. That crater is the canker sore: a raw, exposed patch of tissue that sits right where food, saliva, and your teeth constantly make contact, which is why they hurt so much.
The ulcer itself typically has a white or yellowish center surrounded by a red border. The white area is a layer of proteins that forms over the wound, similar to a scab on skin but in a wet environment. The redness around it is inflammation, with blood vessels dilating as your body sends repair cells to the area. The response to steroid treatments like prednisone confirms that immune activation drives the damage, since steroids suppress that immune response and allow healing.
Why Some People Get Them and Others Don’t
Genetics play a significant role. About one-third of people who get recurring canker sores have a family history of them. Researchers have identified several immune system gene markers that appear more frequently in people prone to canker sores, suggesting that some people simply inherit a hair-trigger immune response in their oral tissue. If one or both of your parents dealt with frequent canker sores, your chances of getting them are considerably higher.
This genetic susceptibility helps explain why two people can eat the same spicy meal or bite their cheek in the same way, but only one of them develops a sore. Your genes set the threshold for how easily your immune system overreacts to minor irritation inside the mouth.
Common Triggers
Even with a genetic predisposition, canker sores usually need a trigger to appear. The most well-established ones include:
- Physical injury: Biting your cheek, jabbing yourself with a toothbrush, dental work, or braces rubbing against tissue can all start the process. The minor wound signals your immune system, which then overreacts and creates an ulcer far worse than the original scratch.
- Certain foods: Acidic foods (citrus, tomatoes), spicy foods, coffee, chocolate, eggs, cheese, peanuts, and almonds are all recognized triggers. These foods can irritate the mucosa directly or provoke an immune response in susceptible people.
- Toothpaste ingredients: Sodium lauryl sulfate, a foaming agent in many toothpastes, strips away the protective mucus layer inside your mouth and can trigger sores in people who are sensitive to it. Switching to an SLS-free toothpaste reduces outbreaks for some people.
- Stress and anxiety: Psychological stress is one of the most consistent triggers. Research published in Contemporary Clinical Dentistry found that canker sore patients had significantly higher levels of anxiety and depression compared to controls. Interestingly, the connection doesn’t appear to work through cortisol (the classic “stress hormone”) but instead through changes in immune system activity. Stress can increase the reactivity of T cells and other immune components, essentially lowering the threshold for that mucosal attack.
- Hormonal shifts: Some women notice canker sores appearing at specific points in their menstrual cycle, suggesting that fluctuations in estrogen and progesterone can influence the immune response in oral tissue.
The Three Types of Canker Sores
Not all canker sores are the same. The vast majority, roughly 80%, are minor canker sores. These are smaller than about 10 millimeters across (about the size of a pencil eraser or smaller), appear one to five at a time, and heal on their own within one to two weeks without leaving a scar. They’re painful but manageable.
Major canker sores are larger, deeper, and far more painful. They can exceed a centimeter in diameter, often appear with irregular borders, and can take six weeks or longer to heal. These sometimes leave scars. If you’re dealing with a sore that’s unusually large or has persisted for more than three weeks, it’s worth getting it evaluated.
The third type, called herpetiform ulcers (despite having nothing to do with the herpes virus), appears as clusters of very small sores, sometimes dozens at once, that can merge into larger irregular ulcers. These are the least common and tend to occur in older adults.
How a Canker Sore Progresses
Most canker sores follow a predictable pattern. In the first day or two, you’ll feel a tingling, burning, or prickling sensation in one spot inside your mouth before anything is visible. This is the prodromal phase, when the immune response is ramping up beneath the surface but hasn’t yet broken through the tissue.
Over the next day or so, a small red spot appears and quickly develops into the characteristic open ulcer with its white or yellow center. Pain peaks during the first three to five days, when the ulcer is fully open and exposed. Eating, drinking, and talking can all aggravate it. After that, the pain gradually decreases as new tissue begins growing from the edges of the sore inward. Most minor sores are fully healed within 7 to 14 days.
When Canker Sores Signal Something Else
Occasional canker sores are extremely common and usually harmless. But frequent or severe outbreaks can sometimes point to an underlying condition. Celiac disease, the autoimmune reaction to gluten, often shows up as recurring canker sores before any digestive symptoms appear. Nutritional deficiencies in iron, B12, zinc, or folate can also cause recurring ulcers, because these nutrients are essential for maintaining the oral lining and regulating immune function.
Behcet’s disease is a rarer condition where the immune system attacks blood vessels throughout the body. Painful mouth sores that look identical to canker sores are often its earliest and most common symptom. The key difference is that Behcet’s also causes genital sores, eye inflammation, skin rashes, and joint pain. The mouth sores in Behcet’s tend to heal within one to three weeks but keep returning.
Inflammatory bowel diseases like Crohn’s can also produce oral ulcers. In some cases, mouth sores appear as the first sign of intestinal inflammation. If your canker sores are unusually frequent (more than three or four episodes a year), consistently large, slow to heal, or accompanied by other symptoms like fatigue, joint pain, digestive issues, or sores elsewhere on your body, those patterns are worth investigating.
What Helps Them Heal
Most canker sores heal on their own without treatment. The goal with any intervention is to reduce pain and avoid making things worse. Over-the-counter topical gels that contain a numbing agent can coat the sore and provide temporary relief, especially before meals. Rinsing with warm salt water several times a day helps keep the area clean and can reduce inflammation.
Avoiding your known trigger foods during an outbreak makes a real difference. Acidic, salty, and spicy foods directly irritate the exposed tissue and can extend healing time. Soft, bland foods are easier on an active sore. If you’ve noticed a pattern with sodium lauryl sulfate in your toothpaste, switching brands is a simple change that can reduce how often sores appear in the first place.
For people with frequent or severe outbreaks, prescription options exist. Topical steroid pastes applied directly to the sore can suppress the local immune response and speed healing. In cases tied to nutritional deficiencies, correcting the underlying shortage (through diet changes or supplements) often reduces or eliminates recurrences entirely. If stress is your primary trigger, the sores themselves become a useful signal that your body is under more pressure than you might realize.