Canker sores are caused by an overreaction of your immune system that destroys small patches of tissue inside your mouth. Unlike cold sores, they aren’t caused by a virus and aren’t contagious. The frustrating part is that no single cause explains every outbreak. Instead, a combination of genetics, nutritional gaps, stress, and physical irritation typically work together to trigger them.
Your Immune System Attacks Healthy Tissue
At the cellular level, canker sores form when your body’s immune defenses mistakenly target the soft lining of your mouth. Certain white blood cells that normally fight infections instead destroy healthy oral tissue, and this damage is fueled by inflammatory signaling molecules that keep the process going. People with active canker sores have higher blood levels of several inflammation markers compared to people who rarely get them.
One leading theory points to a common mouth bacterium called Streptococcus sanguinis. Your immune system may confuse proteins on this bacterium with proteins in your own mouth tissue, launching an attack against both. This misdirected immune response is what creates the painful, open crater you recognize as a canker sore.
Genetics Play a Major Role
If both of your parents get canker sores, you have roughly a 90% chance of getting them too. If neither parent is affected, that drops to about 20%. This strong hereditary pattern suggests that the underlying immune tendency is largely something you’re born with, not something you pick up from your environment. So if you’ve always been prone to canker sores while your friends never get them, your family tree is likely the biggest reason.
Vitamin and Mineral Deficiencies
Low levels of certain nutrients are strongly linked to recurrent canker sores. In one study of 40 people with frequent outbreaks, 75% were deficient in vitamin B12, folate, or both. Iron deficiency is also commonly associated with repeated sores, though it’s studied less precisely.
These nutrients are essential for maintaining the rapidly dividing cells that line your mouth. When your body doesn’t have enough of them, that tissue becomes fragile and more vulnerable to immune-driven damage. If you get canker sores frequently, it’s worth having your B12 and folate levels checked. B12 below 200 pg/mL is considered deficient, and folate below 3 ng/mL raises a red flag. Correcting a deficiency through diet or supplements often reduces how often sores come back.
Stress as a Trigger
People with recurrent canker sores consistently report higher levels of psychological stress than people without them. Research confirms this isn’t just perception: studies comparing canker sore patients with matched control groups find statistically significant differences in stress levels. Stress likely acts as a trigger or amplifier rather than a root cause on its own. It suppresses parts of the immune system while ramping up inflammation, creating conditions where the misdirected immune attack on your mouth tissue is more likely to happen.
This explains why many people notice outbreaks during exam periods, work deadlines, or emotionally difficult stretches. The sores aren’t “caused” by stress in the way a burn is caused by heat, but stress can tip the balance in someone already predisposed.
Foods That Trigger or Worsen Sores
Certain foods can both initiate new sores and make existing ones worse. The main culprits fall into a few categories:
- Acidic foods: Oranges, lemons, grapefruits, tomatoes, tomato sauces, and vinegar-heavy dressings. These create a sharp sting on contact and can slow healing of existing sores.
- Spicy foods: Chili peppers and hot sauces irritate delicate mouth tissue, and the burning sensation often lingers well after the meal.
- Rough or hard textures: Chips, crackers, crusty bread, and nuts can physically scrape the inside of your mouth, reopening healing sores or creating micro-injuries where new ones form.
- Salty or processed foods: High salt content increases irritation on any existing sore.
Very hot beverages can also aggravate sores by increasing blood flow and sensitivity in the area. Alcohol dries out the mouth and slows healing, while carbonated drinks add irritation from both fizz and acidity.
Physical and Chemical Irritation
Biting the inside of your cheek, getting poked by a sharp chip, or having braces rub against your gums can all spark a canker sore in someone who’s predisposed. The physical damage gives the overactive immune response an entry point.
Sodium lauryl sulfate (SLS), a foaming agent in most toothpastes, is another commonly discussed trigger. SLS can irritate the mouth’s mucous lining, and some research has found that switching to an SLS-free toothpaste reduced the number and frequency of sores. However, other studies found no significant change in ulcer patterns after switching. The evidence is mixed, but trying an SLS-free toothpaste is a low-risk experiment if you get frequent outbreaks.
Connections to Digestive Conditions
Canker sores can be a sign of something happening deeper in your digestive system. Celiac disease and inflammatory bowel disease (IBD) are both associated with frequent mouth sores. In an Italian survey of patients with celiac disease or IBD, about 66% reported oral symptoms during active flare-ups of their intestinal disease, and canker sores were the most common complaint by far, affecting 63% of those with oral symptoms.
The connection makes sense biologically. Your mouth is the beginning of your digestive tract, and the same immune dysfunction driving inflammation in the gut can target mouth tissue. For people who get canker sores alongside chronic digestive symptoms like bloating, diarrhea, or unexplained weight loss, the sores may be a visible clue pointing toward an undiagnosed condition. In celiac disease specifically, the nutrient absorption problems caused by intestinal damage can also create the B12 and folate deficiencies that independently trigger sores.
Canker Sores vs. Cold Sores
These two are often confused, but they’re entirely different. The National Institute of Dental and Craniofacial Research highlights two key differences. First, location: canker sores occur inside the mouth, while cold sores (fever blisters) form on the outside, typically around the border of the lips. Second, appearance: canker sores are usually a single round sore, white or yellow with a red border, while cold sores appear as clusters of small, fluid-filled blisters.
Cold sores are caused by the herpes simplex virus and are contagious. Canker sores are not viral and cannot be spread to another person. If your sore is inside your mouth and looks like a single shallow crater, it’s almost certainly a canker sore.
How Long They Last
Most canker sores are the “minor” type, measuring less than a centimeter across. These typically heal on their own within one to two weeks without leaving a scar. Major canker sores are larger, deeper, and significantly more painful. They can take weeks or even longer to resolve and are more likely to leave scar tissue behind. A third, less common type called herpetiform ulcers appears as clusters of tiny sores that may merge together, despite having nothing to do with the herpes virus.
If a canker sore hasn’t healed after three weeks, is unusually large, comes with a fever, or makes it genuinely difficult to eat or drink, that’s a sign something more than a typical outbreak is going on.