Canker sores form when your immune system overreacts to minor irritation inside your mouth. Unlike cold sores, they aren’t caused by a virus and aren’t contagious. The triggers range from something as simple as biting your cheek to ongoing nutritional gaps or genetic predisposition. Understanding what sets them off can help you reduce how often they appear.
What Actually Happens Inside Your Mouth
Canker sores are the result of a T-cell mediated immune response. Your immune cells essentially attack a small area of the soft tissue lining your mouth, breaking down the surface and creating an open ulcer. One leading theory is that people prone to canker sores have a heightened sensitivity to everyday bacteria and viruses in the mouth. When their immune system encounters these microbes, it overreacts, destroying healthy tissue in the process of trying to fight off the perceived threat.
The result is a round white or yellow sore with a red border, typically appearing on the inner cheeks, lips, tongue, or soft palate. Most are small (under a centimeter) and heal on their own within one to two weeks. Larger ones can take significantly longer and occasionally leave scars.
The Most Common Triggers
Physical Injury to Mouth Tissue
The most straightforward cause is tissue damage. Biting the inside of your cheek, scraping your gums with a sharp chip, burning your mouth on hot food, or irritation from braces and dental work can all kick off the immune response that produces an ulcer. Even aggressive tooth brushing can do it.
Stress
Stress is one of the most frequently reported triggers. It suppresses parts of the immune system while activating others, which can tip the balance toward the kind of overreaction that produces canker sores. Many people notice outbreaks during exam periods, major life changes, or prolonged sleep deprivation.
Certain Foods and Drinks
Acidic, spicy, and abrasive foods are well-known culprits. Coffee, alcohol, citrus fruits, tomatoes, and anything containing capsaicin (the compound that makes peppers hot) can all irritate the oral lining enough to trigger or worsen a sore. Nuts are a double threat: their rough texture physically scrapes the tissue, and some people have allergic sensitivities to them as well.
Food sensitivities play a separate role. In some people, dairy proteins, gluten, eggs, shellfish, or certain food additives provoke an immune reaction in the mouth’s lining that leads directly to ulceration. Chocolate can also be a trigger, likely due to its acidity and the allergenic potential of cocoa or its common companion ingredients like milk and sugar. If you notice a pattern between specific foods and outbreaks, that connection is worth paying attention to.
Your Toothpaste
Many toothpastes contain sodium lauryl sulfate (SLS), a foaming agent that strips away the protective mucous layer on your oral tissue. Research published in the American Journal of Dentistry found that people who brushed with SLS-containing toothpaste developed significantly more canker sores than those using SLS-free alternatives. Multiple studies have shown that switching to an SLS-free toothpaste reduces not only how often sores appear but also how long they last and how much they hurt. If you get canker sores regularly, this is one of the simplest changes you can make.
Nutritional Deficiencies
Your mouth’s lining regenerates quickly and depends on a steady supply of specific nutrients to stay intact. Deficiencies in iron, vitamin B12, folic acid, and zinc are all linked to recurrent canker sores. These nutrients support the production of healthy mucosal cells and proper immune function. When levels drop, the tissue becomes more fragile and the immune system more prone to misfiring.
This doesn’t mean every canker sore signals a deficiency. But if you’re getting them repeatedly and can’t identify an obvious trigger like injury or stress, it’s worth looking at your diet. People on restrictive diets, those with absorption issues, and vegetarians or vegans who may be low in B12 are at higher risk.
Genetics Play a Major Role
If both of your parents get canker sores, your chance of developing them is roughly 90%. If neither parent is affected, that drops to about 20%. This strong hereditary component explains why some people seem to get canker sores constantly while others never experience one. Your genetic makeup likely influences how reactive your immune system is to the minor insults your mouth encounters daily.
Canker sores affect somewhere between 5% and 66% of the population depending on the study and population examined, with ulceration often beginning before age five. Frequency tends to increase with age, though some people find their outbreaks decrease in adulthood.
When Canker Sores Signal Something Else
Occasional canker sores are normal. Frequent, severe, or unusually persistent ones can sometimes be the first visible sign of a systemic condition. Crohn’s disease, lupus, celiac disease, and Behçet syndrome all have canker-like oral ulcers among their symptoms. In Crohn’s disease and Behçet syndrome specifically, mouth sores may appear before any other symptoms develop.
Behçet syndrome tends to produce numerous painful ulcers, especially on the soft palate and back of the throat. Crohn’s-related oral changes can also include diffuse swelling of the mouth’s lining and a cobblestone-like texture on the inner cheeks. If your canker sores are large, unusually frequent, slow to heal, or accompanied by other symptoms like digestive problems, joint pain, or skin changes, that pattern is worth investigating.
Canker Sores vs. Cold Sores
People often confuse the two, but they’re fundamentally different. Canker sores appear inside the mouth and are not contagious. Cold sores (fever blisters) appear outside the mouth, typically around the border of the lips, and are caused by the herpes simplex virus. They look different, too: a canker sore is usually a single round ulcer with a white or yellow center, while a cold sore is a cluster of small fluid-filled blisters. You cannot spread a canker sore to another person through kissing, sharing utensils, or any other contact.