Canker sores develop when the protective lining of your mouth breaks down, exposing the tissue underneath to bacteria and irritants. In most cases, a combination of stress, minor injuries, and immune system responses triggers these painful ulcers. About 20% of people get them regularly, and while they’re almost always harmless, recurring outbreaks sometimes signal a nutritional deficiency or underlying health condition worth investigating.
How Canker Sores Form
The inside of your mouth is lined with a thin layer of tissue called mucosa. When that tissue is damaged or weakened, your immune system launches an inflammatory response. White blood cells flood the area, and the body releases inflammatory signals that, somewhat counterproductively, cause your own immune cells to attack the surface layer of tissue. The result is a shallow, open sore surrounded by redness and swelling.
Most canker sores are the minor type: less than 1 centimeter across, shallow, and self-healing within about two weeks. Larger sores (over 10 millimeters) are classified as major aphthous ulcers. These can persist for six weeks or longer, extend to the back of the throat or gums, and leave scars. A third type, called herpetiform ulcers, appears as clusters of many small sores at once. Despite the name, they have nothing to do with the herpes virus.
Stress and Your Immune System
Stress is one of the most consistent triggers for canker sores, and the mechanism is more specific than “stress weakens your immune system.” When you’re under psychological stress, your body ramps up cortisol production. Cortisol suppresses normal immune function by reducing the mobility of white blood cells and altering how your body produces antibodies. At the same time, stress triggers the release of inflammatory molecules (IL-1 and IL-6) that can cause cell death in the lining of your mouth, creating the conditions for an ulcer to form.
Once that initial damage occurs, a cascade follows. Your body sends inflammatory signals that recruit more immune cells to the area, including a type of white blood cell that actively attacks the damaged tissue. This is why canker sores seem to get worse before they get better: your immune system is essentially overreacting to a small wound.
Physical Triggers and Mouth Injuries
Biting the inside of your cheek, scraping your gums with a chip, or irritating tissue with braces or ill-fitting dentures can all set off a canker sore. The injury doesn’t need to be dramatic. Aggressive tooth brushing, dental work, or even a sharp edge on a tooth can create enough micro-damage for an ulcer to develop in someone who’s prone to them.
Sports-related mouth injuries, burns from hot food, and acidic foods like citrus fruits or tomatoes are common physical triggers. Some people also react to chocolate, peanuts, or eggs, though research hasn’t confirmed a direct allergic cause. If you notice sores appearing after specific foods, tracking your outbreaks alongside what you eat can help you identify personal triggers through a process of elimination.
Your Toothpaste Could Be a Factor
Sodium lauryl sulfate (SLS), the foaming agent in most toothpaste, disrupts the surface of oral tissue. Lab studies show SLS significantly increases the permeability of mouth tissue by denaturing the proteins that hold cells together. Essentially, it strips away part of the protective barrier inside your mouth.
The clinical picture is more nuanced than the lab data suggests. Some trials found that switching to SLS-free toothpaste didn’t significantly change ulcer frequency, while others reported shorter ulcer duration and less pain in people who made the switch. If you get canker sores frequently, trying an SLS-free toothpaste for a few months is a low-risk experiment. It may not eliminate outbreaks, but it could reduce their severity.
Nutritional Deficiencies
This is one of the most overlooked causes. In a study of adults with recurring canker sores, 56% had some form of blood-related nutritional deficiency, compared to just 7% in a control group. Over a third of the canker sore group was anemic. The most commonly implicated deficiencies are iron, vitamin B12, folate, and zinc.
If your canker sores keep coming back and you can’t pin them on stress or injury, a simple blood test can check for these deficiencies. Correcting a low iron or B12 level often reduces the frequency of outbreaks significantly.
When Canker Sores Signal Something Bigger
For most people, canker sores are a nuisance and nothing more. But recurring ulcers can occasionally be a symptom of a systemic condition. Understanding the patterns that warrant further investigation can save you from overlooking something important.
Celiac disease causes canker sores in roughly 16% of children and 26% of adults with the condition. If your sores come with bloating, digestive problems, or unexplained weight loss, celiac screening is reasonable. Crohn’s disease produces oral ulcers in up to 20% of patients, often alongside abdominal pain, diarrhea, or fatigue.
Behçet’s disease, a rare inflammatory condition, presents with mouth sores as the first symptom in 80% of cases. The distinguishing feature is that Behçet’s also causes genital ulcers, eye inflammation, and skin lesions. Canker sores that appear on a strict three-week cycle can point to a condition called cyclic neutropenia, where white blood cell counts drop on a regular schedule. Any pattern of recurring sores accompanied by fever should be evaluated, as fever can indicate an infectious cause or an autoinflammatory syndrome.
Managing Outbreaks at Home
Most minor canker sores heal on their own within 10 to 14 days. In the meantime, rinsing with warm salt water several times a day can reduce pain and keep the area clean. Over-the-counter topical gels that contain a numbing agent provide temporary relief, especially before meals. Avoiding spicy, acidic, or crunchy foods while a sore is active prevents further irritation.
For sores that are especially large or painful, prescription-strength topical treatments can speed healing. These are typically anti-inflammatory pastes applied directly to the ulcer several times a day. They work by calming the immune response at the sore’s surface. For people with severe, frequent outbreaks that don’t respond to topical treatment, systemic medications that modulate the immune response are sometimes used, though this is reserved for the most persistent cases.
When a Mouth Sore Needs Attention
A canker sore that hasn’t healed within three weeks deserves a closer look. The same goes for sores that are getting larger rather than smaller, sores that are completely painless (canker sores typically hurt), or any ulcer accompanied by firm or hard tissue underneath. Oral cancers can mimic the appearance of a canker sore, but they tend to be painless, persistent, and sometimes discolored. A dentist or doctor can evaluate the sore visually and by touch, and if anything looks abnormal, a small tissue sample can rule out something more serious.
Multiple sores appearing alongside fever, joint pain, eye redness, genital sores, or a skin rash also warrant a medical evaluation. These combinations suggest the sores may be part of a broader inflammatory or autoimmune process rather than a standalone issue.