Canker sores don’t have a single, definitive cause. They result from a combination of immune system overreaction, nutritional gaps, physical triggers, and genetic predisposition. Unlike cold sores, they are not caused by a virus and are not contagious. Understanding the factors that contribute to them can help you reduce how often they appear and how long they last.
Your Immune System Attacks Mouth Tissue
The most fundamental driver of canker sores is an inappropriate immune response against your own oral tissue. Certain immune cells, particularly T cells that normally fight infection, begin destroying the thin lining inside your mouth. This process is fueled by a cascade of inflammatory signaling molecules that sustain the attack, creating the painful open sore you feel on your cheek, tongue, or gums.
People who get recurrent canker sores show higher levels of several inflammatory markers in their blood, along with an elevated ratio of certain white blood cells compared to people who rarely get them. Their immune systems also show abnormal activity in a pathway that normally helps distinguish harmful invaders from the body’s own tissue, which may explain why the immune response misfires against the mouth lining in the first place. This is why canker sores behave more like a localized autoimmune event than a simple wound.
Vitamin and Mineral Deficiencies
Nutritional shortfalls are one of the most actionable causes of recurrent canker sores. The most commonly implicated deficiencies include:
- Vitamin B12: The most frequently linked deficiency. B12 supports red blood cell production and tissue repair, so low levels lead to poor healing of mouth tissue.
- Folate (vitamin B9): Essential for cell growth and repair. Low folate triggers oral inflammation and cracking at the corners of the lips alongside ulcers.
- Iron: Low iron reduces oxygen delivery to tissues, making the mouth lining more fragile and prone to breakdown.
- Zinc: Supports immune function and tissue repair. Deficiency delays healing and increases the frequency of sores.
- Vitamin C: Strengthens blood vessels and promotes collagen formation. Without enough, wound healing slows throughout the mouth.
If you get canker sores frequently, a simple blood test can check these levels. Correcting a deficiency often reduces recurrence significantly.
Physical Trauma to the Mouth
Mechanical injury is one of the most common immediate triggers. Biting the inside of your cheek, getting poked by a sharp chip or cracker, brushing too aggressively, or having dental work done can all damage the mouth lining enough to set off the immune cascade that produces a canker sore. Braces and ill-fitting dentures are repeat offenders because they create ongoing friction against the same spots.
Not everyone who bites their cheek develops a canker sore, of course. The injury acts as a trigger in people whose immune systems are already primed to overreact. This is why the same minor trauma that heals uneventfully in one person produces a full ulcer in another.
Your Toothpaste May Be a Factor
Sodium lauryl sulfate (SLS), a foaming agent found in most toothpastes, is a well-documented trigger. A systematic review published in the Journal of Oral Pathology and Medicine found that switching to an SLS-free toothpaste significantly reduced the number of ulcers, the duration of each ulcer, the number of episodes, and the level of pain. Participants using SLS-free toothpaste had roughly one fewer ulcer on average per observation period, and their sores healed about two days faster.
SLS strips away the protective mucous layer inside your mouth, leaving the tissue more vulnerable to irritation and immune attack. If you get canker sores regularly, switching to an SLS-free toothpaste is one of the simplest changes you can make. Several major brands now offer SLS-free versions.
Genetics and Family History
Canker sores run in families. If both of your parents get them, your risk is substantially higher than someone with no family history. Studies estimate the overall prevalence of recurrent canker sores at roughly 4% of the population at any given time, though many more people experience them occasionally at some point in their lives. The genetic component likely influences how your immune system responds to minor oral injuries and irritants, which is why some people are far more prone to them than others despite similar diets and habits.
Hormonal Fluctuations
Many women notice canker sores appearing just before or during their period, when estrogen levels drop. Some research has found that estrogen therapy reduced recurrent canker sores in women who had a clear menstrual link. However, the relationship is complicated. A 2024 Mendelian randomization study found no direct causal link between baseline estrogen levels and mouth ulcers, suggesting that it may be the sudden shift in hormones, rather than low levels themselves, that disrupts the immune environment in the mouth and triggers sores.
Underlying Health Conditions
Frequent canker sores can be a symptom of a broader medical issue. Celiac disease is one of the most important to rule out, because mouth ulcers are sometimes the only visible sign of gluten sensitivity before digestive symptoms develop. Crohn’s disease, which causes inflammation throughout the digestive tract, also commonly produces oral ulcers. Other conditions associated with recurrent canker sores include Behçet syndrome (an inflammatory disorder affecting blood vessels), immune deficiencies, and in children, a condition called PFAPA syndrome that causes periodic fevers alongside mouth sores.
If you experience canker sores frequently, especially alongside fatigue, digestive problems, or joint pain, screening for these conditions is worth pursuing.
Canker Sores vs. Cold Sores
These are often confused, but they are entirely different conditions. Canker sores appear inside the mouth as single, round white or yellow sores with a red border. Cold sores (fever blisters) appear outside the mouth, typically along the lip border, as clusters of small fluid-filled blisters. Cold sores are caused by herpes simplex virus and are highly contagious. Canker sores have no viral cause and cannot be spread to another person.
How Long They Last
Most canker sores are minor and heal within about two weeks without any treatment. Pain typically improves within a few days, even if the sore itself is still visible. Major canker sores, which are larger and deeper, can take months to heal and may leave scarring. A less common type called herpetiform canker sores (named for their appearance, not because they involve herpes) appear as clusters of tiny ulcers that typically heal within about two weeks.
If a canker sore hasn’t healed after three weeks, is unusually large, or keeps coming back in the same spot, that pattern warrants a closer look to rule out other conditions.