How Do You Get A Canker Sore? Causes And Triggers

Canker sores are small, shallow lesions that form on the soft tissues inside your mouth, and they are not contagious. Unlike cold sores, which are caused by the herpes virus and spread through contact, canker sores appear to be triggered by a combination of factors including minor injury, stress, certain foods, and nutritional deficiencies. Research has not found a single cause, but evidence points to an overactive immune response targeting the lining of your mouth.

What Exactly Is a Canker Sore and How Is It Different From a Cold Sore?

A canker sore, also known as an aphthous ulcer, is a painful round or oval sore with a white or yellow center and a red border. They form inside the mouth — on the cheeks, lips, tongue, or the floor of the mouth. They are not the same as cold sores.

Cold sores, or fever blisters, are caused by the herpes simplex virus. They appear on the outside of the mouth, usually on the lips. Cold sores are contagious and can spread through kissing or sharing utensils. Canker sores are not contagious. You cannot give one to someone else.

The confusion between the two is common. Many people think every mouth sore is the same. They are not. Knowing the difference matters because treatment and prevention are completely different.

What Are the Most Common Triggers for Canker Sores?

Research shows that canker sores often appear after specific events. The most common trigger is minor mouth injury. This includes accidentally biting your cheek, brushing too hard, or eating sharp foods like chips or pretzels. The trauma creates a small wound, and the body reacts by forming a sore.

Stress is another well-documented trigger. Studies have found that people who report high stress levels are more likely to develop canker sores. The exact mechanism is not fully understood, but stress is known to affect the immune system. Some researchers believe stress causes immune cells to mistakenly attack healthy tissue in the mouth.

Certain foods are frequently reported as triggers. Acidic foods like citrus fruits, tomatoes, and strawberries are common culprits. Spicy foods and chocolate are also mentioned in many patient reports. Strong evidence is limited for specific foods, but many people find that avoiding these items reduces their outbreaks.

Hormonal changes may play a role. Some women report more canker sores during their menstrual cycle. The evidence for this is moderate, but it aligns with the idea that immune function fluctuates with hormone levels.

How Do You Get a Canker Sore From Nutritional Deficiencies?

This is one of the most important areas of research on canker sores. Studies have found strong links between recurrent canker sores and deficiencies in certain vitamins and minerals.

A study published in the Journal of Oral Pathology & Medicine found that people with recurrent canker sores were more likely to have low levels of vitamin B12, folate, and iron. Another study in the same journal found that zinc deficiency was also common in this group.

The connection makes sense. These nutrients are essential for maintaining healthy mucous membranes and supporting immune function. When levels are low, the lining of the mouth becomes more vulnerable to damage and less able to heal.

Some people have a condition called recurrent aphthous stomatitis, which means they get canker sores frequently. For these individuals, checking nutrient levels is a practical first step. A simple blood test can reveal deficiencies that may be driving the problem.

It is worth noting that not everyone with recurrent canker sores has a deficiency. But for those who do, correcting the deficiency often reduces the frequency and severity of outbreaks. The evidence for this is strong enough that many dentists and doctors recommend testing for these nutrients.

What Does Research Say About Genetics and Immune System Links?

Genetics play a clear role in canker sores. Studies have found that up to 40% of people with recurrent canker sores have a family history of the condition. This is not a single gene cause. It is more likely that multiple genes influence how your immune system responds to triggers.

The immune system connection is central to understanding canker sores. Research suggests that canker sores are an autoimmune-like reaction. The body’s immune cells, specifically T-cells, attack the mucous membrane cells in the mouth. This causes the ulcer to form.

This is why canker sores are more common in people with certain autoimmune conditions. Crohn’s disease, Behçet’s disease, and celiac disease are all associated with higher rates of canker sores. In celiac disease specifically, avoiding gluten can dramatically reduce outbreaks for some people.

The immune link also explains why canker sores sometimes appear after illness. When your immune system is already activated fighting an infection, it may be more likely to misfire and target mouth tissue.

Are There Medications or Products That Cause Canker Sores?

Some medications are known to trigger canker sores. Nonsteroidal anti-inflammatory drugs, or NSAIDs, are a common culprit. This includes ibuprofen, naproxen, and aspirin. These drugs can irritate the lining of the mouth in some people.

Beta-blockers, used for high blood pressure, have also been linked to canker sores. The evidence for this is moderate, but enough that doctors sometimes consider switching medications for patients with frequent outbreaks.

Certain dental products can be a trigger. Toothpastes and mouthwashes containing sodium lauryl sulfate, or SLS, have been studied. SLS is a foaming agent. Some research suggests it can irritate the mouth and increase canker sore frequency. A study in the Journal of Oral Pathology & Medicine found that people who used SLS-free toothpaste had fewer outbreaks.

The evidence on SLS is not conclusive. Some studies show a benefit, others do not. But it is a low-cost change to try. Switching to an SLS-free toothpaste for a few months is a reasonable experiment for anyone with recurrent canker sores.

Table: Common Triggers and Strength of Evidence

What Actually Helps Prevent Canker Sores Based on Evidence?

Prevention starts with identifying your personal triggers. This is not a one-size-fits-all situation. What causes sores for one person may not affect another.

Start with a food diary. Write down what you eat and when sores appear. Look for patterns over a few months. Many people find that acidic foods, chocolate, or nuts are their personal triggers.

Check your toothpaste. Switch to an SLS-free version for at least two months. If you see a reduction in sores, stick with it. If not, it was not the problem.

Consider a multivitamin. Given the strong link to B12, folate, and iron, a daily supplement is a low-risk step. A blood test is better. If you have a true deficiency, a multivitamin may not be enough. Targeted supplementation based on lab results is more effective.

Manage stress. This is easier said than done, but the evidence is clear. Exercise, sleep, and relaxation techniques all help. You do not need a complicated plan. Even a 10-minute walk daily can lower stress hormones.

Avoid mouth injury. Be careful when eating hard or sharp foods. Use a soft-bristled toothbrush. Do not chew on pens or other objects. Small changes in daily habits can reduce the number of injuries that trigger sores.

For people with celiac disease or gluten sensitivity, a strict gluten-free diet is essential. The evidence here is strong. Many people with celiac disease find that canker sores disappear entirely when they remove gluten from their diet.

What Are the Most Common Misconceptions About Canker Sores?

The biggest misconception is that canker sores are contagious. They are not. You cannot catch a canker sore from someone else. This confusion comes from mixing them up with cold sores. Cold sores are contagious. Canker sores are not.

Another misconception is that canker sores are caused by poor hygiene. This is false. In fact, some people with recurrent canker sores have excellent oral hygiene. The cause is internal, not external.

Some people believe that canker sores are a sign of a serious disease. For most people, they are not. Recurrent canker sores are usually just an annoying condition. However, if you get sores that are very large, last longer than two weeks, or come with other symptoms like fever or weight loss, see a doctor. These can be signs of an underlying condition.

There is also a belief that certain foods cause canker sores in everyone. This is not true. Triggers are highly individual. Chocolate may cause sores in one person and have no effect on another. The only way to know your triggers is to track them.

Frequently Asked Questions